Manpage of UMASK
UMASKSection: Linux Programmer's Manual (2)
NAMEumask - set file mode creation mask
DESCRIPTIONumask() sets the calling process's file mode creation mask (umask) to mask& 0777 (i.e., only the file permission bits of maskare used), and returns the value of the mask.
The umask is used by open(2), mkdir(2), and other system calls that create files to modify the permissions placed on newly created files or directories. Specifically, permissions in the umask are turned off from the modeargument to open(2) and mkdir(2).
Alternatively, if the parent directory has a default ACL (see acl(5)), the umask is ignored, the default ACL is inherited, the permission bits are set based on the inherited ACL, and permission bits absent in the modeargument are turned off. For example, the following default ACL is equivalent to a umask of 022:
Combining the effect of this default ACL with a modeargument of 0666 (rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).
The constants that should be used to specify maskare described under stat(2).
The typical default value for the process umask is S_IWGRP | S_IWOTH(octal 022). In the usual case where the modeargument to open(2) is specified as:
S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH(octal 0666) when creating a new file, the permissions on the resulting file will be:
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH(because 0666 & ~022 = 0644; i.e., rw-r--r--).
RETURN VALUEThis system call always succeeds and the value of the mask is returned.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTESA child process created via fork(2) inherits its parent's umask. The umask is left unchanged by execve(2).
It is impossible to use umask() to fetch a process's umask without at the same time changing it. A second call to umask() would then be needed to restore the umask. The nonatomicity of these two steps provides the potential for races in multithreaded programs.
Since Linux 4.7, the umask of any process can be viewed via the Umaskfield of /proc/[pid]/status. Inspecting this field in /proc/self/statusallows a process to retrieve its umask without at the same time changing it.
The umask setting also affects the permissions assigned to POSIX IPC objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs (mkfifo(3)), and UNIX domain sockets (unix(7)) created by the process. The umask does not affect the permissions assigned to System V IPC objects created by the process (using msgget(2), semget(2), shmget(2)).
SEE ALSOchmod(2), mkdir(2), open(2), stat(2), acl(5)
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