semget - get a System V semaphore set identifier
#include <sys/types.h>
#include <sys/ipc.h>
#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.
If successful, the return value will be the semaphore set identifier (a
nonnegative integer), otherwise, -1 is returned, with
errno indicating
the error.
On failure,
errno will be set to one of the following:
- 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.
SVr4, POSIX.1-2001.
The inclusion of
<sys/types.h> and
<sys/ipc.h> isn't
required on Linux or by any version of POSIX. However, some old
implementations required the inclusion of these header files, and the SVID
also documented their inclusion. Applications intended to be portable to such
old systems may need to include these header files.
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).
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.
The following limits on semaphore set resources affect the
semget() call:
- SEMMNI
- System-wide limit on the number of semaphore sets. On Linux systems before
version 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. On Linux systems before
version 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() 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
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.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)