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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
semget(2) System Calls Manualsemget(2)semget - get a System V semaphore set identifier
Standard C library (libc,-lc)
#include <sys/sem.h>int semget(key_tkey, intnsems, intsemflg);
Thesemget() system call returns the System V semaphore set identifier associated with the argumentkey. It may be used either to obtain the identifier of a previously created semaphore set (whensemflg is zero andkey does not have the valueIPC_PRIVATE), or to create a new set. A new set ofnsems semaphores is created ifkey has the valueIPC_PRIVATEor if no existing semaphore set is associated withkey andIPC_CREATis specified insemflg. Ifsemflg specifies bothIPC_CREATandIPC_EXCLand a semaphore set already exists forkey, thensemget() fails witherrno set toEEXIST. (This is analogous to the effect of the combinationO_CREAT | O_EXCLforopen(2).) Upon creation, the least significant 9 bits of the argumentsemflg define the permissions (for owner, group, and others) for the semaphore set. These bits have the same format, and the same meaning, as themode argument ofopen(2) (though the execute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values). When creating a new semaphore set,semget() initializes the set's associated data structure,semid_ds (seesemctl(2)), as follows: •sem_perm.cuid andsem_perm.uid are set to the effective user ID of the calling process. •sem_perm.cgid andsem_perm.gid are set to the effective group ID of the calling process. • The least significant 9 bits ofsem_perm.mode are set to the least significant 9 bits ofsemflg. •sem_nsems is set to the value ofnsems. •sem_otime is set to 0. •sem_ctime is set to the current time. The argumentnsems can be 0 (a don't care) when a semaphore set is not being created. Otherwise,nsems must be greater than 0 and less than or equal to the maximum number of semaphores per semaphore set (SEMMSL). If the semaphore set already exists, the permissions are verified.
On success,semget() returns the semaphore set identifier (a nonnegative integer). On failure, -1 is returned, anderrno is set to indicate the error.
EACCESA semaphore set exists forkey, but the calling process does not have permission to access the set, and does not have theCAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.EEXIST IPC_CREATandIPC_EXCLwere specified insemflg, but a semaphore set already exists forkey.EINVALnsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL).EINVALA semaphore set corresponding tokey already exists, butnsems is larger than the number of semaphores in that set.ENOENTNo semaphore set exists forkey andsemflg did not specifyIPC_CREAT.ENOMEMA semaphore set has to be created but the system does not have enough memory for the new data structure.ENOSPCA semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI), or the system wide maximum number of semaphores (SEMMNS), would be exceeded.
POSIX.1-2008.
SVr4, POSIX.1-2001.
IPC_PRIVATEisn't a flag field but akey_t type. If this special value is used forkey, the system call ignores all but the least significant 9 bits ofsemflg and creates a new semaphore set (on success).Semaphore initialization The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 and POSIX.1-2008 are explicit on this point, although POSIX.1-2008 notes that a future version of the standard may require an implementation to initialize the semaphores to 0.) Although Linux, like many other implementations, initializes the semaphore values to 0, a portable application cannot rely on this: it should explicitly initialize the semaphores to the desired values. Initialization can be done usingsemctl(2)SETVALorSETALL operation. Where multiple peers do not know who will be the first to initialize the set, checking for a nonzerosem_otime in the associated data structure retrieved by asemctl(2)IPC_STAT operation can be used to avoid races.Semaphore limits The following limits on semaphore set resources affect thesemget() call:SEMMNISystem-wide limit on the number of semaphore sets. Before Linux 3.19, the default value for this limit was 128. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via the fourth field of/proc/sys/kernel/sem.SEMMSLMaximum number of semaphores per semaphore ID. Before Linux 3.19, the default value for this limit was 250. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via the first field of/proc/sys/kernel/sem.SEMMNSSystem-wide limit on the number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of/proc/sys/kernel/sem). Note that the number of semaphores system-wide is also limited by the product ofSEMMSLandSEMMNI.
The name choiceIPC_PRIVATEwas perhaps unfortunate,IPC_NEWwould more clearly show its function.
The program shown below usessemget() to create a new semaphore set or retrieve the ID of an existing set. It generates thekey forsemget() usingftok(3). The first two command-line arguments are used as thepathname andproj_id arguments forftok(3). The third command-line argument is an integer that specifies thensems argument forsemget(). Command-line options can be used to specify theIPC_CREAT(-c) andIPC_EXCL(-x) flags for the call tosemget(). The usage of this program is demonstrated below. We first create two files that will be used to generate keys usingftok(3), create two semaphore sets using those files, and then list the sets usingipcs(1): $touch mykey mykey2; $./t_semget -c mykey p 1; ID = 9 $./t_semget -c mykey2 p 2; ID = 10 $ipcs -s; ------ Semaphore Arrays -------- key semid owner perms nsems 0x7004136d 9 mtk 600 1 0x70041368 10 mtk 600 2 Next, we demonstrate that whensemctl(2) is given the samekey (as generated by the same arguments toftok(3)), it returns the ID of the already existing semaphore set: $./t_semget -c mykey p 1; ID = 9 Finally, we demonstrate the kind of collision that can occur whenftok(3) is given differentpathname arguments that have the same inode number: $ln mykey link; $ls -i1 link mykey; 2233197 link 2233197 mykey $./t_semget link p 1; # Generates same key as 'mykey' ID = 9Program source /* t_semget.c Licensed under GNU General Public License v2 or later. */ #include <stdio.h> #include <stdlib.h> #include <sys/ipc.h> #include <sys/sem.h> #include <unistd.h> static void usage(const char *pname) { fprintf(stderr, "Usage: %s [-cx] pathname proj-id num-sems\n", pname); fprintf(stderr, " -c Use IPC_CREAT flag\n"); fprintf(stderr, " -x Use IPC_EXCL flag\n"); exit(EXIT_FAILURE); } int main(int argc, char *argv[]) { int semid, nsems, flags, opt; key_t key; flags = 0; while ((opt = getopt(argc, argv, "cx")) != -1) { switch (opt) { case 'c': flags |= IPC_CREAT; break; case 'x': flags |= IPC_EXCL; break; default: usage(argv[0]); } } if (argc != optind + 3) usage(argv[0]); key = ftok(argv[optind], argv[optind + 1][0]); if (key == -1) { perror("ftok"); exit(EXIT_FAILURE); } nsems = atoi(argv[optind + 2]); semid = semget(key, nsems, flags | 0600); if (semid == -1) { perror("semget"); exit(EXIT_FAILURE); } printf("ID = %d\n", semid); exit(EXIT_SUCCESS); }semctl(2),semop(2),ftok(3),capabilities(7),sem_overview(7),sysvipc(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 2025-05-17semget(2)Pages that refer to this page:ipcrm(1), ipcs(1), lsipc(1), pcp-ipcs(1), ipc(2), semctl(2), semop(2), syscalls(2), umask(2), ftok(3), sem_overview(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. | |