Registered: Feb 2003
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Can anyone tell me if there is a standard function I could use in C code to determine whether a named folder exists or not? I know I can do it in shell, but I'd like to do it in C.

Does fopen set errno to something useful if the attempted open is a preexisting folder?

Thankee.

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You'll return NULL if you try to fopen a file for reading that doesn't exist (don't try to open it for writing, or it'll create one if it doesn't exist, or nuke the existing one if it does). I think that you can check for NULL in C, if I recall aright.

NULL is returned if there is any error, however, not just a 'no file of that name exists' error.

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I specifically want to check for whether a given *folder* exists, though. Ideally, I'd like a function which returned different values if you handed it a path to an existing file, a path to a nonexisting file, a path to an existing directory, and a path to a nonesxisting directiory.

Actually, what *does* fopen do if you hand it a path to an existing folder?

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Don't recall much errorhandling from C, but fopen might be written so that you can check against the error to find out what the cause was, specifically.

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Ah, sorry, read that as 'file'.

fopen probably throws some special 'this isn't a file, you moron' error. Or perhaps just returns a pointer to 'moron'.

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There has to be a better way of doing this, but if you ask it to open a nonexistent file (qwertyuiop.asd, say) in a folder of whose existence you're not sure, maybe it throws a different error to the 'can't find file' one (given that it must check along the location string from left to right).

I haven't written C in years and years.

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I think you're right about it setting errno based on what the problem was, but I'm having trouble determining if "this is a folder" is one of the errors.

#define moron TALAROHK

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Hmm. That does seem roundabout, as well as leaving little files around if the folder DOES exist, but I guess it would work in a pinch.

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If you try to open to read, it won't create a file if none exists. Only if you open to write or append, I think.

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You're right.
Upon further looking, fopen might set errno to EISDIR if the named path is a directory and you're trying to write. Not sure what happens if you're trying to read.

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It might be that stat is your friend in this. Not anything I've used, though.

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In general, chapter 8 in Kernighan and Richie is about the Unix interface. There's a function called opendir, too

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Let's be clear, I know fucking nothing about this, really.

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Opendir seems to return null for nonexistent directories, and non-null for existing ones. Perfect.
Thanks, Smug!

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I am not a coder:

code:

#include <sys/stat.h>
int isdirectory(char *path)
  {
    struct stat statbuf;
    if (stat(path, &statbuf) == -1)
      return 0;
    else
      return S_ISDIR(statbuf.st_mode);
  }

code:

#include <sys/stat.h>
#include <stdio.h>

int isdirectory(char *path);

int main()
  {
    if (isdirectory("./"))
        printf("Yes\n");
    else
        printf("No\n");
    if (isdirectory("boob"))
        printf("Yes\n");
    else
        printf("No\n");
  }
---snip--

code:

Yes
No

Last edited by zim on 02-28-2006 at 01:55 AM

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http://www.scit.wlv.ac.uk/~jphb/spo...calls/stat.html the stat() example there does the same sort of thing, from the command line.

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filthy (sexy) nerds.

*PS. Just assume I took a wrong turn at the Gonad Forum.

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That sounds like it would work, too. Many thanks, all.

Ignatz--my doctoral advisor's wife (who was also a professor, with the next office down the hall) had a serious thing for geeks. Anytime she came into the room and he was forearm-deep in eviscerating a computer, or building some sort of MRI doowhatsit, or coding loudly and vigorously, she would get this amazing look on her face. It was beautiful.

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"man 2 open"

int open(const char *path, int flags, mode_t mode);

DESCRIPTION

The file name specified by path is opened for reading and/or writing as
specified by the argument flags and the file descriptor returned to the
calling process. The flags argument may indicate the file is to be cre-
ated if it does not exist (by specifying the O_CREAT flag), in which case
the file is created with mode mode as described in chmod(2) and modified
by the process' umask value (see umask(2)).

The flags specified are formed by or'ing the following values

O_RDONLY open for reading only...

ERRORS

[EISDIR] The named file is a directory, and the arguments specify it is to be opened for writing.

Base your switch statement off of errno.

for further info look up fcntl.h

and don't forget to close(int fd); the file descriptor returned if it's != -1.

Last edited by Thimbles worth of opinion on 02-28-2006 at 08:36 AM

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opendir(and related functions) aren't part of the ANSI standard(although most Unices these days seem to include dirent.h so it might not matter much) so if you want to behave yourself I'd use zim's stat method.

If you plan on creating the directory if it doesn't exist, I'd just skip the stat call and just use open() directly(as there's a vague possibility that between the stat() and open() calls the directory could be created by something else. Unlikely with a local filesystem but with network filesystems that use attribute cacheing your data could be several thousand ms stale). It'll barf in a non-spectacular manner which you can catch if the directory already exists.

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Or you'd write opendir yourself, if you didn't have it and wanted to use it. It's small.

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<sys/stat.h> isn't in the ANSI standard, itself, it seems, anyhow.

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But it's part of POSIX, as is dirent.h(which contains opendir), so I guess if you're going to use stat, you may as well use opendir.

My advice? Use perl. Perl doesn't suffer from this sillyness.

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Use C#.

Embrace the beast.