semget(2) System Calls Manual semget(2)
NAME
semget - get a System V semaphore set identifier
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
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_PRI-
VATE 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 al-
ready 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 de-
fine 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 mean-
ingful for semaphores, and write permissions mean permission to alter
semaphore values).
When creating a new semaphore set, semget() initializes the set's asso-
ciated 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.
RETURN VALUE
On success, semget() returns the semaphore set identifier (a nonnega-
tive integer). On failure, -1 is returned, and errno is set to indi-
cate the error.
ERRORS
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.
STANDARDS
POSIX.1-2008.
HISTORY
SVr4, POSIX.1-2001.
NOTES
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 ini-
tialize 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 struc-
ture 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 sema-
phores system-wide is also limited by the product of SEMMSL and
SEMMNI.
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
clearly show its function.
EXAMPLES
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 ar-
gument 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 pro-
gram 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 gen-
erated by the same arguments to ftok(3)), it returns the ID of the al-
ready 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);
}
SEE ALSO
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
sysvipc(7)
Linux man-pages 6.7 2023-10-31 semget(2)
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