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sem_open - initialize and open a named semaphore
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
sem_t *sem_open(const char *name, int oflag);
sem_t *sem_open(const char *name, int oflag,
mode_t mode, unsigned int value);
Link with -lrt or -pthread.
sem_open() creates a new POSIX semaphore or opens an existing
semaphore. The semaphore is identified by name. For details of the
construction of name, see sem_overview(7).
The oflag argument specifies flags that control the operation of the
call. (Definitions of the flags values can be obtained by including
<fcntl.h>.) If O_CREAT is specified in oflag, then the semaphore is
created if it does not already exist. The owner (user ID) of the
semaphore is set to the effective user ID of the calling process. The
group ownership (group ID) is set to the effective group ID of the
calling process. If both O_CREAT and O_EXCL are specified in oflag,
then an error is returned if a semaphore with the given name already
If O_CREAT is specified in oflag, then two additional arguments must be
supplied. The mode argument specifies the permissions to be placed on
the new semaphore, as for open(2). (Symbolic definitions for the
permissions bits can be obtained by including <sys/stat.h>.) The
permissions settings are masked against the process umask. Both read
and write permission should be granted to each class of user that will
access the semaphore. The value argument specifies the initial value
for the new semaphore. If O_CREAT is specified, and a semaphore with
the given name already exists, then mode and value are ignored.
On success, sem_open() returns the address of the new semaphore; this
address is used when calling other semaphore-related functions. On
error, sem_open() returns SEM_FAILED, with errno set to indicate the
EACCES The semaphore exists, but the caller does not have permission to
EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a semaphore
with this name already exists.
EINVAL value was greater than SEM_VALUE_MAX.
EINVAL name consists of just "/", followed by no other characters.
EMFILE The process already has the maximum number of files and open.
name was too long.
ENFILE The system limit on the total number of open files has been
ENOENT The O_CREAT flag was not specified in oflag and no semaphore
with this name exists; or, O_CREAT was specified, but name
wasn't well formed.
ENOMEM Insufficient memory.
sem_close(3), sem_getvalue(3), sem_post(3), sem_unlink(3), sem_wait(3),
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