SEMCTL(2) Linux Programmer's Manual SEMCTL(2)
NAME
semctl - semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);
DESCRIPTION
The function semctl performs the control operation specified by cmd on the semaphore set identified by semid, or on the semnum-th semaphore
of that set. (Semaphores are numbered starting at 0.)
This function has three or four arguments. When there are four, the call is semctl(semid,semnum,cmd,arg); where the fourth argument arg has
a type union semun defined as follows:
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
/* union semun is defined by including <sys/sem.h> */
#else
/* according to X/OPEN we have to define it ourselves */
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT, IPC_SET */
unsigned short *array; /* array for GETALL, SETALL */
/* Linux specific part: */
struct seminfo *__buf; /* buffer for IPC_INFO */
};
#endif
Legal values for cmd are:
IPC_STAT Copy info from the semaphore set data structure into the structure pointed to by arg.buf. The argument semnum is ignored. The
calling process must have read access privileges on the semaphore set.
IPC_SET Write the values of some members of the semid_ds structure pointed to by arg.buf to the semaphore set data structure, updating
also its sem_ctime member. Considered members from the user supplied struct semid_ds pointed to by arg.buf are
sem_perm.uid
sem_perm.gid
sem_perm.mode /* only lowest 9-bits */
The effective user-ID of the calling process must be that of the super-user, or match the creator or owner of the semaphore
set. The argument semnum is ignored.
IPC_RMID Immediately remove the semaphore set and its data structures awakening all waiting processes (with an error return and errno
set to EIDRM). The effective user-ID of the calling process must be that of the super-user, or match the creator or owner of
the semaphore set. The argument semnum is ignored.
GETALL Return semval for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have
read access privileges on the semaphore set.
GETNCNT The system call returns the value of semncnt for the semnum-th semaphore of the set (i.e. the number of processes waiting for
an increase of semval for the semnum-th semaphore of the set). The calling process must have read access privileges on the
semaphore set.
GETPID The system call returns the value of sempid for the semnum-th semaphore of the set (i.e. the pid of the process that executed
the last semop call for the semnum-th semaphore of the set). The calling process must have read access privileges on the sema-
phore set.
GETVAL The system call returns the value of semval for the semnum-th semaphore of the set. The calling process must have read access
privileges on the semaphore set.
GETZCNT The system call returns the value of semzcnt for the semnum-th semaphore of the set (i.e. the number of processes waiting for
semval of the semnum-th semaphore of the set to become 0). The calling process must have read access privileges on the sema-
phore set.
SETALL Set semval for all semaphores of the set using arg.array, updating also the sem_ctime member of the semid_ds structure associ-
ated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the wait queue are
awakened if some semval becomes 0 or increases. The argument semnum is ignored. The calling process must have alter access
privileges on the semaphore set.
SETVAL Set the value of semval to arg.val for the semnum-th semaphore of the set, updating also the sem_ctime member of the semid_ds
structure associated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the
wait queue are awakened if semval becomes 0 or increases. The calling process must have alter access privileges on the sema-
phore set.
RETURN VALUE
On failure semctl returns -1 with errno indicating the error. Otherwise the system call returns a nonnegative value depending on cmd as
follows:
GETNCNT the value of semncnt.
GETPID the value of sempid.
GETVAL the value of semval.
GETZCNT the value of semzcnt.
All other cmd values return 0 on success.
ERRORS
On failure, errno will be set to one of the following:
EACCES The calling process has no access permissions needed to execute cmd.
EFAULT The address pointed to by arg.buf or arg.array isn't accessible.
EIDRM The semaphore set was removed.
EINVAL Invalid value for cmd or semid.
EPERM The argument cmd has value IPC_SET or IPC_RMID but the calling process has insufficient privileges to execute the command.
ERANGE The argument cmd has value SETALL or SETVAL and the value to which semval has to be set (for some semaphore of the set) is less
than 0 or greater than the implementation value SEMVMX.
NOTES
The IPC_INFO, SEM_STAT and SEM_INFO control calls are used by the ipcs(8) program to provide information on allocated resources. In the
future these can be modified as needed or moved to a proc file system interface.
Various fields in a struct semid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recom-
pilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)
The following system limit on semaphore sets affects a semctl call:
SEMVMX Maximum value for semval: implementation dependent (32767).
For greater portability it is best to always call semctl with four arguments.
Under Linux, the function semctl is not a system call, but is implemented via the system call ipc(2).
CONFORMING TO
SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.
SEE ALSO
ipc(2), shmget(2), shmat(2), shmdt(2), ipc(5)
Linux 2.4.1 2001-12-21 SEMCTL(2)