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FORK(2)                     BSD System Calls Manual                    FORK(2)

     fork -- create a new process

     #include <unistd.h>


     Fork() causes creation of a new process.  The new process (child process)
     is an exact copy of the calling process (parent process) except for the

           oo   The child process has a unique process ID.

           oo   The child process has a different parent process ID (i.e., the
               process ID of the parent process).

           oo   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 an lseek(2) 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 cre-ated created
               ated processes as well as to set up pipes.

           oo   The child processes resource utilizations are set to 0; see

     Upon successful completion, fork() 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 indi-cate indicate
     cate the error.

     Fork() will fail and no child process will be created if:

     [EAGAIN]           The system-imposed limit on the total number of pro-cesses processes
                        cesses under execution would be exceeded.  This limit
                        is configuration-dependent.

     [EAGAIN]           The system-imposed limit MAXUPRC (<sys/param.h>) 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.

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

     The include file <sys/types.h> is necessary.

     execve(2), sigaction(2), wait(2), compat(5)

     A fork() function call appeared in Version 6 AT&T UNIX.

     There are limits to what you can do in the child process.  To be totally
     safe you should restrict yourself to only executing async-signal safe
     operations until such time as one of the exec functions is called.  All
     APIs, including global data symbols, in any framework or library should
     be assumed to be unsafe after a fork() unless explicitly documented to be
     safe or async-signal safe.  If you need to use these frameworks in the
     child process, you must exec.  In this situation it is reasonable to exec

4th Berkeley Distribution        June 4, 1993        4th Berkeley Distribution