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28 .\" @(#)fork.2 8.1 (Berkeley) 6/4/93
29 .\" $FreeBSD: src/lib/libc/sys/fork.2,v 1.9.2.6 2002/07/30 19:04:25 silby Exp $
30 .\" $DragonFly: src/lib/libc/sys/fork.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
37 .Nd create a new process
47 causes creation of a new process.
48 The new process (child process) is an exact copy of the
49 calling process (parent process) except for the following:
50 .Bl -bullet -offset indent
52 The child process has a unique process ID.
54 The child process has a different parent
55 process ID (i.e., the process ID of the parent process).
57 The child process has its own copy of the parent's descriptors.
58 These descriptors reference the same underlying objects, so that,
59 for instance, file pointers in file objects are shared between
60 the child and the parent, so that an
62 on a descriptor in the child process can affect a subsequent
67 This descriptor copying is also used by the shell to
68 establish standard input and output for newly created processes
69 as well as to set up pipes.
71 The child process' resource utilizations
75 All interval timers are cleared; see
79 Upon successful completion,
82 of 0 to the child process and returns the process ID of the child
83 process to the parent process. Otherwise, a value of -1 is returned
84 to the parent process, no child process is created, and the global
87 is set to indicate the error.
88 .Sh MULTI-THREADING CONSIDERATIONS
90 can create severe issues for multi-threaded programs due to the fact that
91 the memory state of the child process will record the asynchronous state
92 of the threads that are running in the parent.
94 will only be synchronous for the specific thread making the call.
96 locks used internally by
100 can be caught in a bad state.
101 To deal with these issues, the pthreads library goes to great lengths
102 to synchronize internal locks when
106 The threaded program itself as well as third party libraries used by the
107 program might or might not properly handle these issues when it comes to
108 their own internal state.
110 If at all possible, programs should use
114 when forking for the purposes of issuing an exec of some sort.
115 Attempting to fork a threaded program without issuing an exec is not
117 Attempting to bypass pthreads and implement threading manually is also
118 not recommended as it is doubtful that homegrown implementations could
119 properly deal with rtld races.
122 will fail and no child process will be created if:
125 The system-imposed limit on the total
126 number of processes under execution would be exceeded.
127 The limit is given by the
131 (The limit is actually ten less than this
132 except for the super user).
134 The user is not the super user, and
135 the system-imposed limit
136 on the total number of
137 processes under execution by a single user would be exceeded.
138 The limit is given by the
141 .Dv KERN_MAXPROCPERUID .
143 The user is not the super user, and
144 the soft resource limit corresponding to the resource parameter
146 would be exceeded (see
149 There is insufficient swap space for the new process.
161 function call appeared in