CREATSEM(S) XENIX System V CREATSEM(S)
Name
creatsem - Creates an instance of a binary semaphore.
Syntax
int = creatsem(sem_name,mode)
char *sem_name;
int mode;
Description
creatsem defines a binary semaphore named by sem_name to be
used by waitsem(S) and sigsem(S) to manage mutually
exclusive access to a resource, shared variable, or critical
section of a program. creatsem returns a unique semaphore
number, sem_num, which may then be used as the parameter in
waitsem and sigsem calls. Semaphores are special files of 0
length. The filename space is used to provide unique
identifiers for semaphores. mode sets the accessibility of
the semaphore using the same format as file access bits.
Access to a semaphore is granted only on the basis of the
read access bit; the write and execute bits are ignored.
A semaphore can be operated on only by a synchronizing
primitive, such as waitsem or sigsem, by creatsem which
initializes it to some value, or by opensem which opens the
semaphore for use by a process. Synchronizing primitives
are guaranteed to be executed without interruption once
started. These primitives are used by associating a
semaphore with each resource (including critical code
sections) to be protected.
The process controlling the semaphore should issue:
sem_num = creatsem("semaphore", mode);
to create, initialize, and open the semaphore for that
process. All other processes using the semaphore should
issue:
sem_num = opensem("semaphore");
to access the semaphore's identification value. Note that a
process cannot open and use a semaphore that has not been
initialized by a call to creatsem, nor should a process open
a semaphore more than once in one period of execution. Both
the creating and opening processes use waitsem and sigsem to
use the semaphore sem_num.
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CREATSEM(S) XENIX System V CREATSEM(S)
Compatibility
creatsem can only be used to define
version 3.0 semaphores, not System V semaphores.
See Also
opensem(S), waitsem(S), sigsem(S)
Diagnostics
creatsem returns the value -1 if an error occurs. If the
semaphore named by sem_name is already open for use by other
processes, errno is set to EEXIST. If the file specified
exists but is not a semaphore type, errno is set to ENOTNAM.
If the semaphore has not been initialized by a call to
creatsem, errno is set to ENAVAIL.
Notes
After a creatsem you must do a waitsem to gain control of a
given resource.
This feature is a XENIX specific enhancement and may not be
present in all UNIX implementations. This function must be
linked with the linker option -lx.
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