The Linux Programming Interface

302 Chapter 15

Listing 15-5 illustrates the use of umask() in conjunction with open() and

mkdir(). When we run this program, we see the following:

$ ./t_umask

Requested file perms: rw-rw---- This is what we asked for

Process umask: ----wx-wx This is what we are denied

Actual file perms: rw-r----- So this is what we end up with

Requested dir. perms: rwxrwxrwx

Process umask: ----wx-wx

Actual dir. perms: rwxr--r--

In Listing 15-5, we employ the mkdir() and rmdir() system calls to create and

remove a directory, and the unlink() system call to remove a file. We describe

these system calls in Chapter 18.

Listing 15-5: Using umask()

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––– files/t_umask.c

#include <sys/stat.h>

#include <fcntl.h>

#include "file_perms.h"

#include "tlpi_hdr.h"

#define MYFILE "myfile"

#define MYDIR "mydir"

#define FILE_PERMS (S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP)

#define DIR_PERMS (S_IRWXU | S_IRWXG | S_IRWXO)

#define UMASK_SETTING (S_IWGRP | S_IXGRP | S_IWOTH | S_IXOTH)

int

main(int argc, char *argv[])

{

int fd;

struct stat sb;

mode_t u;

umask(UMASK_SETTING);

fd = open(MYFILE, O_RDWR | O_CREAT | O_EXCL, FILE_PERMS);

if (fd == -1)

errExit("open-%s", MYFILE);

if (mkdir(MYDIR, DIR_PERMS) == -1)

errExit("mkdir-%s", MYDIR);

u = umask(0); /* Retrieves (and clears) umask value */

if (stat(MYFILE, &sb) == -1)

errExit("stat-%s", MYFILE);

printf("Requested file perms: %s\n", filePermStr(FILE_PERMS, 0));

printf("Process umask: %s\n", filePermStr(u, 0));

printf("Actual file perms: %s\n\n", filePermStr(sb.st_mode, 0));