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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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semget(2) System Calls Manual semget(2)
semget - get a System V semaphore set identifier
Standard C library (libc, -lc)
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
The semget() system call returns the System V semaphore set
identifier associated with the argument key. It may be used
either to obtain the identifier of a previously created semaphore
set (when semflg is zero and key does not have the value
IPC_PRIVATE), or to create a new set.
A new set of nsems semaphores is created if key has the value
IPC_PRIVATE or if no existing semaphore set is associated with
key and IPC_CREAT is specified in semflg.
If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore
set already exists for key, then semget() fails with errno set to
EEXIST. (This is analogous to the effect of the combination
O_CREAT | O_EXCL for open(2).)
Upon creation, the least significant 9 bits of the argument
semflg define the permissions (for owner, group, and others) for
the semaphore set. These bits have the same format, and the same
meaning, as the mode argument of open(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 (see semctl(2)), as follows:
• sem_perm.cuid and sem_perm.uid are set to the effective user
ID of the calling process.
• sem_perm.cgid and sem_perm.gid are set to the effective group
ID of the calling process.
• The least significant 9 bits of sem_perm.mode are set to the
least significant 9 bits of semflg.
• sem_nsems is set to the value of nsems.
• sem_otime is set to 0.
• sem_ctime is set to the current time.
The argument nsems 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, and errno is
set to indicate the error.
EACCES A semaphore set exists for key, but the calling process
does not have permission to access the set, and does not
have the CAP_IPC_OWNER capability in the user namespace
that governs its IPC namespace.
EEXIST IPC_CREAT and IPC_EXCL were specified in semflg, but a
semaphore set already exists for key.
EINVAL nsems is less than 0 or greater than the limit on the
number of semaphores per semaphore set (SEMMSL).
EINVAL A semaphore set corresponding to key already exists, but
nsems is larger than the number of semaphores in that set.
ENOENT No semaphore set exists for key and semflg did not specify
IPC_CREAT.
ENOMEM A semaphore set has to be created but the system does not
have enough memory for the new data structure.
ENOSPC A 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_PRIVATE isn't a flag field but a key_t type. If this special
value is used for key, the system call ignores all but the least
significant 9 bits of semflg 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 using semctl(2) SETVAL or SETALL
operation. Where multiple peers do not know who will be the
first to initialize the set, checking for a nonzero sem_otime in
the associated data structure retrieved by a semctl(2) IPC_STAT
operation can be used to avoid races.
Semaphore limits
The following limits on semaphore set resources affect the
semget() call:
SEMMNI System-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.
SEMMSL Maximum 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.
SEMMNS System-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 of SEMMSL and SEMMNI.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
The program shown below uses semget() to create a new semaphore
set or retrieve the ID of an existing set. It generates the key
for semget() using ftok(3). The first two command-line arguments
are used as the pathname and proj_id arguments for ftok(3). The
third command-line argument is an integer that specifies the
nsems argument for semget(). Command-line options can be used to
specify the IPC_CREAT (-c) and IPC_EXCL (-x) flags for the call
to semget(). The usage of this program is demonstrated below.
We first create two files that will be used to generate keys
using ftok(3), create two semaphore sets using those files, and
then list the sets using ipcs(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 when semctl(2) is given the same key
(as generated by the same arguments to ftok(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 when
ftok(3) is given different pathname 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 = 9
Program 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)
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2024-06-26. If you discover any rendering problems in this HTML
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Linux man-pages 6.9.1 2024-06-15 semget(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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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