semop() -- perform array of operations on semaphore set

SYNOPSIS

#include <sys/sem.h>

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

The semop() function performs atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier that semid specifies. sops is a pointer to the array of semaphore-operation structures. nsops is the number of semaphore-operation 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 identified by semid and sem_num. The variable sem_op specifies one of three semaphore operations as follows, depending upon whether its value is negative, positive, or zero.

If sem_op is a negative integer and the calling process has alter permission, one of the following occurs.

If semval 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 non-zero, the absolute value of sem_op is added to the semadj value of the calling process for the specified semaphore.

If semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is non-zero, semop() returns immediately.

If semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is zero, sem_op increments the semncnt associated with the specified semaphore and suspends execution of the calling thread until one of the following conditions occur:
1. 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 non-zero, the absolute value of sem_op is added to the semadj value of the calling process for the specified semaphore.
2. The semid for which the calling process is awaiting action is removed from the system. When this occurs, semop() sets errno equal to EIDRM and returns a value of -1.
3. 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.

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 non-zero, the value of sem_op is subtracted from the semadj value of the calling process for the specified semaphore.

If sem_op is zero and the calling process has read permission, one of the following occurs:

If semval is zero, semop() returns immediately.

If semval is non-zero and (sem_flg & IPC_NOWAIT) is non-zero, semop() returns immediately.

If semval is non-zero and (sem_flg & IPC_NOWAIT) is zero, semop() increments the semzcnt associated with the specified semaphore and suspends execution of the calling thread until one of the following occurs:
1. semval becomes zero, at which time the value of semzcnt associated with the specified semaphore is decremented.
2. 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.
3. 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 sigaction().

When semop() successfully completes, 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.

PARAMETERS

semid: Is a unique positive integer created by a semget() call.
sops: Points to the array of semaphore-operation structures.
nsops: Is the number of semaphore-operation structures in sops.

RETURN VALUES

If successful, semop() returns a value of 0. On failure, semop() returns a value of -1 and sets errno to one of the following values:

E2BIG

nsops is greater than the system-imposed maximum.

EACCES

Operation permission is denied to the calling process.

EAGAIN

The operation would result in suspension of the calling process but (sem_flg & IPC_NOWAIT) is non-zero.

EFAULT

sops points to an illegal address.

EFBIG

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

EIDRM

A semid was removed from the system.

EINTR

A signal interrupted the call.

EINVAL

semid is not a valid semaphore identifier.

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

ENOSPC

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

ERANGE

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

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

semop()
perform array of operations on semaphore set

Function

SYNOPSIS

DESCRIPTION

PARAMETERS

RETURN VALUES

CONFORMANCE

MULTITHREAD SAFETY LEVEL

PORTING ISSUES

AVAILABILITY

SEE ALSO

semop() perform array of operations on semaphore set

Function

semop()
perform array of operations on semaphore set