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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 call-ing calling
     ing process (parent process) except for the following:

           •   The child process has a unique process ID.

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

           •   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 created processes as well as to
               set up pipes.

           •   The child processes resource utilizations are set to 0; see setrlimit(2).

     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 indicate the error.

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

     [EAGAIN]           The system-imposed limit on the total number of processes 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 your-self yourself
     self 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 your-self. yourself.

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

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