Umask function in Linux

The umask function creates a blocked word for the process setting file mode and returns the previous value. Function prototype:

# Include <sys/stat. h>

Mode_t umask (mode_t cmask );

Cmask is composed of several bitwise OR constants listed in the following table.

S_IRUSR User Read

S_IWUSR User Write

S_IXUSR user execution

S_IRGRP Group read

S_IWGRP group write

S_IXGRP group execution

S_IROTH other reads

S_IWOTH other writes

S_IXOTH other executions

In Linux, the permissions of a file are divided into nine permissions in three groups: users, groups, and others. For example:

Ls-l test

-Rw-1 shmily 0 May 6 17:16 test

Indicates that the test file has the following permissions: User readable and writable, group readable and writable, and other readable and writable (r indicates readable w indicates writable x indicates executable-Indicates not set)

The three-digit octal number indicates that the test permission is 666.

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Umask is mainly used to createFileHourSetOr block some permissions on the file.

Specify the permission of the file when creating a file. The last mode parameter of the open function is not the permission to be set. It must perform the following operations:

Mode &(~ Cmask)

For example, the 4_3 function in APUE

#include "apue.h"#include <fcntl.h>  #define RWRWRW (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)  int main(void){     umask(0);     if (creat("foo", RWRWRW) < 0)        err_sys("create error for foo");     umask(S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);     if (creat("bar", RWRWRW) < 0)         err_sys("creat error for bar");     return 0;}        

In the initial cmask is 0, that is, 000 000 000

Set the mode to 666 when using the creat function.

Mode &(~ Cmask) = 110 110 110 & 111 111 111 = 110 110 110

So the permission for the foo file is rw-

Then cmask is 066, that is, 000 110 110

The mode of the crear function is still 666.

Mode &(~ Cmask) = 110 110 110 & 111 001 001 = 110 000 000

So the bar File Permission is rw -------

Shmily @ pc-Shmily :~ /Code/UnixCode $ ls-l foo bar
-Rw ------- 1 shmily 0 May 6 17:16 bar
-Rw-1 shmily 0 May 6 17:16 foo