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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

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SEMOP(2)						      BSD System Calls Manual							  SEMOP(2)

NAME
semop -- atomic array of operations on a semaphore set SYNOPSIS
#include <sys/sem.h> int semop(int semid, struct sembuf *sops, size_t nsops); DESCRIPTION
The semop() system call atomically performs the array of operations indicated by sops on the semaphore set indicated by semid. The length of sops is indicated by nsops. 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 */ }; For each element in sops, sem_op and sem_flg determine an operation to be performed on semaphore number sem_num in the set. The values 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: o 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. o The behavior when sem_op is negative and the process has alter permission, depends on the current value of the semaphore: o 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. o If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens: o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN. o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied: o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a return value of EIDRM. o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2). o The semaphore's value is greater than or equal to the absolute value of sem_op. When this condition becomes true, the sema- phore'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. A negative value for sem_op generally means that a process is waiting for a resource to become available. o When sem_op is zero and the process has read permission, one of the following will occur: o If the current value of the semaphore is equal to zero then semop() can return immediately. o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN. o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied: o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a return value of EIDRM. o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2). o The semaphore's value becomes zero. For each semaphore a process has in use, the kernel maintains an ``adjust on exit'' value, as alluded to earlier. When a process exits, either voluntarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore's value. This can be used to insure that a resource is released if a process terminates unexpectedly. RETURN VALUES
The semop() 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
The semop() system call will fail if: [E2BIG] Too many operations are specified. [SEMOPM] [EACCES] Permission is denied, due to a mismatch between the operation and the mode of the semaphore set. [EAGAIN] The semaphore's value would result in the process being put to sleep and IPC_NOWAIT is specified. [EFBIG] sem_num is not in the range of valid semaphores for the set. [EIDRM] The semaphore set is removed from the system. [EINTR] The semop() system call is interrupted by a signal. [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. [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. LEGACY SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> The include files <sys/types.h> and <sys/ipc.h> are necessary. SEE ALSO
semctl(2), semget(2), sigaction(2), compat(5) BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT was specified. BSD
September 22, 1995 BSD
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