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SEMOP(2) FreeBSD System Calls Manual SEMOP(2)
NAME semop - atomic array of operations on a semaphore set
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
int semop(int semid, struct sembuf *array, size_t nops);
DESCRIPTION The semop() system call atomically performs the array of operations indicated by array on the semaphore set indicated by semid. The length of array is indicated by nops. Each operation is encoded in a struct sembuf, which is defined as follows:
struct sembuf { u_short sem_num; /* semaphore # */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */ };
For each element in array, sem_op and sem_flg determine an operation to be performed on semaphore number sem_num in the set. The values SEM_UNDO and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify the behavior of the given operation.
The operation performed depends as follows on the value of sem_op:
o When sem_op is positive and the process has alter permission, the semaphore's value is incremented by sem_op's value. If SEM_UNDO is specified, the semaphore's adjust on exit value is decremented by sem_op's value. A positive value for sem_op generally corresponds to a process releasing a resource associated with the semaphore.
o The behavior when sem_op is negative and the process has alter permission, depends on the current value of the semaphore:
o If the current value of the semaphore is greater than or equal to the absolute value of sem_op, then the value is decremented by the absolute value of sem_op. If SEM_UNDO is specified, the semaphore's adjust on exit value is incremented by the absolute value of sem_op.
o If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens:
o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:

o The semaphore's value is greater than or equal to the absolute value of sem_op. When this condition becomes true, the semaphore's value is decremented by the absolute value of sem_op, the semaphore's adjust on exit value is incremented by the absolute value of sem_op.
A negative value for sem_op generally means that a process is waiting for a resource to become available.
o When sem_op is zero and the process has read permission, one of the following will occur:
o If the current value of the semaphore is equal to zero then semop() can return immediately.
o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a return value of EIDRM.
o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
o The semaphore's value becomes zero.
For each semaphore a process has in use, the kernel maintains an "adjust on exit" value, as alluded to earlier. When a process exits, either voluntarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore's value. This can be used to ensure that a resource is released if a process terminates unexpectedly.
RETURN VALUES The semop() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS The semop() system call will fail if:
[EINVAL] No semaphore set corresponds to semid, or the process would exceed the system-defined limit for the number of per-process SEM_UNDO structures.
[EACCES] Permission denied due to mismatch between operation and mode of semaphore set.
[EAGAIN] The semaphore's value would have resulted in the process being put to sleep and IPC_NOWAIT was specified.
[E2BIG] Too many operations were specified. [SEMOPM]
[ENOSPC] The system SEM_UNDO pool [SEMMNU] is full.
[ERANGE] The requested operation would cause either the semaphore's current value [SEMVMX] or its adjust on exit value [SEMAEM] to exceed the system-imposed limits.
SEE ALSO semctl(2), semget(2), sigaction(2)
BUGS The semop() system call may block waiting for memory even if IPC_NOWAIT was specified.
FreeBSD 14.2-RELEASE September 22, 1995 FreeBSD 14.2-RELEASE