umask(2) - f38

1umask(2)                      System Calls Manual                     umask(2)
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NAME

6       umask - set file mode creation mask
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <sys/stat.h>
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14       mode_t umask(mode_t mask);
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DESCRIPTION

17       umask()  sets  the calling process's file mode creation mask (umask) to
18       mask & 0777 (i.e., only the file permission bits of mask are used), and
19       returns the previous value of the mask.
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21       The  umask  is  used  by open(2), mkdir(2), and other system calls that
22       create files to modify the permissions placed on newly created files or
23       directories.   Specifically,  permissions  in  the umask are turned off
24       from the mode argument to open(2) and mkdir(2).
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26       Alternatively, if the parent directory has a default ACL (see  acl(5)),
27       the umask is ignored, the default ACL is inherited, the permission bits
28       are set based on the inherited ACL, and permission bits absent  in  the
29       mode  argument  are turned off.  For example, the following default ACL
30       is equivalent to a umask of 022:
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32           u::rwx,g::r-x,o::r-x
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34       Combining the effect of this default ACL with a mode argument  of  0666
35       (rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).
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37       The  constants that should be used to specify mask are described in in‐
38       ode(7).
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40       The typical default value for the process umask is  S_IWGRP  |  S_IWOTH
41       (octal  022).   In the usual case where the mode argument to open(2) is
42       specified as:
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44           S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
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46       (octal 0666) when creating a new file, the permissions on the resulting
47       file will be:
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49           S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
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51       (because 0666 & ~022 = 0644; i.e. rw-r--r--).
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RETURN VALUE

54       This  system call always succeeds and the previous value of the mask is
55       returned.
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STANDARDS

58       POSIX.1-2008.
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HISTORY

61       POSIX.1-2001, SVr4, 4.3BSD.
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NOTES

64       A child process created via fork(2) inherits its parent's  umask.   The
65       umask is left unchanged by execve(2).
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67       It  is  impossible to use umask() to fetch a process's umask without at
68       the same time changing it.  A second call  to  umask()  would  then  be
69       needed  to restore the umask.  The nonatomicity of these two steps pro‐
70       vides the potential for races in multithreaded programs.
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72       Since Linux 4.7, the umask of any process can be viewed via  the  Umask
73       field  of /proc/pid/status.  Inspecting this field in /proc/self/status
74       allows a process to retrieve its umask without at the same time  chang‐
75       ing it.
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77       The  umask  setting  also affects the permissions assigned to POSIX IPC
78       objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs (mkfifo(3)),  and
79       UNIX  domain  sockets (unix(7)) created by the process.  The umask does
80       not affect the permissions assigned to System V IPC objects created  by
81       the process (using msgget(2), semget(2), shmget(2)).
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