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# semget() - Unix, Linux System Call

# NAME

semget - get a semaphore set identifier

# SYNOPSIS

    #include <sys/types.h>

    #include <sys/ipc.h>

    #include <sys/sem.h>

int semget(key_t key, int nsems, int semflg);

# DESCRIPTION

The semget() system call returns the semaphore set identifier associated with the argument key. 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).

The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 is explicit on this point.) Although Linux, like many other implementations, initialises the semaphore values to 0, a portable application cannot rely on this: it should explicitly initialise the semaphores to the desired values.

When creating a new semaphore set, semget() initialises the set’s associated data structure, semid_ds (see semctl(2)), as follows:

Tag | Description
--- | ---
| 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

If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicating the error.

# ERRORS

On failure errno will be set to one of the following:

Tag | Description
--- | ---
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.
EEXIST | A semaphore set exists for key and semflg specified both IPC_CREAT and IPC_EXCL.
EINVAL | nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corresponding to key already exists, and 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.

# 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 everything but the least significant 9 bits of semflg and creates a new semaphore set (on success).

The following limits on semaphore set resources affect the semget() call:

Tag | Description
--- | ---
SEMMNI | System wide maximum number of semaphore sets: policy dependent (on Linux, this limit can be read and modified via the fourth field of /proc/sys/kernel/sem).
SEMMSL | Maximum number of semaphores per semid: implementation dependent (on Linux, this limit can be read and modified via the first field of /proc/sys/kernel/sem).
SEMMNS | System wide maximum number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of /proc/sys/kernel/sem). Values greater than SEMMSL * SEMMNI makes it irrelevant.

# BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

The semaphores in a set are not initialised by semget(). In order to initialise the semaphores, semctl(2) must be used to perform a SETVAL or a SETALL operation on the semaphore set. (Where multiple peers do not know who will be the first to initialise the set, checking for a non-zero sem_otime in the associated data structure retrieved by a semctl() IPC_STAT operation can be used to avoid races.)

# CONFORMING TO

SVr4, POSIX.1-2001.

# SEE ALSO

- semctl (2)

- semop (2)

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