HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |
NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |VERSIONS |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
semop(2) System Calls Manualsemop(2)semop, semtimedop - System V semaphore operations
Standard C library (libc,-lc)
#include <sys/sem.h>int semop(intsemid, struct sembuf *sops, size_tnsops);int semtimedop(intsemid, struct sembuf *sops, size_tnsops,const struct timespec *_Nullabletimeout); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):semtimedop(): _GNU_SOURCE
Each semaphore in a System V semaphore set has the following associated values: unsigned short semval; /* semaphore value */ unsigned short semzcnt; /* # waiting for zero */ unsigned short semncnt; /* # waiting for increase */ pid_t sempid; /* PID of process that last modified the semaphore value */semop() performs operations on selected semaphores in the set indicated bysemid. Each of thensops elements in the array pointed to bysops is a structure that specifies an operation to be performed on a single semaphore. The elements of this structure are of typestruct sembuf, containing the following members: unsigned short sem_num; /* semaphore number */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */ Flags recognized insem_flg areIPC_NOWAITandSEM_UNDO. If an operation specifiesSEM_UNDO, it will be automatically undone when the process terminates. The set of operations contained insops is performed inarrayorder, andatomically, that is, the operations are performed either as a complete unit, or not at all. The behavior of the system call if not all operations can be performed immediately depends on the presence of theIPC_NOWAITflag in the individualsem_flg fields, as noted below. Each operation is performed on thesem_num-th semaphore of the semaphore set, where the first semaphore of the set is numbered 0. There are three types of operation, distinguished by the value ofsem_op. Ifsem_op is a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, ifSEM_UNDOis specified for this operation, the system subtracts the valuesem_op from the semaphore adjustment (semadj) value for this semaphore. This operation can always proceed—it never forces a thread to wait. The calling process must have alter permission on the semaphore set. Ifsem_op is zero, the process must have read permission on the semaphore set. This is a "wait-for-zero" operation: ifsemval is zero, the operation can immediately proceed. Otherwise, ifIPC_NOWAITis specified insem_flg,semop() fails witherrno set toEAGAIN(and none of the operations insops is performed). Otherwise,semzcnt (the count of threads waiting until this semaphore's value becomes zero) is incremented by one and the thread sleeps until one of the following occurs: •semval becomes 0, at which time the value ofsemzcnt is decremented. • The semaphore set is removed:semop() fails, witherrno set toEIDRM. • The calling thread catches a signal: the value ofsemzcnt is decremented andsemop() fails, witherrno set toEINTR. Ifsem_op is less than zero, the process must have alter permission on the semaphore set. Ifsemval is greater than or equal to the absolute value ofsem_op, the operation can proceed immediately: the absolute value ofsem_op is subtracted fromsemval, and, ifSEM_UNDOis specified for this operation, the system adds the absolute value ofsem_op to the semaphore adjustment (semadj) value for this semaphore. If the absolute value ofsem_op is greater thansemval, andIPC_NOWAITis specified insem_flg,semop() fails, witherrno set toEAGAIN(and none of the operations insops is performed). Otherwise,semncnt (the counter of threads waiting for this semaphore's value to increase) is incremented by one and the thread sleeps until one of the following occurs: •semval becomes greater than or equal to the absolute value ofsem_op: the operation now proceeds, as described above. • The semaphore set is removed from the system:semop() fails, witherrno set toEIDRM. • The calling thread catches a signal: the value ofsemncnt is decremented andsemop() fails, witherrno set toEINTR. On successful completion, thesempid value for each semaphore specified in the array pointed to bysops is set to the caller's process ID. In addition, thesem_otime is set to the current time.semtimedop()semtimedop() behaves identically tosemop() except that in those cases where the calling thread would sleep, the duration of that sleep is limited by the amount of elapsed time specified by thetimespec structure whose address is passed in thetimeout argument. (This sleep interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the interval may overrun by a small amount.) If the specified time limit has been reached,semtimedop() fails witherrno set toEAGAIN(and none of the operations insops is performed). If thetimeout argument is NULL, thensemtimedop() behaves exactly likesemop(). Note that ifsemtimedop() is interrupted by a signal, causing the call to fail with the errorEINTR, the contents oftimeout are left unchanged.
On success,semop() andsemtimedop() return 0. On failure, they return -1, and seterrno to indicate the error.
E2BIGThe argumentnsops is greater thanSEMOPM, the maximum number of operations allowed per system call.EACCESThe calling process does not have the permissions required to perform the specified semaphore operations, and does not have theCAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.EAGAINAn operation could not proceed immediately and eitherIPC_NOWAITwas specified insem_flg or the time limit specified intimeout expired.EFAULTAn address specified in either thesops or thetimeout argument isn't accessible.EFBIGFor some operation the value ofsem_num is less than 0 or greater than or equal to the number of semaphores in the set.EIDRMThe semaphore set was removed.EINTRWhile blocked in this system call, the thread caught a signal; seesignal(7).EINVALThe semaphore set doesn't exist, orsemid is less than zero, ornsops has a nonpositive value.ENOMEMThesem_flg of some operation specifiedSEM_UNDOand the system does not have enough memory to allocate the undo structure.ERANGEFor some operationsem_op+semval is greater thanSEMVMX, the implementation dependent maximum value forsemval.
POSIX.1-2008.
Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3. POSIX.1-2001, SVr4.
Thesem_undo structures of a process aren't inherited by the child produced byfork(2), but they are inherited across anexecve(2) system call.semop() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of theSA_RESTART flag when establishing a signal handler. A semaphore adjustment (semadj) value is a per-process, per- semaphore integer that is the negated sum of all operations performed on a semaphore specifying theSEM_UNDOflag. Each process has a list ofsemadj values—one value for each semaphore on which it has operated usingSEM_UNDO. When a process terminates, each of its per-semaphoresemadj values is added to the corresponding semaphore, thus undoing the effect of that process's operations on the semaphore (but see BUGS below). When a semaphore's value is directly set using theSETVALorSETALL request tosemctl(2), the correspondingsemadj values in all processes are cleared. Theclone(2)CLONE_SYSVSEMflag allows more than one process to share asemadj list; seeclone(2) for details. Thesemval,sempid,semzcnt, andsemnct values for a semaphore can all be retrieved using appropriatesemctl(2) calls.Semaphore limits The following limits on semaphore set resources affect thesemop() call:SEMOPMMaximum number of operations allowed for onesemop() call. Before Linux 3.19, the default value for this limit was 32. Since Linux 3.19, the default value is 500. On Linux, this limit can be read and modified via the third field of/proc/sys/kernel/sem.Note: this limit should not be raised above 1000, because of the risk of thatsemop() fails due to kernel memory fragmentation when allocating memory to copy thesops array.SEMVMXMaximum allowable value forsemval: implementation dependent (32767). The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM), the system wide maximum number of undo structures (SEMMNU) and the per-process maximum number of undo entries system parameters.
When a process terminates, its set of associatedsemadj structures is used to undo the effect of all of the semaphore operations it performed with theSEM_UNDOflag. This raises a difficulty: if one (or more) of these semaphore adjustments would result in an attempt to decrease a semaphore's value below zero, what should an implementation do? One possible approach would be to block until all the semaphore adjustments could be performed. This is however undesirable since it could force process termination to block for arbitrarily long periods. Another possibility is that such semaphore adjustments could be ignored altogether (somewhat analogously to failing whenIPC_NOWAITis specified for a semaphore operation). Linux adopts a third approach: decreasing the semaphore value as far as possible (i.e., to zero) and allowing process termination to proceed immediately. In Linux 2.6.x, x <= 10, there is a bug that in some circumstances prevents a thread that is waiting for a semaphore value to become zero from being woken up when the value does actually become zero. This bug is fixed in Linux 2.6.11.
The following code segment usessemop() to atomically wait for the value of semaphore 0 to become zero, and then increment the semaphore value by one. struct sembuf sops[2]; int semid; /* Code to setsemid omitted */ sops[0].sem_num = 0; /* Operate on semaphore 0 */ sops[0].sem_op = 0; /* Wait for value to equal 0 */ sops[0].sem_flg = 0; sops[1].sem_num = 0; /* Operate on semaphore 0 */ sops[1].sem_op = 1; /* Increment value by one */ sops[1].sem_flg = 0; if (semop(semid, sops, 2) == -1) { perror("semop"); exit(EXIT_FAILURE); } A further example of the use ofsemop() can be found inshmop(2).clone(2),semctl(2),semget(2),sigaction(2),capabilities(7),sem_overview(7),sysvipc(7),time(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17semop(2)Pages that refer to this page:ipcs(1), lsipc(1), pcp-ipcs(1), clone(2), fork(2), ipc(2), semctl(2), semget(2), syscalls(2), unshare(2), proc_sys_kernel(5), pthreads(7), sem_overview(7), signal(7), sysvipc(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |