Quickest way to check whether or not file exists [duplicate] - c

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This question already has answers here:
What's the best way to check if a file exists in C?
(8 answers)
Closed 5 years ago.
The way I'm using just involves trying to fopen() the file to be checked,
/* --- does file exist??? --- */
char fname[999] = "whatever"; /* constructed during execution */
FILE *fp = NULL; /* try to fopen(fname,"r") */
int isfilefound = 0; /* set true if fopen() succeeds */
if ( (fp = fopen(fname,"r")) /* try to fopen() for read */
!= NULL ) { /* succeeded */
isfilefound = 1; /* set file found flag */
fclose(fp); } /* and just close the file */
Is there a quicker, less resource-intensive, way?... A specific way for unix/linux? A Windows way? And preferably, a portable posix-compliant way (as above presumably is)? It's being done lots (1000's) of times, so I'd prefer not to be unnecessarily opening and closing files for no good reason.
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Edit Okay, based on answers below, I put together the following little function intended to check whether or not file (already:) exists in a posix,windows,other portable way...
/* ==========================================================================
* Function: isfilexists ( path )
* Purpose: check whether file at path exists
* --------------------------------------------------------------------------
* Arguments: path (I) pointer to null-terminated char string
* containing "path/filename.ext" of
* file whose existence is to be determined
* (path is relative to pwd unless explicitly
* absolute by initial '/' or other syntax)
* --------------------------------------------------------------------------
* Returns: ( int ) 1 if file at path exists, or 0 if not
* --------------------------------------------------------------------------
* Notes: o conditional compiles for various systems,
* depending on whether POSIX or WINDOWS is #define'ed...
* o ...method used:
* 1: use access() on Posix systems,
* 2: PathFileExists() on Windows systems,
* 3: fopen() on any other systems.
* ======================================================================= */
/* --- entry point --- */
int isfilexists ( char *path )
{
/* ---
* allocations and declarations
* ------------------------------- */
int isexists = 0; /* set true if file at path exists */
FILE *fp = NULL; /* fopen() for non-posix,windows */
#define POSIX /* just for testing */
/* ---
* determine whether file at path already exists
* ------------------------------------------------ */
#if defined(POSIX) /* posix-compliant system... */
#include <unistd.h>
if ( access(path,F_OK) == 0 ) /* file at path exists */
isexists = 1; /* so set file exists flag */
#else
#if defined(WINDOWS) /* Windows system... */
isexists = PathFileExists(path); /* set flag if file at path exists */
#else
/* --- fopen() for any other non-posix, non-windows system --- */
if ( (fp = fopen(path,"r")) /* try to fopen() for read */
!= NULL ) { /* succeeded */
isexists = 1; /* set file exists flag */
fclose(fp); } /* and just close the file */
#endif
#endif
return ( isexists ); /* back to caller with 1 if file at path exists */
} /* --- end-of-function isfilexists() --- */
The access() and fopen() methods tested and work okay. Unable to test PathFileExists() for windows. And I still want to figure out what #define'ed symbols to automatically and unambiguously check for conditional compiles.

Answer 1

You are thinking about the problem the wrong way. You shouldn't ever "check whether a file already exists", because that has an inherent TOCTOU race — in between the time you check whether the file exists, and the time you act on that information, another process may come along and change whether the file exists, rendering the check invalid.
What you do instead depends on why you want to know. One very common case is that you only want to create the file if it doesn't already exist, in which case you use the lower-level open function in O_EXCL mode:
int fd = open("whatever", O_WRONLY|O_CREAT|O_EXCL, 0666);
if (fd == -1 && errno == EEXIST) {
/* the file already exists */
} else if (fd == -1) {
/* report that some other error happened */
} else {
FILE *fp = fdopen(fd, "w");
/* write data to fp here */
}
Another very common case is that you want to create the file if it doesn't exist, or append new data to the file if it does; this can be done with the "a" mode to fopen or O_APPEND flag to open.

Answer 2

On Windows, there is PathFileExists().
On a POSIX system, you have stat() or access().
That said, if you check for existence of the file because your code needs the file, this is the wrong approach -- file systems are out of your program's control, so this would be a race condition, the only correct way would be to properly handle errors when opening the file.