1FORK(2)                    Linux Programmer's Manual                   FORK(2)
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NAME

6       fork - create a child process
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
11
12       pid_t fork(void);
13

DESCRIPTION

15       fork()  creates  a new process by duplicating the calling process.  The
16       new process is referred to as the child process.  The  calling  process
17       is referred to as the parent process.
18
19       The child process and the parent process run in separate memory spaces.
20       At the time of fork() both memory spaces have the same content.  Memory
21       writes,  file  mappings (mmap(2)), and unmappings (munmap(2)) performed
22       by one of the processes do not affect the other.
23
24       The child process is an exact duplicate of the  parent  process  except
25       for the following points:
26
27       *  The child has its own unique process ID, and this PID does not match
28          the ID of any existing process group (setpgid(2)) or session.
29
30       *  The child's parent process ID is the same as  the  parent's  process
31          ID.
32
33       *  The  child  does  not  inherit  its parent's memory locks (mlock(2),
34          mlockall(2)).
35
36       *  Process resource utilizations (getrusage(2)) and CPU  time  counters
37          (times(2)) are reset to zero in the child.
38
39       *  The  child's  set  of  pending  signals is initially empty (sigpend‐
40          ing(2)).
41
42       *  The child does not inherit semaphore  adjustments  from  its  parent
43          (semop(2)).
44
45       *  The  child does not inherit process-associated record locks from its
46          parent (fcntl(2)).  (On the other hand,  it  does  inherit  fcntl(2)
47          open file description locks and flock(2) locks from its parent.)
48
49       *  The  child  does  not  inherit timers from its parent (setitimer(2),
50          alarm(2), timer_create(2)).
51
52       *  The child does not inherit outstanding asynchronous  I/O  operations
53          from its parent (aio_read(3), aio_write(3)), nor does it inherit any
54          asynchronous I/O contexts from its parent (see io_setup(2)).
55
56       The process attributes in the  preceding  list  are  all  specified  in
57       POSIX.1.   The parent and child also differ with respect to the follow‐
58       ing Linux-specific process attributes:
59
60       *  The child does not inherit directory change notifications  (dnotify)
61          from its parent (see the description of F_NOTIFY in fcntl(2)).
62
63       *  The  prctl(2)  PR_SET_PDEATHSIG  setting  is reset so that the child
64          does not receive a signal when its parent terminates.
65
66       *  The default timer slack value is set to the parent's  current  timer
67          slack value.  See the description of PR_SET_TIMERSLACK in prctl(2).
68
69       *  Memory mappings that have been marked with the madvise(2) MADV_DONT‐
70          FORK flag are not inherited across a fork().
71
72       *  Memory in address ranges that have been marked with  the  madvise(2)
73          MADV_WIPEONFORK  flag  is  zeroed in the child after a fork().  (The
74          MADV_WIPEONFORK setting remains in place for those address ranges in
75          the child.)
76
77       *  The   termination  signal  of  the  child  is  always  SIGCHLD  (see
78          clone(2)).
79
80       *  The port access permission bits set by ioperm(2) are  not  inherited
81          by the child; the child must turn on any bits that it requires using
82          ioperm(2).
83
84       Note the following further points:
85
86       *  The child process is created  with  a  single  thread—the  one  that
87          called  fork().   The  entire virtual address space of the parent is
88          replicated in the child, including the states of mutexes,  condition
89          variables,  and other pthreads objects; the use of pthread_atfork(3)
90          may be helpful for dealing with problems that this can cause.
91
92       *  After a fork() in a multithreaded program, the child can safely call
93          only  async-signal-safe  functions (see signal-safety(7)) until such
94          time as it calls execve(2).
95
96       *  The child inherits copies of the parent's set of open file  descrip‐
97          tors.   Each  file  descriptor  in the child refers to the same open
98          file description (see open(2)) as the corresponding file  descriptor
99          in  the parent.  This means that the two file descriptors share open
100          file status flags, file offset,  and  signal-driven  I/O  attributes
101          (see the description of F_SETOWN and F_SETSIG in fcntl(2)).
102
103       *  The  child inherits copies of the parent's set of open message queue
104          descriptors (see mq_overview(7)).  Each file descriptor in the child
105          refers to the same open message queue description as the correspond‐
106          ing file descriptor in the parent.  This means that the two file de‐
107          scriptors share the same flags (mq_flags).
108
109       *  The  child  inherits  copies  of  the parent's set of open directory
110          streams (see opendir(3)).  POSIX.1 says that the  corresponding  di‐
111          rectory  streams  in  the  parent  and child may share the directory
112          stream positioning; on Linux/glibc they do not.
113

RETURN VALUE

115       On success, the PID of the child process is returned in the parent, and
116       0  is returned in the child.  On failure, -1 is returned in the parent,
117       no child process is created, and errno is set appropriately.
118

ERRORS

120       EAGAIN A system-imposed limit on the number of threads was encountered.
121              There are a number of limits that may trigger this error:
122
123              *  the  RLIMIT_NPROC soft resource limit (set via setrlimit(2)),
124                 which limits the number of processes and threads for  a  real
125                 user ID, was reached;
126
127              *  the kernel's system-wide limit on the number of processes and
128                 threads,  /proc/sys/kernel/threads-max,  was   reached   (see
129                 proc(5));
130
131              *  the  maximum  number  of  PIDs, /proc/sys/kernel/pid_max, was
132                 reached (see proc(5)); or
133
134              *  the PID limit (pids.max) imposed by the cgroup "process  num‐
135                 ber" (PIDs) controller was reached.
136
137       EAGAIN The caller is operating under the SCHED_DEADLINE scheduling pol‐
138              icy and does not have the reset-on-fork flag set.  See sched(7).
139
140       ENOMEM fork() failed to allocate the necessary  kernel  structures  be‐
141              cause memory is tight.
142
143       ENOMEM An attempt was made to create a child process in a PID namespace
144              whose "init" process has terminated.  See pid_namespaces(7).
145
146       ENOSYS fork() is not supported on this platform (for example,  hardware
147              without a Memory-Management Unit).
148
149       ERESTARTNOINTR (since Linux 2.6.17)
150              System  call  was interrupted by a signal and will be restarted.
151              (This can be seen only during a trace.)
152

CONFORMING TO

154       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
155

NOTES

157       Under Linux, fork() is implemented using copy-on-write  pages,  so  the
158       only  penalty  that it incurs is the time and memory required to dupli‐
159       cate the parent's page tables, and to create a  unique  task  structure
160       for the child.
161
162   C library/kernel differences
163       Since  version  2.3.3,  rather than invoking the kernel's fork() system
164       call, the glibc fork() wrapper that is provided as  part  of  the  NPTL
165       threading  implementation  invokes clone(2) with flags that provide the
166       same effect as the traditional system  call.   (A  call  to  fork()  is
167       equivalent  to  a  call  to clone(2) specifying flags as just SIGCHLD.)
168       The glibc wrapper invokes any fork handlers that have been  established
169       using pthread_atfork(3).
170

EXAMPLES

172       See pipe(2) and wait(2).
173

SEE ALSO

175       clone(2),   execve(2),  exit(2),  setrlimit(2),  unshare(2),  vfork(2),
176       wait(2), daemon(3), pthread_atfork(3), capabilities(7), credentials(7)
177

COLOPHON

179       This page is part of release 5.10 of the Linux  man-pages  project.   A
180       description  of  the project, information about reporting bugs, and the
181       latest    version    of    this    page,    can     be     found     at
182       https://www.kernel.org/doc/man-pages/.
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186Linux                             2020-06-09                           FORK(2)
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