semctl(2) [freebsd man page]
SEMCTL(2) BSD System Calls Manual SEMCTL(2) NAME
semctl -- control 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 semctl(int semid, int semnum, int cmd, ...); DESCRIPTION
The semctl() system call performs the operation indicated by cmd on the semaphore set indicated by semid. A fourth argument, a union semun arg, is required for certain values of cmd. For the commands that use the arg argument, union semun is defined as follows: union semun { int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */ u_short *array; /* array for GETALL & SETALL */ }; Commands are performed as follows: IPC_STAT Fetch the semaphore set's struct semid_ds, storing it in the memory pointed to by arg.buf. IPC_SET Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode members of the semaphore set's struct semid_ds to match those of the struct pointed to by arg.buf. The calling process's effective uid must match either sem_perm.uid or sem_perm.cuid, or it must have superuser privileges. IPC_RMID Immediately removes the semaphore set from the system. The calling process's effective uid must equal the semaphore set's sem_perm.uid or sem_perm.cuid, or the process must have superuser privileges. GETVAL Return the value of semaphore number semnum. SETVAL Set the value of semaphore number semnum to arg.val. Outstanding adjust on exit values for this semaphore in any process are cleared. GETPID Return the pid of the last process to perform an operation on semaphore number semnum. GETNCNT Return the number of processes waiting for semaphore number semnum's value to become greater than its current value. GETZCNT Return the number of processes waiting for semaphore number semnum's value to become 0. GETALL Fetch the value of all of the semaphores in the set into the array pointed to by arg.array. SETALL Set the values of all of the semaphores in the set to the values in the array pointed to by arg.array. Outstanding adjust on exit values for all semaphores in this set, in any process are cleared. The struct semid_ds is defined as follows: struct semid_ds { struct ipc_perm sem_perm; /* operation permission struct */ struct sem *sem_base; /* pointer to first semaphore in set */ u_short sem_nsems; /* number of sems in set */ time_t sem_otime; /* last operation time */ time_t sem_ctime; /* last change time */ /* Times measured in secs since */ /* 00:00:00 GMT, Jan. 1, 1970 */ }; RETURN VALUES
On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT, semctl() returns the corresponding value; otherwise, 0 is returned. On failure, -1 is returned, and errno is set to indicate the error. ERRORS
The semctl() system call will fail if: [EINVAL] No semaphore set corresponds to semid. [EINVAL] The semnum argument is not in the range of valid semaphores for given semaphore set. [EPERM] The calling process's effective uid does not match the uid of the semaphore set's owner or creator. [EACCES] Permission denied due to mismatch between operation and mode of semaphore set. [ERANGE] SETVAL or SETALL attempted to set a semaphore outside the allowable range [0 .. SEMVMX]. SEE ALSO
semget(2), semop(2) BUGS
SETALL may update some semaphore elements before returning an error. BSD
September 12, 1995 BSD
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SEMCTL(2) BSD System Calls Manual SEMCTL(2) NAME
semctl -- semaphore control operations LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/sem.h> int semctl(int semid, int semnum, int cmd, ...); DESCRIPTION
The semctl() system call provides a number of control operations on the semaphore specified by semnum and semid. The operation to be per- formed is specified in cmd (see below). The fourth argument is optional and depends upon the operation requested. If required, it is a union of the following fields: int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */ u_short *array; /* array for GETALL & SETALL */ The semid_ds structure used in the IPC_SET and IPC_STAT commands is defined in <sys/sem.h> and contains the following members: struct ipc_perm sem_perm; /* operation permissions */ unsigned short sem_nsems; /* number of sems in set */ time_t sem_otime; /* last operation time */ time_t sem_ctime; /* last change time */ The ipc_perm structure used inside the semid_ds structure is defined in <sys/ipc.h> and contains the following members: uid_t cuid; /* creator user id */ gid_t cgid; /* creator group id */ uid_t uid; /* user id */ gid_t gid; /* group id */ mode_t mode; /* permission (lower 9 bits) */ semctl() provides the following operations: GETVAL Return the value of the semaphore. SETVAL Set the value of the semaphore to arg.val, where arg is the fourth argument to semctl(). GETPID Return the pid of the last process that did an operation on this semaphore. GETNCNT Return the number of processes waiting to acquire the semaphore. GETZCNT Return the number of processes waiting for the value of the semaphore to reach 0. GETALL Return the values of all the semaphores associated with semid. SETALL Set the values of all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array, where arg is the fourth argument to semctl(). IPC_STAT Gather information about a semaphore and place the information in the structure pointed to by arg.buf, where arg is the fourth argument to semctl(). IPC_SET Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the structure associated with the semaphore. The val- ues are taken from the corresponding fields in the structure pointed to by arg.buf, there arg is the fourth argument to semctl(). This operation can only be executed by the super-user, or a process that has an effective user id equal to either sem_perm.cuid or sem_perm.uid in the data structure associated with the semaphore. IPC_RMID Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the super- user or a process with an effective uid equal to the sem_perm.cuid or sem_perm.uid values in the data structure associated with the semaphore can do this. The permission to read or change a semaphore (see semop(2)) is determined by the sem_perm.mode field in the same way as is done with files (see chmod(2)), but the effective uid can match either the sem_perm.cuid field or the sem_perm.uid field, and the effective gid can match either sem_perm.cgid or sem_perm.gid. RETURN VALUES
For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns one of the values described above if successful. All other opera- tions will make semctl() return 0 if no errors occur. Otherwise -1 is returned and errno set to reflect the error. ERRORS
semctl() will fail if: [EACCES] The caller has no operation permission for this semaphore. [EFAULT] arg.buf or arg.array specifies an invalid address. [EINVAL] semid is not a valid message semaphore identifier. cmd is not a valid command. [EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the sem_perm.uid or sem_perm.cuid fields of the data structure associated with the semaphore. [ERANGE] cmd is equal to SETVAL or SETALL and the value to be set is greater than the system semaphore maximum value. SEE ALSO
semget(2), semop(2) STANDARDS
The semctl system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Semaphores appeared in the first release of AT&T System V UNIX. BSD
August 25, 1999 BSD