bsd/man/man2/fork.2

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  34 .\"     @(#)fork.2      8.1 (Berkeley) 6/4/93
  35 .\"
  36 .Dd June 4, 1993
  37 .Dt FORK 2
  38 .Os BSD 4
  39 .Sh NAME
  40 .Nm fork
  41 .Nd create a new process
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/types.h>
  44 .Fd #include <unistd.h>
  45 .Ft pid_t
  46 .Fn fork void
  47 .Sh DESCRIPTION
  48 .Fn Fork
  49 causes creation of a new process.
  50 The new process (child process) is an exact copy of the
  51 calling process (parent process) except for the following:
  52 .Bl -bullet -offset indent
  53 .It
  54 The child process has a unique process ID.
  55 .It
  56 The child process has a different parent
  57 process ID (i.e., the process ID of the parent process).
  58 .It
  59 The child process has its own copy of the parent's descriptors.
  60 These descriptors reference the same underlying objects, so that,
  61 for instance, file pointers in file objects are shared between
  62 the child and the parent, so that an
  63 .Xr lseek 2
  64 on a descriptor in the child process can affect a subsequent
  65 .Xr read
  66 or
  67 .Xr write
  68 by the parent.
  69 This descriptor copying is also used by the shell to
  70 establish standard input and output for newly created processes
  71 as well as to set up pipes.
  72 .It
  73 The child processes resource utilizations
  74 are set to 0; see
  75 .Xr setrlimit 2 .
  76 .El
  77 .Sh RETURN VALUES
  78 Upon successful completion,
  79 .Fn fork
  80 returns a value
  81 of 0 to the child process and returns the process ID of the child
  82 process to the parent process.  Otherwise, a value of -1 is returned
  83 to the parent process, no child process is created, and the global
  84 variable
  85 .Va errno
  86 is set to indicate the error.
  87 .Sh ERRORS
  88 .Fn Fork
  89 will fail and no child process will be created if:
  90 .Bl -tag -width Er
  91 .It Bq Er EAGAIN
  92 The system-imposed limit on the total
  93 number of processes under execution would be exceeded.
  94 This limit is configuration-dependent.
  95 .It Bq Er EAGAIN
  96 The system-imposed limit
  97 .Dv MAXUPRC
  98 .Pq Aq Pa sys/param.h
  99 on the total number of
 100 processes under execution by a single user would be exceeded.
 101 .It Bq Er ENOMEM
 102 There is insufficient swap space for the new process.
 103 .El
 104 .Sh SEE ALSO
 105 .Xr execve 2 ,
 106 .Xr wait 2
 107 .Sh HISTORY
 108 A
 109 .Fn fork
 110 function call appeared in
 111 .At v6 .