SAS/C Library Reference, Vol. 1

chmod -- Change Directory or File Mode

SYNOPSIS

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

 int chmod(const char *pathname, mode_t mode);

DESCRIPTION

chmod changes the mode bits for the directory or file specified by pathname.

The following symbols are defined in the <sys/stat.h> include file:

S_ISUID: sets the execution user ID. When the specified file is processed through an exec function, the user id of the file owner becomes the effective user ID of the process.
S_ISGID: sets execution group ID. When the specified file is processed through an exec function, the group ID that owns the file becomes the effective group ID of the process.
S_ISVTX: specifies shared text.
S_IRUSR: sets file owner permission to read.
S_IWUSR: sets file owner permission to write.
S_IXUSR: sets file owner permission to execute.
S_IRWXU: sets file owner permission to read, write, and execute.
S_IRGRP: sets group permission to read.
S_IWGRP: sets group permission to write.
S_IXGRP: sets group permission to execute.
S_IRWXG: sets group permission to read, write, and execute.
S_IROTH: sets general permission to read.
S_IWOTH: sets general permission to write.
S_IXOTH: sets general permission to execute.
S_IRWXO: sets general permission to read, write, and execute.

A process can set mode bits if it has superuser authority, or if the user ID is the same as that of the file's owner. The S_ISGID bit in the file's mode bits is cleared if

the calling process does not have superuser authority.
the group ID of the file does not match the effective group ID or any of the process' supplementary group IDs.

RETURN VALUE

chmod returns 0 if it is successful and - 1 if it is not successful.

EXAMPLE

This example changes a file (named by the function argument) so that it can be executed by any user who can read it.

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

  int chexec(const char *name) {
     struct stat stat_data;
     mode_t newmode;
     int rc;

     rc = stat(name, &stat_data);
     if (rc != 0) {
        perror("stat failure");
        return -1;
     }
     newmode = stat_data.st_mode;
     if (newmode & S_IRUSR) newmode |= S_IXUSR;
     if (newmode & S_IRGRP) newmode |= S_IXGRP;
     if (newmode & S_IROTH) newmode |= S_IXOTH;

        /* If the mode bits changed, make them effective. */
     if (newmode != stat_data.st_mode) {
        rc = chmod(name, newmode);
        if (rc != 0) perror("chmod failure");
        return rc;
     }
     return(0);               /* No change was necessary. */
  }

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