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NAME

semop — semaphore operations

SYNOPSIS

#include <sys/sem.h> int semop( int semid, struct sembuf *sops, size_t nsops );

DESCRIPTION

semop() is used to atomically perform an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. sops is a pointer to the array of semaphore-operation structures. nsops is the number of such structures in the array. The contents of each structure includes the following members:

ushort sem_num; /* semaphore number */ short sem_op /* semaphore operation */ short sem_flg; /* operation flags */

Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num. Semaphore array operations are atomic in that none of the semaphore operations are performed until blocking conditions on all of the semaphores in the array have been removed.

sem_op specifies one of three semaphore operations as follows:

  • If sem_op is a negative integer, one of the following occurs:

    • If semval (see semaphore identifier in glossary(9)) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. Also, if (sem_flg & SEM_UNDO) is ``true'', the absolute value of sem_op is added to the calling process's semadj value (see glossary(9) and exit(2)) for the specified semaphore.

    • If semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is ``true'', semop() returns immediately.

    • If semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is ``false'', semop() increments the semncnt associated with the specified semaphore and suspend execution of the calling process until one of the following conditions occur:

      • semval becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of semncnt associated with the specified semaphore is decremented, the absolute value of sem_op is subtracted from semval and, if (sem_flg & SEM_UNDO) is ``true'', the absolute value of sem_op is added to the calling process's semadj value for the specified semaphore.

      • The semid for which the calling process is awaiting action is removed from the system (see semctl(2)). When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.

      • The calling process receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in signal(5).

  • If sem_op is a positive integer, the value of sem_op is added to semval and, if (sem_flg & SEM_UNDO) is ``true'', the value of sem_op is subtracted from the calling process's semadj value for the specified semaphore.

  • If sem_op is zero, one of the following occurs:

    • If semval is zero, semop() proceeds to the next semaphore operation specified by sops, or returns immediately if this is the last operation.

    • If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is ``true'', semop() returns immediately.

    • If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is ``false'', semop() increments the semzcnt associated with the specified semaphore and suspends execution of the calling process until one of the following occurs:

      • semval becomes zero, at which time the value of semzcnt associated with the specified semaphore is decremented.

      • The semid for which the calling process is awaiting action is removed from the system. When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.

      • The calling process receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in signal(5).

EXAMPLES

The following call to semop() atomically performs a "P" or "get" operation on the second semaphore in the semaphore set and a "V" or "release" operation on the third semaphore in the set. This example assumes the process has a valid semid which represents a set of 4 semaphores as shown on the semget(2) manual page. It also assumes that the semvals of the semaphores in the set have been initialized as shown in the semctl(2) manual entry.

struct sembuf sops[4]; sops[0].sem_num = 1; sops[0].sem_op = -1; /* P (get) */ sops[0].sem_flg = 0; sops[1].sem_num = 2; sops[1].sem_op = 1; /* V (release) */ sops[1].sem_flg = 0; semop (mysemid, sops, 2);

RETURN VALUE

If semop() returns due to the receipt of a signal, a value of -1 is returned to the calling process and errno is set to EINTR. If it returns due to the removal of a semid from the system, a value of -1 is returned and errno is set to EIDRM.

Upon successful completion, a non-negative value is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS

semop() fails if one or more of the following is true for any of the semaphore operations specified by sops:

[EINVAL]

semid is not a valid semaphore identifier.

[EFBIG]

sem_num is less than zero or greater than or equal to the number of semaphores in the set associated with semid.

[E2BIG]

nsops is greater than the system-imposed maximum.

[EACCES]

Operation permission is denied to the calling process (see glossary(9)).

[EAGAIN]

The operation would result in suspension of the calling process but (sem_flg & IPC_NOWAIT) is ``true''.

[ENOSPC]

The limit on the number of individual processes requesting an SEM_UNDO would be exceeded.

[EINVAL]

The number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the limit.

[ERANGE]

An operation would cause a semval to overflow the system-imposed limit.

[ERANGE]

An operation would cause a semadj value to overflow the system-imposed limit.

[EFAULT]

sops points to an illegal address. The reliable detection of this error will be implementation dependent.

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops is set equal to the process ID of the calling process. The value of sem_otime in the data structure associated with the semaphore identifier will be set to the current time.

WARNINGS

Check all references to signal(5) for appropriateness on systems that support sigvector(2). sigvector(2) can affect the behavior described on this page.

SEE ALSO

ipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C), signal(5).

STANDARDS CONFORMANCE

semop(): SVID2, SVID3, XPG2, XPG3, XPG4



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