8.2 access: Testing File Permissions

The access system call determines whether the calling process has access permission to a file. It can check any combination of read, write, and execute permission, and it can also check for a file's existence.

The access call takes two arguments. The first is the path to the file to check. The second is a bitwise or of R_OK, W_OK, and X_OK, corresponding to read, write, and execute permission. The return value is 0 if the process has all the specified permissions. If the file exists but the calling process does not have the specified permissions, access returns -1 and sets errno to EACCES (or EROFS, if write permission was requested for a file on a read-only file system).

If the second argument is F_OK, access simply checks for the file's existence. If the file exists, the return value is 0; if not, the return value is -1 and errno is set to ENOENT. Note that errno may instead be set to EACCES if a directory in the file path is inaccessible.

The program shown in Listing 8.1 uses access to check for a file's existence and to determine read and write permissions. Specify the name of the file to check on the command line.

Listing 8.1 (check-access.c) Check File Access Permissions

#include <errno.h> 

#include <stdio.h> 

#include <unistd.h> 

int main (int argc, char* argv[]) 

{

  char* path = argv[1]; 

  int rval; 

  /* Check file existence.  */ 

  rval = access (path, F_OK); 

  if (rval == 0) 

    printf ("%s exists\n", path); 

  else {

    if (errno == ENOENT) 

      printf ("%s does not exist\n", path); 

    else if (errno == EACCES) 

      printf ("%s is not accessible\n", path); 

    return 0; 

  } 

  /* Check read access.  */ 

  rval = access (path, R_OK); 

  if (rval == 0) 

    printf ("%s is readable\n", path); 

  else 

    printf ("%s is not readable (access denied)\n", path); 

  /* Check write access. */ 

  rval = access (path, W_OK); 

  if (rval == 0) 

    printf ("%s is writable\n", path); 

  else if (errno == EACCES) 

    printf ("%s is not writable (access denied)\n", path); 

  else if (errno == EROFS) 

    printf ("%s is not writable (read-only filesystem)\n", path); 

  return 0; 

} 

For example, to check access permissions for a file named README on a CD-ROM, invoke it like this:

% ./check-access /mnt/cdrom/README 

/mnt/cdrom/README exists 

/mnt/cdrom/README is readable 

/mnt/cdrom/README is not writable (read-only filesystem)