SEM_INIT(3)                                                             Linux Programmer's Manual                                                            SEM_INIT(3)

NAME
       sem_init - initialize an unnamed semaphore

SYNOPSIS
       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

       Link with -pthread.

DESCRIPTION
       sem_init() initializes the unnamed semaphore at the address pointed to by sem.  The value argument specifies the initial value for the semaphore.

       The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.

       If  pshared  has the value 0, then the semaphore is shared between the threads of a process, and should be located at some address that is visible to all threads
       (e.g., a global variable, or a variable allocated dynamically on the heap).

       If pshared is nonzero, then the semaphore is shared between processes, and should be located in  a  region  of  shared  memory  (see  shm_open(3),  mmap(2),  and
       shmget(2)).   (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.)  Any process that can access the shared
       memory region can operate on the semaphore using sem_post(3), sem_wait(3), and so on.

       Initializing a semaphore that has already been initialized results in undefined behavior.

RETURN VALUE
       sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

ERRORS
       EINVAL value exceeds SEM_VALUE_MAX.

       ENOSYS pshared is nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                                                                             │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │sem_init()                                                                                                                            │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO
       POSIX.1-2001.

NOTES
       Bizarrely, POSIX.1-2001 does not specify the value that should be returned by a successful call to sem_init().  POSIX.1-2008 rectifies this, specifying the  zero
       return on success.

EXAMPLES
       See shm_open(3) and sem_wait(3).

SEE ALSO
       sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

Linux                                                                          2021-03-22                                                                    SEM_INIT(3)