semop(2) [netbsd man page]
SEMOP(2) BSD System Calls Manual SEMOP(2) NAME
semop -- semaphore operations LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/sem.h> int semop(int semid, struct sembuf *sops, size_t nsops); DESCRIPTION
semop() provides a number of atomic operations on a set of semaphores. The semaphore set is specified by semid, sops is an array of sema- phore operations, and nsops is the number of operations in this array. The sembuf structures in the array contain the following members: unsigned short sem_num; /* semaphore # */ short sem_op; /* semaphore operation */ short sem_flg; /* operation flags */ Each operation (specified in sem_op) is applied to semaphore number sem_num in the set of semaphores specified by semid. The value of sem_op determines the action taken in the following way: o sem_op is less than 0. The current process is blocked until the value of the semaphore is greater than or equal to the absolute value of sem_op. The absolute value of sem_op is then subtracted from the value of the semaphore, and the calling process continues. Negative values of sem_op are thus used to enter critical regions. o sem_op is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions. o sem_op is equal to 0. The calling process is blocked until the value of the specified semaphore reaches 0. The behaviour of each operation is influenced by the flags set in sem_flg in the following way: IPC_NOWAIT In the case where the calling process would normally block, waiting for a semaphore to reach a certain value, IPC_NOWAIT makes the call return immediately, returning a value of -1 and setting errno to EAGAIN. SEM_UNDO Keep track of the changes that this call makes to the value of a semaphore, so that they can be undone when the calling process terminates. This is useful to prevent other processes waiting on a semaphore to block forever, should the process that has the semaphore locked terminate in a critical section. RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error. ERRORS
semop() will fail if: [EINVAL] There is no semaphore associated with semid. [EIDRM] The semaphore set was removed while the process was waiting for one of its semaphores to reach a certain value. [EACCES] The calling process has no permission to access the specified semaphore set. [E2BIG] The value of nsops is too big. The maximum is defined as MAX_SOPS in <sys/sem.h>. [EFBIG] sem_num in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set speci- fied by semid. [ENOSPC] SEM_UNDO was requested, and there is not enough space left in the kernel to store the undo information. [EAGAIN] The requested operation can not immediately be performed, and IPC_NOWAIT was set in sem_flg. [EFAULT] sops points to an illegal address. SEE ALSO
semctl(2), semget(2) STANDARDS
The semop 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
November 3, 2005 BSD
Check Out this Related Man Page
semop(2) System Calls semop(2) NAME
semop, semtimedop - semaphore operations SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semop(int semid, struct sembuf *sops, size_t nsops); int semtimedop(int semid, struct sembuf *sops, size_t nsops, const struct timespec *timeout); DESCRIPTION
The semop() function is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. The sops argument is a pointer to the array of semaphore-operation structures. The nsops argument is the number of such structures in the array. Each sembuf structure contains 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. The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are inter- preted as follows: 00400 READ by user 00200 ALTER by user 00040 READ by group 00020 ALTER by group 00004 READ by others 00002 ALTER by others See the Semaphore Operation Permissions section of intro(2) for more information. A process maintains a value, semadj, for each semaphore it modifies. This value contains the cumulative effect of operations the process has performed on an individual semaphore with the SEM_UNDO flag set (so that they can be undone if the process terminates unexpectedly). The value of semadj can affect the behavior of calls to semop(), semtimedop(), exit(), and _exit() (the latter two functions documented on exit(2)), but is otherwise unobservable. See below for details. The sem_op member specifies one of three semaphore operations: 1. The sem_op member is a negative integer; {ALTER} o 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's semadj value (see exit(2)) for the specified semaphore. o If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately. o If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is false, semop() increments the semncnt associated with the specified semaphore and suspends execution of the calling thread until one of the following conditions occur: o The value of 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's semadj value for the specified semaphore. o The semid for which the calling thread is awaiting action is removed from the system (see semctl(2)). When this occurs, errno is set to EIDRM and -1 is returned. o The calling thread receives a signal that is to be caught. When this occurs, the value of semncnt associated with the speci- fied semaphore is decremented, and the calling thread resumes execution in the manner prescribed in sigaction(2). 2. The sem_op member is a positive integer; {ALTER} 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's semadj value for the specified semaphore. 3. The sem_op member is 0; {READ} o If semval is 0, semop() returns immediately. o If semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately. o If semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is false, semop() increments the semzcnt associated with the specified sema- phore and suspends execution of the calling thread until one of the following occurs: o The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore is set to 0 and all processes waiting on semval to become 0 are awakened. o The semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno is set to EIDRM and -1 is returned. o The calling thread receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the speci- fied semaphore is decremented, and the calling thread resumes execution in the manner prescribed in sigaction(2). Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process. The semtimedop() function behaves as semop() except when it must suspend execution of the calling process to complete its operation. If semtimedop() must suspend the calling process after the time interval specified in timeout expires, or if the timeout expires while the process is suspended, semtimedop() returns with an error. If the timespec structure pointed to by timeout is zero-valued and semtimedop() needs to suspend the calling process to complete the requested operation(s), it returns immediately with an error. If timeout is the NULL pointer, the behavior of semtimedop() is identical to that of semop(). RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The semop() and semtimedop() functions will fail if: E2BIG The nsops argument is greater than the system-imposed maximum. See NOTES. 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. EFAULT The sops argument points to an illegal address. EFBIG The value of sem_num is less than 0 or greater than or equal to the number of semaphores in the set associated with semid. EIDRM A semid was removed from the system. EINTR A signal was received. EINVAL The semid argument is not a valid semaphore identifier, or the number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the limit. ENOSPC The limit on the number of individual processes requesting an SEM_UNDO would be exceeded. ERANGE An operation would cause a semval or a semadj value to overflow the system-imposed limit. The semtimedop() function will fail if: EAGAIN The timeout expired before the requested operation could be completed. The semtimedop() function will fail if one of the following is detected: EFAULT The timeout argument points to an illegal address. EINVAL The timeout argument specified a tv_sec or tv_nsec value less than 0, or a tv_nsec value greater than or equal to 1000 mil- lion. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |semop() is Standard. | +-----------------------------+-----------------------------+ SEE ALSO
ipcs(1), rctladm(1M), intro(2), exec(2), exit(2), fork(2), semctl(2), semget(2), setrctl(2), sigaction(2), attributes(5), standards(5) NOTES
The system-imposed maximum on nsops for a semaphore identifier is the minimum enforced value of the process.max-sem-ops resource control of the creating process at the time semget(2) was used to allocate the identifier. See rctladm(1M) and setrctl(2) for information about using resource controls. SunOS 5.10 13 May 2003 semop(2)