NAME

creatsem − create an instance of a binary semaphore

SYNOPSIS

cc [flag . . .] file . . . -lx
int creatsem(char ∗sem_name, int mode);

DESCRIPTION

creatsem defines a binary semaphore named by sem_name to be used by waitsem and sigsem 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. 

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 ­EINVAL. 

SEE ALSO

opensem(2), sigsem(2), waitsem(2)

NOTES

After a creatsem, you must do a waitsem to gain control of a given resource. 

  —  Application Compatibility Package