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 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:

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}

• 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.

• 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 . 

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.  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, the value of sem_op is added to semval .  ­{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(2). 

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 VALUE

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. 

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

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 intro(2)). 

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

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

EFAULT sops points to an illegal address. 

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

SEE ALSO

exec(2), exit(2), fork(2), intro(2), semctl(2), semget(2), signal(2)

BUGS

The SEM_UNDO function is not implemented. 

Semaphores always exit with EINTR when a signal is received.  The signal cannot be caught inside the semaphore. 

Sprite version 1.0  —  21 November 1987