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SEMOP(2) | 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¶
Thesemop
() 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 */ };
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:
- 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. - The behavior when sem_op is negative and
the process has alter permission, depends on the current value of the
semaphore:
- 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. - If the current value of the semaphore is less than the absolute value
of sem_op, one of the following
happens:
- If
IPC_NOWAIT
was specified, thensemop
() returns immediately with a return value ofEAGAIN
. - Otherwise, the calling process is put to sleep until one of the
following conditions is satisfied:
- Some other process removes the semaphore with the
IPC_RMID
option of semctl(2). In this case,semop
() returns immediately with a return value ofEIDRM
. - The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
- 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.
- Some other process removes the semaphore with the
- If
- 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
- When sem_op is zero and the process has
read permission, one of the following will occur:
- If the current value of the semaphore is equal to zero then
semop
() can return immediately. - If
IPC_NOWAIT
was specified, thensemop
() returns immediately with a return value ofEAGAIN
. - Otherwise, the calling process is put to sleep until one of the
following conditions is satisfied:
- Some other process removes the semaphore with the
IPC_RMID
option of semctl(2). In this case,semop
() returns immediately with a return value ofEIDRM
. - The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
- The semaphore's value becomes zero.
- Some other process removes the semaphore with the
- If the current value of the semaphore is equal to zero then
RETURN VALUES¶
Thesemop
() 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¶
Thesemop
() 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
] - [
EFBIG
] - sem_num was not in the range of valid semaphores for the set.
- [
EIDRM
] - The semaphore set was removed from the system.
- [
EINTR
] - The
semop
() system call was interrupted by a signal. - [
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¶
Thesemop
() system call may block waiting for
memory even if IPC_NOWAIT
was
specified.September 22, 1995 | Debian |