SEMOP(2)

NAME
semop - semaphore operations

SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semop (semid, sops, nsops)
int semid;
struct sembuf *sops;
int nsops;

DESCRIPTION
semop is used to automatically 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:

    short	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.

sem_op specifies one of three semaphore operations as follows:

    If sem_op is a negative integer, one of the following will occur:

        If semval (see kedef(2)) 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 exit(2)) for the specified semaphore. SEM_UNDO is defined in <sys/sem.h>.

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

        If semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is ``false'', semop will increment 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(2).

    If sem_op is a positive integer and the calling process has alter permission, 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 will occur:

        If semval is zero, semop will return immediately.

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

        If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is ``false'', semop will increment the semzcnt associated with the specified semaphore and suspend 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(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, the value of semval at the time of the call for the last operation in the array pointed to by sops is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS
semop will fail if one or more of the following are 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 kedef(2)).

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 or semadj value to overflow the system-imposed limit.

EFAULT
    sops points to an illegal address.

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.

SEE ALSO
exec(2), exit(2), fork(2), kedef(2), semctl(2), semget(2).