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 <unistd.h>
  44 .Ft pid_t
  45 .Fo fork
  46 .Fa void
  47 .Fc
  48 .Sh DESCRIPTION
  49 .Fn fork
  50 causes creation of a new process.
  51 The new process (child process) is an exact copy of the
  52 calling process (parent process) except for the following:
  53 .Bl -bullet -offset indent
  54 .It
  55 The child process has a unique process ID.
  56 .It
  57 The child process has a different parent
  58 process ID (i.e., the process ID of the parent process).
  59 .It
  60 The child process has its own copy of the parent's descriptors.
  61 These descriptors reference the same underlying objects, so that,
  62 for instance, file pointers in file objects are shared between
  63 the child and the parent, so that an
  64 .Xr lseek 2
  65 on a descriptor in the child process can affect a subsequent
  66 .Xr read
  67 or
  68 .Xr write
  69 by the parent.
  70 This descriptor copying is also used by the shell to
  71 establish standard input and output for newly created processes
  72 as well as to set up pipes.
  73 .It
  74 The child processes resource utilizations
  75 are set to 0; see
  76 .Xr setrlimit 2 .
  77 .El
  78 .Sh RETURN VALUES
  79 Upon successful completion,
  80 .Fn fork
  81 returns a value of 0 to the child process
  82 and returns the process ID of the child process to the parent process.
  83 Otherwise, a value of -1 is returned to the parent process,
  84 no child process is created, and the global 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 .\" ==========
  92 .It Bq Er EAGAIN
  93 The system-imposed limit
  94 on the total number of processes under execution
  95 would be exceeded.
  96 This limit is configuration-dependent.
  97 .\" ==========
  98 .It Bq Er EAGAIN
  99 The system-imposed limit
 100 .Dv MAXUPRC
 101 .Pq Aq Pa sys/param.h
 102 on the total number of
 103 processes under execution by a single user would be exceeded.
 104 .\" ==========
 105 .It Bq Er ENOMEM
 106 There is insufficient swap space for the new process.
 107 .El
 108 .Sh LEGACY SYNOPSIS
 109 .Fd #include <sys/types.h>
 110 .Fd #include <unistd.h>
 111 .Pp
 112 The include file
 113 .In sys/types.h
 114 is necessary.
 115 .Sh SEE ALSO
 116 .Xr execve 2 ,
 117 .Xr sigaction 2 ,
 118 .Xr wait 2 ,
 119 .Xr compat 5
 120 .Sh HISTORY
 121 A
 122 .Fn fork
 123 function call appeared in
 124 .At v6 .
 125 .Sh CAVEATS
 126 There are limits to what you can do in the child process.
 127 To be totally safe you should restrict yourself to only
 128 executing async-signal safe operations until such time
 129 as one of the exec functions is called.  All APIs, including
 130 global data symbols, in any framework or library should be
 131 assumed to be unsafe after a
 132 .Fn fork
 133 unless explicitly documented to be safe or async-signal
 134 safe.  If you need to use these frameworks in the child
 135 process, you must exec.  In this situation it is reasonable
 136 to exec yourself.