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semctl(3p) — Linux manual page

PROLOG \|NAME \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|EXAMPLES \|APPLICATION USAGE \|RATIONALE \|FUTURE DIRECTIONS \|SEE ALSO \|COPYRIGHT

SEMCTL(3P)              POSIX Programmer's ManualSEMCTL(3P)

PROLOG top

       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 top

       semctl — XSI semaphore control operations

SYNOPSIS top

       #include <sys/sem.h>       int semctl(intsemid, intsemnum, intcmd, ...);

DESCRIPTION top

Thesemctl() 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 inSection2.8,Realtime. Thesemctl() function provides a variety of semaphore control operations as specified bycmd. The fourth argument is optional and depends upon the operation requested. If required, it is of typeunion 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 bycmd are executed with respect to the semaphore specified bysemid andsemnum. The level of permission required for each operation is shown with each command; seeSection 2.7,XSI InterprocessCommunication. The symbolic names for the values ofcmd are defined in the<sys/sem.h> header: GETVAL Return the value ofsemval; see<sys/sem.h>. Requires read permission. SETVAL Set the value ofsemval toarg.val, wherearg is the value of the fourth argument tosemctl(). When this command is successfully executed, thesemadj value corresponding to the specified semaphore in all processes is cleared. Also, thesem_ctime timestamp shall be set to the current time, as described inSection 2.7.1,IPC General Description. Requires alter permission; seeSection 2.7,XSI InterprocessCommunication. GETPID Return the value ofsempid. Requires read permission. GETNCNT Return the value ofsemncnt. Requires read permission. GETZCNT Return the value ofsemzcnt. Requires read permission. The following values ofcmd operate on eachsemval in the set of semaphores: GETALL Return the value ofsemval for each semaphore in the semaphore set and place into the array pointed to byarg.array, wherearg is the fourth argument tosemctl(). Requires read permission. SETALL Set the value ofsemval for each semaphore in the semaphore set according to the array pointed to byarg.array, wherearg is the fourth argument tosemctl(). When this command is successfully executed, thesemadj values corresponding to each specified semaphore in all processes are cleared. Also, thesem_ctime timestamp shall be set to the current time, as described inSection 2.7.1,IPC GeneralDescription. Requires alter permission. The following values ofcmd are also available: IPC_STAT Place the current value of each member of thesemid_ds data structure associated withsemid into the structure pointed to byarg.buf, wherearg is the fourth argument tosemctl(). The contents of this structure are defined in<sys/sem.h>. Requires read permission. IPC_SET Set the value of the following members of thesemid_ds data structure associated withsemid to the corresponding value found in the structure pointed to byarg.buf, wherearg is the fourth argument tosemctl(): sem_perm.uid sem_perm.gid sem_perm.mode The mode bits specified inSection 2.7.1,IPC GeneralDescription are copied into the corresponding bits of thesem_perm.mode associated withsemid. The stored values of any other bits are unspecified. Thesem_ctime timestamp shall be set to the current time, as described inSection 2.7.1,IPC GeneralDescription. 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 ofsem_perm.cuid orsem_perm.uid in thesemid_dsdata structure associated withsemid. IPC_RMID Remove the semaphore identifier specified bysemid from the system and destroy the set of semaphores andsemid_dsdata 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 ofsem_perm.cuid orsem_perm.uid in thesemid_dsdata structure associated withsemid.

RETURN VALUE top

       If successful, the value returned bysemctl() depends oncmd as       follows:       GETVAL      The value ofsemval.       GETPID      The value ofsempid.       GETNCNT     The value ofsemncnt.       GETZCNT     The value ofsemzcnt.       All others  0.       Otherwise,semctl() shall return -1 and seterrno to indicate the       error.

ERRORS top

       Thesemctl() function shall fail if:EACCESOperation permission is denied to the calling process; seeSection 2.7,XSI Interprocess Communication.EINVALThe value ofsemid is not a valid semaphore identifier, or              the value ofsemnum is less than 0 or greater than or equal              tosem_nsems, or the value ofcmd is not a valid command.EPERMThe argumentcmd 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 ofsem_perm.cuid orsem_perm.uid in the              data structure associated withsemid.ERANGEThe argumentcmd is equal to SETVAL or SETALL and the value              to whichsemval is to be set is greater than the system-              imposed maximum.The following sections are informative.

EXAMPLES top

       Refer tosemop(3p).

APPLICATION USAGE top

       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 unionsemun (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 inSection 2.7,XSI Interprocess Communication       can be easily modified to use the alternative interfaces.

RATIONALE top

       None.

FUTURE DIRECTIONS top

       None.

COPYRIGHT top

       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  athttp://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,  seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group                2017SEMCTL(3P)

Pages that refer to this page:sys_sem.h(0p), ipcrm(1p), sem_close(3p), sem_destroy(3p), semget(3p), sem_getvalue(3p), semop(3p), sem_open(3p), sem_post(3p), sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p)