How can I detect if a file exists?
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I'm programming in C and trying to write portable code.
My question is, how can I tell if a file exists and is readable?
I am currently using the code:
f = fopen(filename, "r");
if (f) printf("File exists!");
In my program, filename is set by the user, and should be considered as untrusted input (e.g. filename could be maliciously crafted).
My issue is that the above code is not robust. For example, when used on windows with the filename "PRN" it will print "File exists!" even though no such file exists on the filesystem.
I know I could filter out the reserved filenames on Windows, as there is only about a dozen of them, but that feels like a hack. Also, I only know what I know. Maybe there are other "reserved" or "special" names that I don't know about.
Is there any simple and portable way to determine if a file exists in C?
Alternatively, if I have to use an OS API, which function should I use?
c windows filesystems
asked Jan 4 at 6:16
jimjim
192
|
show 10 more comments
I'm programming in C and trying to write portable code.
My question is, how can I tell if a file exists and is readable?
I am currently using the code:
f = fopen(filename, "r");
if (f) printf("File exists!");
In my program, filename is set by the user, and should be considered as untrusted input (e.g. filename could be maliciously crafted).
My issue is that the above code is not robust. For example, when used on windows with the filename "PRN" it will print "File exists!" even though no such file exists on the filesystem.
I know I could filter out the reserved filenames on Windows, as there is only about a dozen of them, but that feels like a hack. Also, I only know what I know. Maybe there are other "reserved" or "special" names that I don't know about.
Is there any simple and portable way to determine if a file exists in C?
Alternatively, if I have to use an OS API, which function should I use?
c windows filesystems
asked Jan 4 at 6:16
jimjim
192
2
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
3
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as usingaccess()orstat()orlstat()are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.
– Jonathan Leffler
Jan 4 at 6:39
1
You probably want one of the File Management Functions, and probably one in theGetFileAttributes*family.
– jxh
Jan 4 at 6:53
1
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
1
@Tom's Did you read the question? Windows has reserved filenames. If you usefopen("COM1.jpg", "r")on Windows, it will open a pipe to a serial port. The fileCOM1.jpgdoes not (and indeed isn't allow to) exist as an actual file.
– jim
Jan 4 at 16:12
|
show 10 more comments
I'm programming in C and trying to write portable code.
My question is, how can I tell if a file exists and is readable?
I am currently using the code:
f = fopen(filename, "r");
if (f) printf("File exists!");
In my program, filename is set by the user, and should be considered as untrusted input (e.g. filename could be maliciously crafted).
My issue is that the above code is not robust. For example, when used on windows with the filename "PRN" it will print "File exists!" even though no such file exists on the filesystem.
I know I could filter out the reserved filenames on Windows, as there is only about a dozen of them, but that feels like a hack. Also, I only know what I know. Maybe there are other "reserved" or "special" names that I don't know about.
Is there any simple and portable way to determine if a file exists in C?
Alternatively, if I have to use an OS API, which function should I use?
c windows filesystems
asked Jan 4 at 6:16
jimjim
192
I'm programming in C and trying to write portable code.
My question is, how can I tell if a file exists and is readable?
I am currently using the code:
f = fopen(filename, "r");
if (f) printf("File exists!");
In my program, filename is set by the user, and should be considered as untrusted input (e.g. filename could be maliciously crafted).
My issue is that the above code is not robust. For example, when used on windows with the filename "PRN" it will print "File exists!" even though no such file exists on the filesystem.
I know I could filter out the reserved filenames on Windows, as there is only about a dozen of them, but that feels like a hack. Also, I only know what I know. Maybe there are other "reserved" or "special" names that I don't know about.
Is there any simple and portable way to determine if a file exists in C?
Alternatively, if I have to use an OS API, which function should I use?
c windows filesystems
c windows filesystems
asked Jan 4 at 6:16
jimjim
192
asked Jan 4 at 6:16
jimjim
192
asked Jan 4 at 6:16
jimjim
192
asked Jan 4 at 6:16
jimjim
192
asked Jan 4 at 6:16
jimjim
192
192
2
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
3
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as usingaccess()orstat()orlstat()are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.
– Jonathan Leffler
Jan 4 at 6:39
1
You probably want one of the File Management Functions, and probably one in theGetFileAttributes*family.
– jxh
Jan 4 at 6:53
1
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
1
@Tom's Did you read the question? Windows has reserved filenames. If you usefopen("COM1.jpg", "r")on Windows, it will open a pipe to a serial port. The fileCOM1.jpgdoes not (and indeed isn't allow to) exist as an actual file.
– jim
Jan 4 at 16:12
|
show 10 more comments
2
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
3
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as usingaccess()orstat()orlstat()are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.
– Jonathan Leffler
Jan 4 at 6:39
1
You probably want one of the File Management Functions, and probably one in theGetFileAttributes*family.
– jxh
Jan 4 at 6:53
1
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
1
@Tom's Did you read the question? Windows has reserved filenames. If you usefopen("COM1.jpg", "r")on Windows, it will open a pipe to a serial port. The fileCOM1.jpgdoes not (and indeed isn't allow to) exist as an actual file.
– jim
Jan 4 at 16:12
2
2
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
3
3
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as using
access() or stat() or lstat() are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.– Jonathan Leffler
Jan 4 at 6:39
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as using
access() or stat() or lstat() are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.– Jonathan Leffler
Jan 4 at 6:39
1
1
You probably want one of the File Management Functions, and probably one in the
GetFileAttributes* family.– jxh
Jan 4 at 6:53
You probably want one of the File Management Functions, and probably one in the
GetFileAttributes* family.– jxh
Jan 4 at 6:53
1
1
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
1
1
@Tom's Did you read the question? Windows has reserved filenames. If you use
fopen("COM1.jpg", "r") on Windows, it will open a pipe to a serial port. The file COM1.jpg does not (and indeed isn't allow to) exist as an actual file.– jim
Jan 4 at 16:12
@Tom's Did you read the question? Windows has reserved filenames. If you use
fopen("COM1.jpg", "r") on Windows, it will open a pipe to a serial port. The file COM1.jpg does not (and indeed isn't allow to) exist as an actual file.– jim
Jan 4 at 16:12
|
show 10 more comments
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2
Just FYI: Portable code is a myth. There are applications which are ported successfully, but really there is nothing called universally portable.
– Sourav Ghosh
Jan 4 at 6:17
3
Attempting to open the file is one of the most reliable and portable ways to see whether the file can be opened. It typically isn't subject to the TOCTOU (Time Of Check, Time Of Use) attacks that alternatives such as using
access()orstat()orlstat()are vulnerable to. And those alternatives are POSIX-specific which may limit their usefulness for you.– Jonathan Leffler
Jan 4 at 6:39
1
You probably want one of the File Management Functions, and probably one in the
GetFileAttributes*family.– jxh
Jan 4 at 6:53
1
@jim No, fopen doesn't magically succeed when the file doesn't exist. Either the file exist (try to use an absolute path) or your code is brken (then show an MCVE).
– Tom's
Jan 4 at 15:07
1
@Tom's Did you read the question? Windows has reserved filenames. If you use
fopen("COM1.jpg", "r")on Windows, it will open a pipe to a serial port. The fileCOM1.jpgdoes not (and indeed isn't allow to) exist as an actual file.– jim
Jan 4 at 16:12