Why does printf not flush after the call unless a newline is in the format string?
Why does printf not flush after the call unless a newline is in the format string? Is this POSIX behavior? How might I have printf immediately flush every time?
c printf flush
edited Jul 24 '18 at 17:18
K DawG
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
add a comment |
Why does printf not flush after the call unless a newline is in the format string? Is this POSIX behavior? How might I have printf immediately flush every time?
c printf flush
edited Jul 24 '18 at 17:18
K DawG
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
2
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
6
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fixsetvbuf(stdout, (char*)NULL, _IONBF, 0)works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~
– Steve Pitchers
Jan 8 '13 at 14:10
9
To clarify the title of the question:printf(..)does not do any flushing itself, it's the buffering ofstdoutthat may flush when seeing a newline (if it's line-buffered). It would react the same way toputchar('n');, soprintf(..)is not special in this regard. This is in contrast withcout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.
– Evgeni Sergeev
Apr 5 '16 at 14:02
1
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31
add a comment |
Why does printf not flush after the call unless a newline is in the format string? Is this POSIX behavior? How might I have printf immediately flush every time?
c printf flush
edited Jul 24 '18 at 17:18
K DawG
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
Why does printf not flush after the call unless a newline is in the format string? Is this POSIX behavior? How might I have printf immediately flush every time?
c printf flush
c printf flush
edited Jul 24 '18 at 17:18
K DawG
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
edited Jul 24 '18 at 17:18
K DawG
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
edited Jul 24 '18 at 17:18
K DawG
6,92682358
edited Jul 24 '18 at 17:18
K DawG
6,92682358
edited Jul 24 '18 at 17:18
K DawG
6,92682358
6,92682358
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
asked Nov 11 '09 at 16:22
Crazy ChenzCrazy Chenz
4,808104057
4,808104057
2
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
6
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fixsetvbuf(stdout, (char*)NULL, _IONBF, 0)works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~
– Steve Pitchers
Jan 8 '13 at 14:10
9
To clarify the title of the question:printf(..)does not do any flushing itself, it's the buffering ofstdoutthat may flush when seeing a newline (if it's line-buffered). It would react the same way toputchar('n');, soprintf(..)is not special in this regard. This is in contrast withcout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.
– Evgeni Sergeev
Apr 5 '16 at 14:02
1
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31
add a comment |
2
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
6
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fixsetvbuf(stdout, (char*)NULL, _IONBF, 0)works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~
– Steve Pitchers
Jan 8 '13 at 14:10
9
To clarify the title of the question:printf(..)does not do any flushing itself, it's the buffering ofstdoutthat may flush when seeing a newline (if it's line-buffered). It would react the same way toputchar('n');, soprintf(..)is not special in this regard. This is in contrast withcout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.
– Evgeni Sergeev
Apr 5 '16 at 14:02
1
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31
2
2
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
6
6
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fix
setvbuf(stdout, (char*)NULL, _IONBF, 0) works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~– Steve Pitchers
Jan 8 '13 at 14:10
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fix
setvbuf(stdout, (char*)NULL, _IONBF, 0) works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~– Steve Pitchers
Jan 8 '13 at 14:10
9
9
To clarify the title of the question:
printf(..) does not do any flushing itself, it's the buffering of stdout that may flush when seeing a newline (if it's line-buffered). It would react the same way to putchar('n');, so printf(..) is not special in this regard. This is in contrast with cout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.– Evgeni Sergeev
Apr 5 '16 at 14:02
To clarify the title of the question:
printf(..) does not do any flushing itself, it's the buffering of stdout that may flush when seeing a newline (if it's line-buffered). It would react the same way to putchar('n');, so printf(..) is not special in this regard. This is in contrast with cout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.– Evgeni Sergeev
Apr 5 '16 at 14:02
1
1
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31
add a comment |
9 Answers
9
active
oldest
votes
The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:
Print to stderr instead using fprintf:
fprintf(stderr, "I will be printed immediately");
Flush stdout whenever you need it to using fflush:
printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer
Edit: From Andy Ross's comment below, you can also disable buffering on stdout by using setbuf:
setbuf(stdout, NULL);
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
242
Or, to disable buffering entirely:setbuf(stdout, NULL);
– Andy Ross
Nov 11 '09 at 17:42
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
I feel that I should add: I've just been testing this theory, and I am finding that usingsetlinebuf()on a stream which is not directed to a terminal is flushing at the end of each line.
– Doddy
Sep 6 '11 at 19:06
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
|
show 8 more comments
No, it's not POSIX behaviour, it's ISO behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO).
Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:
myprog >myfile.txt
This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.
The first is that it's not very efficient. The second is that the original ANSI C mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO nowadays treads very carefully when changing existing rules in the standards.
As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.
Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:
setvbuf (stdout, NULL, _IONBF, BUFSIZ);
Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.
ISO C99 section 7.19.3/3 is the relevant bit:
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.
When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.
When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.
Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.
Support for these characteristics is implementation-defined, and may be affected via the
setbufandsetvbuffunctions.
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output withmyprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.
– paxdiablo
Apr 29 '12 at 0:20
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
add a comment |
It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:
program A output
program B output
program B output
program A output
program B output
This stinks, but it's better than
proprogrgraam m AB ououtputputt
prproogrgram amB A ououtputtput
program B output
Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
add a comment |
To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
25
Keep in mindfflush(NULL);is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.
– R..
Jun 9 '11 at 13:57
add a comment |
Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediatelly to terminal"):
http://msdn.microsoft.com/en-us/library/86cebhfs.aspx
edited Oct 14 '13 at 20:36
Jack
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
2
Worse thanprintfgoing immediately to the terminal in the "normal" case is the fact thatprintfandfprintfget more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.
– supercat
Aug 13 '15 at 18:37
add a comment |
stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
- Print to stderr.
- Make stdout unbuffered.
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
6
Orfflush(stdout).
– RastaJedi
Feb 22 '16 at 22:47
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write tostderr(this answer mentions later),fflush(stdout),fflush(NULL).
– chux
Dec 22 '17 at 10:20
add a comment |
by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
answered Jul 29 '10 at 2:02
wosowoso
20136
add a comment |
You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
add a comment |
Use setbuf(stdout, NULL); to disable buffering.
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
add a comment |
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9 Answers
9
active
oldest
votes
9 Answers
9
active
oldest
votes
active
oldest
votes
active
oldest
votes
The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:
Print to stderr instead using fprintf:
fprintf(stderr, "I will be printed immediately");
Flush stdout whenever you need it to using fflush:
printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer
Edit: From Andy Ross's comment below, you can also disable buffering on stdout by using setbuf:
setbuf(stdout, NULL);
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
242
Or, to disable buffering entirely:setbuf(stdout, NULL);
– Andy Ross
Nov 11 '09 at 17:42
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
I feel that I should add: I've just been testing this theory, and I am finding that usingsetlinebuf()on a stream which is not directed to a terminal is flushing at the end of each line.
– Doddy
Sep 6 '11 at 19:06
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
|
show 8 more comments
The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:
Print to stderr instead using fprintf:
fprintf(stderr, "I will be printed immediately");
Flush stdout whenever you need it to using fflush:
printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer
Edit: From Andy Ross's comment below, you can also disable buffering on stdout by using setbuf:
setbuf(stdout, NULL);
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
242
Or, to disable buffering entirely:setbuf(stdout, NULL);
– Andy Ross
Nov 11 '09 at 17:42
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
I feel that I should add: I've just been testing this theory, and I am finding that usingsetlinebuf()on a stream which is not directed to a terminal is flushing at the end of each line.
– Doddy
Sep 6 '11 at 19:06
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
|
show 8 more comments
The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:
Print to stderr instead using fprintf:
fprintf(stderr, "I will be printed immediately");
Flush stdout whenever you need it to using fflush:
printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer
Edit: From Andy Ross's comment below, you can also disable buffering on stdout by using setbuf:
setbuf(stdout, NULL);
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
The stdout stream is buffered, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:
Print to stderr instead using fprintf:
fprintf(stderr, "I will be printed immediately");
Flush stdout whenever you need it to using fflush:
printf("Buffered, will be flushed");
fflush(stdout); // Will now print everything in the stdout buffer
Edit: From Andy Ross's comment below, you can also disable buffering on stdout by using setbuf:
setbuf(stdout, NULL);
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
edited Nov 11 '09 at 20:24
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
answered Nov 11 '09 at 17:04
Rudd ZwolinskiRudd Zwolinski
15.2k144858
15.2k144858
242
Or, to disable buffering entirely:setbuf(stdout, NULL);
– Andy Ross
Nov 11 '09 at 17:42
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
I feel that I should add: I've just been testing this theory, and I am finding that usingsetlinebuf()on a stream which is not directed to a terminal is flushing at the end of each line.
– Doddy
Sep 6 '11 at 19:06
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
|
show 8 more comments
242
Or, to disable buffering entirely:setbuf(stdout, NULL);
– Andy Ross
Nov 11 '09 at 17:42
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
I feel that I should add: I've just been testing this theory, and I am finding that usingsetlinebuf()on a stream which is not directed to a terminal is flushing at the end of each line.
– Doddy
Sep 6 '11 at 19:06
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
242
242
Or, to disable buffering entirely:
setbuf(stdout, NULL);– Andy Ross
Nov 11 '09 at 17:42
Or, to disable buffering entirely:
setbuf(stdout, NULL);– Andy Ross
Nov 11 '09 at 17:42
67
67
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush.
– hora
Mar 5 '11 at 23:10
4
4
I feel that I should add: I've just been testing this theory, and I am finding that using
setlinebuf() on a stream which is not directed to a terminal is flushing at the end of each line.– Doddy
Sep 6 '11 at 19:06
I feel that I should add: I've just been testing this theory, and I am finding that using
setlinebuf() on a stream which is not directed to a terminal is flushing at the end of each line.– Doddy
Sep 6 '11 at 19:06
7
7
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/…
– Seppo Enarvi
May 22 '15 at 7:23
1
1
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments...
– HostileFork
Apr 8 '16 at 22:08
|
show 8 more comments
No, it's not POSIX behaviour, it's ISO behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO).
Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:
myprog >myfile.txt
This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.
The first is that it's not very efficient. The second is that the original ANSI C mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO nowadays treads very carefully when changing existing rules in the standards.
As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.
Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:
setvbuf (stdout, NULL, _IONBF, BUFSIZ);
Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.
ISO C99 section 7.19.3/3 is the relevant bit:
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.
When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.
When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.
Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.
Support for these characteristics is implementation-defined, and may be affected via the
setbufandsetvbuffunctions.
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output withmyprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.
– paxdiablo
Apr 29 '12 at 0:20
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
add a comment |
No, it's not POSIX behaviour, it's ISO behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO).
Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:
myprog >myfile.txt
This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.
The first is that it's not very efficient. The second is that the original ANSI C mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO nowadays treads very carefully when changing existing rules in the standards.
As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.
Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:
setvbuf (stdout, NULL, _IONBF, BUFSIZ);
Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.
ISO C99 section 7.19.3/3 is the relevant bit:
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.
When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.
When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.
Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.
Support for these characteristics is implementation-defined, and may be affected via the
setbufandsetvbuffunctions.
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output withmyprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.
– paxdiablo
Apr 29 '12 at 0:20
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
add a comment |
No, it's not POSIX behaviour, it's ISO behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO).
Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:
myprog >myfile.txt
This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.
The first is that it's not very efficient. The second is that the original ANSI C mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO nowadays treads very carefully when changing existing rules in the standards.
As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.
Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:
setvbuf (stdout, NULL, _IONBF, BUFSIZ);
Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.
ISO C99 section 7.19.3/3 is the relevant bit:
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.
When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.
When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.
Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.
Support for these characteristics is implementation-defined, and may be affected via the
setbufandsetvbuffunctions.
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
No, it's not POSIX behaviour, it's ISO behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO).
Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:
myprog >myfile.txt
This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.
The first is that it's not very efficient. The second is that the original ANSI C mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO nowadays treads very carefully when changing existing rules in the standards.
As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.
Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:
setvbuf (stdout, NULL, _IONBF, BUFSIZ);
Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.
ISO C99 section 7.19.3/3 is the relevant bit:
When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.
When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.
When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.
Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.
Support for these characteristics is implementation-defined, and may be affected via the
setbufandsetvbuffunctions.
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
edited Nov 17 '10 at 3:58
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
answered Nov 17 '10 at 3:52
paxdiablopaxdiablo
643k17512661687
643k17512661687
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output withmyprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.
– paxdiablo
Apr 29 '12 at 0:20
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
add a comment |
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output withmyprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.
– paxdiablo
Apr 29 '12 at 0:20
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
7
7
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
I just came across a scenario where even there is a 'n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why 'n' failed to flush the buffer in printf().
– Qiang Xu
Apr 28 '12 at 19:45
9
9
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output with
myprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.– paxdiablo
Apr 29 '12 at 0:20
@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output with
myprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation.– paxdiablo
Apr 29 '12 at 0:20
2
2
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx
– Piotr Lopusiewicz
Feb 5 '15 at 10:02
add a comment |
It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:
program A output
program B output
program B output
program A output
program B output
This stinks, but it's better than
proprogrgraam m AB ououtputputt
prproogrgram amB A ououtputtput
program B output
Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
add a comment |
It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:
program A output
program B output
program B output
program A output
program B output
This stinks, but it's better than
proprogrgraam m AB ououtputputt
prproogrgram amB A ououtputtput
program B output
Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
add a comment |
It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:
program A output
program B output
program B output
program A output
program B output
This stinks, but it's better than
proprogrgraam m AB ououtputputt
prproogrgram amB A ououtputtput
program B output
Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:
program A output
program B output
program B output
program A output
program B output
This stinks, but it's better than
proprogrgraam m AB ououtputputt
prproogrgram amB A ououtputtput
program B output
Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
answered Nov 11 '09 at 17:54
Southern HospitalitySouthern Hospitality
1,0171711
1,0171711
add a comment |
add a comment |
To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
25
Keep in mindfflush(NULL);is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.
– R..
Jun 9 '11 at 13:57
add a comment |
To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
25
Keep in mindfflush(NULL);is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.
– R..
Jun 9 '11 at 13:57
add a comment |
To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
edited Sep 25 '15 at 1:10
Cristian Ciupitu
14.9k54264
14.9k54264
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
answered Nov 11 '09 at 16:26
AaronAaron
7,93843538
7,93843538
25
Keep in mindfflush(NULL);is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.
– R..
Jun 9 '11 at 13:57
add a comment |
25
Keep in mindfflush(NULL);is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.
– R..
Jun 9 '11 at 13:57
25
25
Keep in mind
fflush(NULL); is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.– R..
Jun 9 '11 at 13:57
Keep in mind
fflush(NULL); is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks.– R..
Jun 9 '11 at 13:57
add a comment |
Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediatelly to terminal"):
http://msdn.microsoft.com/en-us/library/86cebhfs.aspx
edited Oct 14 '13 at 20:36
Jack
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
2
Worse thanprintfgoing immediately to the terminal in the "normal" case is the fact thatprintfandfprintfget more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.
– supercat
Aug 13 '15 at 18:37
add a comment |
Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediatelly to terminal"):
http://msdn.microsoft.com/en-us/library/86cebhfs.aspx
edited Oct 14 '13 at 20:36
Jack
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
2
Worse thanprintfgoing immediately to the terminal in the "normal" case is the fact thatprintfandfprintfget more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.
– supercat
Aug 13 '15 at 18:37
add a comment |
Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediatelly to terminal"):
http://msdn.microsoft.com/en-us/library/86cebhfs.aspx
edited Oct 14 '13 at 20:36
Jack
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediatelly to terminal"):
http://msdn.microsoft.com/en-us/library/86cebhfs.aspx
edited Oct 14 '13 at 20:36
Jack
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
edited Oct 14 '13 at 20:36
Jack
110k26187293
edited Oct 14 '13 at 20:36
Jack
110k26187293
edited Oct 14 '13 at 20:36
Jack
110k26187293
110k26187293
answered Oct 26 '10 at 20:47
RenatoRenato
14112
answered Oct 26 '10 at 20:47
RenatoRenato
14112
answered Oct 26 '10 at 20:47
RenatoRenato
14112
14112
2
Worse thanprintfgoing immediately to the terminal in the "normal" case is the fact thatprintfandfprintfget more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.
– supercat
Aug 13 '15 at 18:37
add a comment |
2
Worse thanprintfgoing immediately to the terminal in the "normal" case is the fact thatprintfandfprintfget more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.
– supercat
Aug 13 '15 at 18:37
2
2
Worse than
printf going immediately to the terminal in the "normal" case is the fact that printf and fprintf get more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.– supercat
Aug 13 '15 at 18:37
Worse than
printf going immediately to the terminal in the "normal" case is the fact that printf and fprintf get more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each.– supercat
Aug 13 '15 at 18:37
add a comment |
stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
- Print to stderr.
- Make stdout unbuffered.
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
6
Orfflush(stdout).
– RastaJedi
Feb 22 '16 at 22:47
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write tostderr(this answer mentions later),fflush(stdout),fflush(NULL).
– chux
Dec 22 '17 at 10:20
add a comment |
stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
- Print to stderr.
- Make stdout unbuffered.
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
6
Orfflush(stdout).
– RastaJedi
Feb 22 '16 at 22:47
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write tostderr(this answer mentions later),fflush(stdout),fflush(NULL).
– chux
Dec 22 '17 at 10:20
add a comment |
stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
- Print to stderr.
- Make stdout unbuffered.
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
- Print to stderr.
- Make stdout unbuffered.
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
answered Nov 11 '09 at 16:25
Douglas LeederDouglas Leeder
44.2k878120
44.2k878120
6
Orfflush(stdout).
– RastaJedi
Feb 22 '16 at 22:47
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write tostderr(this answer mentions later),fflush(stdout),fflush(NULL).
– chux
Dec 22 '17 at 10:20
add a comment |
6
Orfflush(stdout).
– RastaJedi
Feb 22 '16 at 22:47
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write tostderr(this answer mentions later),fflush(stdout),fflush(NULL).
– chux
Dec 22 '17 at 10:20
6
6
Or
fflush(stdout).– RastaJedi
Feb 22 '16 at 22:47
Or
fflush(stdout).– RastaJedi
Feb 22 '16 at 22:47
2
2
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write to
stderr (this answer mentions later), fflush(stdout), fflush(NULL).– chux
Dec 22 '17 at 10:20
"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write to
stderr (this answer mentions later), fflush(stdout), fflush(NULL).– chux
Dec 22 '17 at 10:20
add a comment |
by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
answered Jul 29 '10 at 2:02
wosowoso
20136
add a comment |
by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
answered Jul 29 '10 at 2:02
wosowoso
20136
add a comment |
by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
answered Jul 29 '10 at 2:02
wosowoso
20136
by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
answered Jul 29 '10 at 2:02
wosowoso
20136
answered Jul 29 '10 at 2:02
wosowoso
20136
answered Jul 29 '10 at 2:02
wosowoso
20136
answered Jul 29 '10 at 2:02
wosowoso
20136
20136
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You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
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You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
add a comment |
You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
answered Nov 11 '09 at 16:26
Rasmus KajRasmus Kaj
3,1511521
3,1511521
add a comment |
add a comment |
Use setbuf(stdout, NULL); to disable buffering.
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
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Use setbuf(stdout, NULL); to disable buffering.
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
add a comment |
Use setbuf(stdout, NULL); to disable buffering.
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
Use setbuf(stdout, NULL); to disable buffering.
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
answered May 31 '15 at 3:22
dnahc araknayirpdnahc araknayirp
8114
8114
add a comment |
add a comment |
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2
did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program.
– PypeBros
Nov 12 '09 at 16:50
6
Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fix
setvbuf(stdout, (char*)NULL, _IONBF, 0)works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~– Steve Pitchers
Jan 8 '13 at 14:10
9
To clarify the title of the question:
printf(..)does not do any flushing itself, it's the buffering ofstdoutthat may flush when seeing a newline (if it's line-buffered). It would react the same way toputchar('n');, soprintf(..)is not special in this regard. This is in contrast withcout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all.– Evgeni Sergeev
Apr 5 '16 at 14:02
1
writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons.
– hanshenrik
Aug 17 '17 at 23:31