What's the best way to check if a file exists in C?

Look up the access() function, found in unistd.h. You can replace your function with

if (access(fname, F_OK) == 0) {
    // file exists
} else {
    // file doesn't exist
}

Under Windows (VC) unistd.h does not exist. To make it work it is necessary to define:

#ifdef WIN32
#include <io.h>
#define F_OK 0
#define access _access
#endif

You can also use R_OK, W_OK, and X_OK in place of F_OK to check for read permission, write permission, and execute permission (respectively) rather than existence, and you can OR any of them together (i.e. check for both read and write permission using R_OK|W_OK)

Update: Note that on Windows, you can't use W_OK to reliably test for write permission, since the access function does not take DACLs into account. access( fname, W_OK ) may return 0 (success) because the file does not have the read-only attribute set, but you still may not have permission to write to the file.

Use stat like this:

#include <sys/stat.h>   // stat
#include <stdbool.h>    // bool type

bool file_exists (char *filename) {
  struct stat   buffer;   
  return (stat (filename, &buffer) == 0);
}

and call it like this:

#include <stdio.h>      // printf

int main(int ac, char **av) {
    if (ac != 2)
        return 1;

    if (file_exists(av[1]))
        printf("%s exists\n", av[1]);
    else
        printf("%s does not exist\n", av[1]);

    return 0;
}

Usually when you want to check if a file exists, it's because you want to create that file if it doesn't. Graeme Perrow's answer is good if you don't want to create that file, but it's vulnerable to a race condition if you do: another process could create the file in between you checking if it exists, and you actually opening it to write to it. (Don't laugh... this could have bad security implications if the file created was a symlink!)

If you want to check for existence and create the file if it doesn't exist, atomically so that there are no race conditions, then use this:

#include <fcntl.h>
#include <errno.h>

fd = open(pathname, O_CREAT | O_WRONLY | O_EXCL, S_IRUSR | S_IWUSR);
if (fd < 0) {
  /* failure */
  if (errno == EEXIST) {
    /* the file already existed */
    ...
  }
} else {
  /* now you can use the file */
}

Yes. Use stat(). See the man page forstat(2).

stat() will fail if the file doesn't exist, otherwise most likely succeed. If it does exist, but you have no read access to the directory where it exists, it will also fail, but in that case any method will fail (how can you inspect the content of a directory you may not see according to access rights? Simply, you can't).

Oh, as someone else mentioned, you can also use access(). However I prefer stat(), as if the file exists it will immediately get me lots of useful information (when was it last updated, how big is it, owner and/or group that owns the file, access permissions, and so on).

FILE *file;
    if((file = fopen("sample.txt","r"))!=NULL)
        {
            // file exists
            fclose(file);
        }
    else
        {
            //File not found, no memory leak since 'file' == NULL
            //fclose(file) would cause an error
        }

You can use realpath() function.

resolved_file = realpath(file_path, NULL);
if (!resolved_keyfile) {
   /*File dosn't exists*/
   perror(keyfile);
   return -1;
}

I think that access() function, which is found in unistd.h is a good choice for Linux (you can use stat too).

You can Use it like this:

#include <stdio.h>
#include <stdlib.h>
#include<unistd.h>

void fileCheck(const char *fileName);

int main (void) {
    char *fileName = "/etc/sudoers";

    fileCheck(fileName);
    return 0;
}

void fileCheck(const char *fileName){

    if(!access(fileName, F_OK )){
        printf("The File %s\t was Found\n",fileName);
    }else{
        printf("The File %s\t not Found\n",fileName);
    }

    if(!access(fileName, R_OK )){
        printf("The File %s\t can be read\n",fileName);
    }else{
        printf("The File %s\t cannot be read\n",fileName);
    }

    if(!access( fileName, W_OK )){
        printf("The File %s\t it can be Edited\n",fileName);
    }else{
        printf("The File %s\t it cannot be Edited\n",fileName);
    }

    if(!access( fileName, X_OK )){
        printf("The File %s\t is an Executable\n",fileName);
    }else{
        printf("The File %s\t is not an Executable\n",fileName);
    }
}

And you get the following Output:

The File /etc/sudoers    was Found
The File /etc/sudoers    cannot be read
The File /etc/sudoers    it cannot be Edited
The File /etc/sudoers    is not an Executable

From the Visual C++ help, I'd tend to go with

/* ACCESS.C: This example uses _access to check the
 * file named "ACCESS.C" to see if it exists and if
 * writing is allowed.
 */

#include  <io.h>
#include  <stdio.h>
#include  <stdlib.h>

void main( void )
{
   /* Check for existence */
   if( (_access( "ACCESS.C", 0 )) != -1 )
   {
      printf( "File ACCESS.C exists\n" );
      /* Check for write permission */
      if( (_access( "ACCESS.C", 2 )) != -1 )
         printf( "File ACCESS.C has write permission\n" );
   }
}

Also worth noting mode values of _access(const char *path,int mode):

00: Existence only

02: Write permission

04: Read permission

06: Read and write permission

As your fopen could fail in situations where the file existed but could not be opened as requested.

Edit: Just read Mecki's post. stat() does look like a neater way to go. Ho hum.