SEMOP(2) SysV SEMOP(2)
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;
unsigned nsops;
DESCRIPTION
semop performs a variety of semaphore operations on the set of semaphores
associated with the semaphore identifier semid. sops is an array of
semaphore operation structures. nsops is the number of such structures
in the array. The contents of each structure includes 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.
Sem_op specifies one of the three following semaphore operations:
1. If sem_op is a negative integer, one of the following will occur:
{ALTER}
7 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' semadj value (see
exit(2)) for the specified semaphore.
7 If semval is less than the absolute value of sem_op and (sem_flg &
IPC_NOWAIT) is "true", semop will return immediately.
7 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 occurs:
- 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' 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(2).
2. 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' semadj value for the specified
semaphore. {ALTER}
3. If sem_op is zero, one of the following will occur: {READ}
7 If semval is 0, semop will return immediately.
7 If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is
"true", semop will return immediately.
7 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).
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.
[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".
[ENOSPC] The limit on the number of individual processes requesting an
SEM_UNDO would be exceeded.
[EINVAL] The number of individual semaphores for which the calling
process requests a SEM_UNDO would exceed the limit.
[ERANGE] An operation would cause a semval to overflow the system-
imposed limit.
[ERANGE] An operation would cause a semadj value to overflow the
system-imposed limit.
[EFAULT] sops points to an illegal address.
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.
SEE ALSO
exec(2), exit(2), fork(2), intro(2), semctl(2), semget(2).
DIAGNOSTICS
If semop returns due to the receipt of a signal, a value of -1 is
returned to the calling process and errno is set to EINTR. If it returns
due to the removal of a semid from the system, a value of -1 is returned
and errno is set to EIDRM.
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error.