--- name: "semctl" section: "3p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/semctl/3p" --- # semctl(3p) XSI semaphore control operations ## PROLOG This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. ## NAME semctl — XSI semaphore control operations ## SYNOPSIS ``` #include ``` int semctl(int *semid*, int *semnum*, int *cmd*, ...); ## DESCRIPTION The *semctl*() function operates on XSI semaphores (see the Base Definitions volume of POSIX.1‐2017, *Section 4.17*, *Semaphore*). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in *Section 2.8*, *Realtime*. The *semctl*() function provides a variety of semaphore control operations as specified by *cmd*. The fourth argument is optional and depends upon the operation requested. If required, it is of type **union semun**, which the application shall explicitly declare: ``` union semun { int val; struct semid_ds *buf; unsigned short *array; } arg; ``` Each operation shall be performed atomically. The following semaphore control operations as specified by *cmd* are executed with respect to the semaphore specified by *semid* and *semnum*. The level of permission required for each operation is shown with each command; see *Section 2.7*, *XSI Interprocess Communication*. The symbolic names for the values of *cmd* are defined in the *\* header: - GETVAL Return the value of *semval*; see *\*. Requires read permission. - SETVAL Set the value of *semval* to *arg.val*, where *arg* is the value of the fourth argument to *semctl*(). When this command is successfully executed, the *semadj* value corresponding to the specified semaphore in all processes is cleared. Also, the *sem\_ctime* timestamp shall be set to the current time, as described in *Section 2.7.1*, *IPC General Description*. Requires alter permission; see *Section 2.7*, *XSI Interprocess Communication*. - GETPID Return the value of *sempid*. Requires read permission. - GETNCNT Return the value of *semncnt*. Requires read permission. - GETZCNT Return the value of *semzcnt*. Requires read permission. The following values of *cmd* operate on each *semval* in the set of semaphores: - GETALL Return the value of *semval* for each semaphore in the semaphore set and place into the array pointed to by *arg.array*, where *arg* is the fourth argument to *semctl*(). Requires read permission. - SETALL Set the value of *semval* for each semaphore in the semaphore set according to the array pointed to by *arg.array*, where *arg* is the fourth argument to *semctl*(). When this command is successfully executed, the *semadj* values corresponding to each specified semaphore in all processes are cleared. Also, the *sem\_ctime* timestamp shall be set to the current time, as described in *Section 2.7.1*, *IPC General Description*. Requires alter permission.\ The following values of *cmd* are also available: - IPC\_STAT Place the current value of each member of the **semid\_ds** data structure associated with *semid* into the structure pointed to by *arg.buf*, where *arg* is the fourth argument to *semctl*(). The contents of this structure are defined in *\*. Requires read permission. - IPC\_SET Set the value of the following members of the **semid\_ds** data structure associated with *semid* to the corresponding value found in the structure pointed to by *arg.buf*, where *arg* is the fourth argument to *semctl*(): ``` sem_perm.uid sem_perm.gid sem_perm.mode ``` The mode bits specified in *Section 2.7.1*, *IPC General Description* are copied into the corresponding bits of the *sem\_perm.mode* associated with *semid*. The stored values of any other bits are unspecified. The *sem\_ctime* timestamp shall be set to the current time, as described in *Section 2.7.1*, *IPC General Description*. This command can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of *sem\_perm.cuid* or *sem\_perm.uid* in the **semid\_ds** data structure associated with *semid*. - IPC\_RMID Remove the semaphore identifier specified by *semid* from the system and destroy the set of semaphores and **semid\_ds** data structure associated with it. This command can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of *sem\_perm.cuid* or *sem\_perm.uid* in the **semid\_ds** data structure associated with *semid*. ## RETURN VALUE If successful, the value returned by *semctl*() depends on *cmd* as follows: - GETVAL The value of *semval*. - GETPID The value of *sempid*. - GETNCNT The value of *semncnt*. - GETZCNT The value of *semzcnt*. - All others 0\. Otherwise, *semctl*() shall return -1 and set *errno* to indicate the error. ## ERRORS The *semctl*() function shall fail if: - **EACCES** Operation permission is denied to the calling process; see *Section 2.7*, *XSI Interprocess Communication*. - **EINVAL** The value of *semid* is not a valid semaphore identifier, or the value of *semnum* is less than 0 or greater than or equal to *sem\_nsems*, or the value of *cmd* is not a valid command. - **EPERM** The argument *cmd* is equal to IPC\_RMID or IPC\_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of *sem\_perm.cuid* or *sem\_perm.uid* in the data structure associated with *semid*. - **ERANGE** The argument *cmd* is equal to SETVAL or SETALL and the value to which *semval* is to be set is greater than the system-imposed maximum. *The following sections are informative.* ## EXAMPLES Refer to *semop()*. ## APPLICATION USAGE The fourth parameter in the SYNOPSIS section is now specified as **"..."** in order to avoid a clash with the ISO C standard when referring to the union *semun* (as defined in Issue 3) and for backwards-compatibility. The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in *Section 2.7*, *XSI Interprocess Communication* can be easily modified to use the alternative interfaces. ## RATIONALE None. ## FUTURE DIRECTIONS None. ## SEE ALSO *Section 2.7*, *XSI Interprocess Communication*, *Section 2.8*, *Realtime*, *semget()*, *semop()*, *sem\_close()*, *sem\_destroy()*, *sem\_getvalue()*, *sem\_init()*, *sem\_open()*, *sem\_post()*, *sem\_trywait()*, *sem\_unlink()* The Base Definitions volume of POSIX.1‐2017, *Section 4.17*, *Semaphore*, ***\*** ## COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .