SEMOP(2) — SYSTEM CALLS

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 perform atomically 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:

shortsem_num;/∗ semaphore number ∗/
shortsem_op;/∗ semaphore operation ∗/
shortsem_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: [ALTER] (see semctl(2))

• If semval (see intro(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(2V)) for the specified semaphore.

• 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(3V).

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. [ALTER]

If sem_op() is zero, one of the following will occur: [READ]

• 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(3V).

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.

RETURN VALUES

semop() returns:

0 on success.

−1 on failure and sets errno to indicate the error.

ERRORS

E2BIG nsops is greater than the system-imposed maximum.

EACCES Operation permission is denied to the calling process (see intro(2)).

EAGAIN The operation would result in suspension of the calling process but (sem_flg & IPC_NOWAIT) is “true”.

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 The set of semaphores referred to by msqid was removed from the system.

EINTR The call was interrupted by the delivery of a signal.

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 an SEM_UNDO would be exceeded.

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

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