semop(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO

NAME

semop- semaphore operations

SYNOPSIS

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

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

DESCRIPTION

The semop() function is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. The sops argument is a pointer to the array of semaphore-operation structures. The nsops argument is the number of such structures in the array.

Each sembuf structure contains 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. The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

---

00400    READ by user
00200    ALTER by user
00040    READ by group
00020    ALTER by group
00004    READ by others
00002    ALTER by others

---

See the Semaphore Operation Permissions section of intro(2) for more information.

The sem_op member specifies one of three semaphore operations:

1. The sem_op member is a negative integer; {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. 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.

   • 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 suspends execution of the calling process until one of the following conditions occur:

     • The value of 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 to EIDRM and -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(3C).

2. The sem_op member is a positive integer; {ALTER}

   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.

3. The sem_op member is 0; {READ}

   • If semval is 0, semop() returns immediately.

   • If semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.

   • If semval is not equal to 0 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:

     • The value of semval becomes 0, 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 to EIDRM and -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(3C).

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process.

RETURN VALUES

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

The semop() function will fail if:

E2BIG

The nsops argument 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

The sops argument points to an illegal address.

EFBIG

The value of sem_num is less than 0 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 was received.

EINVAL

The semid argument is not a valid semaphore identifier, or 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 a semadj value to overflow the system-imposed limit.

SEE ALSO

ipcs(1), intro(2), exec(2), exit(2), fork(2), semctl(2), semget(2)

SunOS 5.7  Last Revised 3 Feb 1994

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO