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Semaphore

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Old 06-14-2012
Semaphore
If I create a semaphore and then I fork a number of child processes then all the child process use that same semaphore.
Since the process address spaces are different rfom each other then how all the child process are able to access the same semaphore?
I understand that semaphore/mutex is at os level out of process context.
Am I correct?
Where in which part of type/memory is this semaphore located?
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LEARN ABOUT ULTRIX

fork

fork(2) 							System Calls Manual							   fork(2)

Name
       fork - create a new process

Syntax
       #include <sys/types.h>
       #include <unistd.h>

       pid = fork()
       pid_t pid;

Description
       The  system  call causes creation of a new process.  The new process (child process) is an exact copy of the calling process except for the
       following:

       o    The child process has a unique process ID.

       o    The child process has a different parent process ID (that is, the process ID of the parent process).

       o    The child process has its own copy of the parent's descriptors.  These descriptors reference the same underlying objects, so that, for
	    instance,  file  pointers  in file objects are shared between the child and the parent, so that a on a descriptor in the child process
	    can affect a subsequent read or write by the parent.  This descriptor copying is also used by the shell to	establish  standard  input
	    and output for newly created processes as well as to set up pipes.

       o    The child processes resource utilizations are set to 0.  For further information, see

Return Values
       Upon  successful  completion,  returns  a  value  of  0	to the child process and returns the process ID of the child process to the parent
       process.  Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set	to
       indicate the error.

Diagnostics
       The system call fails and no child process are created under the following conditions:

       [EAGAIN]       The system-imposed limit {PROC_MAX} on the total number of processes under execution would be exceeded.

       [EAGAIN]       The system-imposed limit {CHILD_MAX} on the total number of processes under execution by a single user would be exceeded.

       [ENOMEM]       There is insufficient swap space for the new process.

See Also
       execve(2), wait(2)

																	   fork(2)