semop

semop, semtimedop - semaphore operations

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
 I int semop(int  semid , struct sembuf * sops , unsigned  nsops );
 I int semtimedop(int  semid , struct sembuf * sops , unsigned  nsops ,
I                struct timespec * timeout );

Each semaphore in a semaphore set has the following associated values:

unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* process that did last op */
 

R semop () performs operations on selected semaphores in the set indicated by R semid . Each of the nsops elements in the array pointed to by sops specifies an operation to be performed on a single semaphore. The elements of this structure are of type R struct sembuf , containing the following members:

unsigned short sem_num;  /* semaphore number */
short          sem_op;   /* semaphore operation */
short          sem_flg;  /* operation flags */
 

Flags recognized in sem_flg are IPC_NOWAIT and R SEM_UNDO . If an operation specifies R SEM_UNDO , it will be automatically undone when the process terminates.

The set of operations contained in sops is performed in R array order , and R atomically , that is, the operations are performed either as a complete unit, or not at all. The behavior of the system call if not all operations can be performed immediately depends on the presence of the IPC_NOWAIT flag in the individual sem_flg fields, as noted below. Each operation is performed on the R sem_num -th semaphore of the semaphore set, where the first semaphore of the set is numbered 0. There are three types of operation, distinguished by the value of R sem_op .

If sem_op is a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, if SEM_UNDO is specified for this operation, the system updates the process undo count (semadj) for this semaphore. This operation can always proceed it never forces a process to wait. The calling process must have alter permission on the semaphore set.

If sem_op is zero, the process must have read permission on the semaphore set. This is a "wait-for-zero" operation: if semval is zero, the operation can immediately proceed. Otherwise, if IPC_NOWAIT is specified in R sem_flg , R semop () fails with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semzcnt (the count of processes waiting until this semaphore's value becomes zero) is incremented by one and the process sleeps until one of the following occurs:

semval becomes 0, at which time the value of semzcnt is decremented.

The semaphore set is removed: R semop () fails, with errno set to R EIDRM .

The calling process catches a signal: the value of semzcnt is decremented and R semop () fails, with errno set to R EINTR .

The time limit specified by timeout in a R semtimedop () call expires: R semop () fails, with errno set to R EAGAIN .

If sem_op is less than zero, the process must have alter permission on the semaphore set. If semval is greater than or equal to the absolute value of R sem_op , the operation can proceed immediately: the absolute value of sem_op is subtracted from R semval , and, if SEM_UNDO is specified for this operation, the system updates the process undo count (semadj) for this semaphore. If the absolute value of sem_op is greater than R semval , and IPC_NOWAIT is specified in R sem_flg , R semop () fails, with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semncnt (the counter of processes waiting for this semaphore's value to increase) is incremented by one and the process sleeps until one of the following occurs:

semval becomes greater than or equal to the absolute value of R sem_op , at which time the value of semncnt is decremented, the absolute value of sem_op is subtracted from semval and, if SEM_UNDO is specified for this operation, the system updates the process undo count (semadj) for this semaphore.

The semaphore set is removed from the system: R semop () fails, with errno set to R EIDRM .

The calling process catches a signal: the value of semncnt is decremented and R semop () fails, with errno set to R EINTR .

The time limit specified by timeout in a R semtimedop () call expires: the system call fails, with errno set to R EAGAIN .

On successful completion, the sempid value for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process. In addition, the sem_otime is set to the current time.

R semtimedop () behaves identically to R semop () except that in those cases were the calling process would sleep, the duration of that sleep is limited by the amount of elapsed time specified by the timespec structure whose address is passed in the timeout parameter. If the specified time limit has been reached, R semtimedop () fails with errno set to EAGAIN (and none of the operations in sops is performed). If the timeout parameter is NULL, then R semtimedop () behaves exactly like R semop ().

If successful R semop () and R semtimedop () return 0; otherwise they return -1 with errno indicating the error.

On failure, errno is set to one of the following:

E2BIG

EACCES

EAGAIN

EFAULT

EFBIG

EIDRM

EINTR

EINVAL

ENOMEM

ERANGE

R semtimedop () first appeared in Linux 2.5.52, and was subsequently backported into kernel 2.4.22. Glibc support for R semtimedop () first appeared in version 2.3.3.

SVr4, POSIX.1-2001.

The sem_undo structures of a process aren't inherited across a fork(2) system call, but they are inherited across an execve(2) system call.

R semop () is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.

semadj is a per-process integer which is simply the (negative) count of all semaphore operations performed specifying the SEM_UNDO flag. When a semaphore's value is directly set using the SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes are cleared.

The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using appropriate semctl(2) calls.

The following limits on semaphore set resources affect the R semop () call:

SEMOPM

SEMVMX

The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM), the system wide maximum number of undo structures (SEMMNU) and the per-process maximum number of undo entries system parameters.

When a process terminates, its set of associated semadj structures is used to undo the effect of all of the semaphore operations it performed with the SEM_UNDO flag. This raises a difficulty: if one (or more) of these semaphore adjustments would result in an attempt to decrease a semaphore's value below zero, what should an implementation do? One possible approach would be to block until all the semaphore adjustments could be performed. This is however undesirable since it could force process termination to block for arbitrarily long periods. Another possibility is that such semaphore adjustments could be ignored altogether (somewhat analogously to failing when IPC_NOWAIT is specified for a semaphore operation). Linux adopts a third approach: decreasing the semaphore value as far as possible (i.e., to zero) and allowing process termination to proceed immediately. In kernels 2.6.x, x <= 10, there is a bug that in some circumstances prevents a process that is waiting for a semaphore value to become zero from being woken up when the value does actually become zero. This bug is fixed in kernel 2.6.11.

semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), svipc(7)