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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd August 5, 2021 29.Dt FORK 2 30.Os 31.Sh NAME 32.Nm fork 33.Nd create a new process 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In unistd.h 38.Ft pid_t 39.Fn fork void 40.Ft pid_t 41.Fn _Fork void 42.Sh DESCRIPTION 43The 44.Fn fork 45function causes creation of a new process. 46The new process (child process) is an exact copy of the 47calling process (parent process) except for the following: 48.Bl -bullet -offset indent 49.It 50The child process has a unique process ID. 51.It 52The child process has a different parent 53process ID (i.e., the process ID of the parent process). 54.It 55The child process has its own copy of the parent's descriptors, 56except for descriptors returned by 57.Xr kqueue 2 , 58which are not inherited from the parent process. 59These descriptors reference the same underlying objects, so that, 60for instance, file pointers in file objects are shared between 61the child and the parent, so that an 62.Xr lseek 2 63on a descriptor in the child process can affect a subsequent 64.Xr read 2 65or 66.Xr write 2 67by the parent. 68This descriptor copying is also used by the shell to 69establish standard input and output for newly created processes 70as well as to set up pipes. 71.It 72The child process' resource utilizations 73are set to 0; see 74.Xr setrlimit 2 . 75.It 76All interval timers are cleared; see 77.Xr setitimer 2 . 78.It 79The robust mutexes list (see 80.Xr pthread_mutexattr_setrobust 3 ) 81is cleared for the child. 82.It 83The atfork handlers established with the 84.Xr pthread_atfork 3 85function are called as appropriate before fork in the parent process, 86and after the child is created, in parent and child. 87.It 88The child process has only one thread, 89corresponding to the calling thread in the parent process. 90If the process has more than one thread, 91locks and other resources held by the other threads are not released 92and therefore only async-signal-safe functions 93(see 94.Xr sigaction 2 ) 95are guaranteed to work in the child process until a call to 96.Xr execve 2 97or a similar function. 98The 99.Fx 100implementation of 101.Fn fork 102provides a usable 103.Xr malloc 3 , 104and 105.Xr rtld 1 106services in the child process. 107.El 108.Pp 109The 110.Fn fork 111function is not async-signal safe and creates a cancellation point 112in the parent process. 113It cannot be safely used from signal handlers, and the atfork handlers 114established by 115.Xr pthread_atfork 3 116do not need to be async-signal safe either. 117.Pp 118The 119.Fn _Fork 120function creates a new process, similarly to 121.Fn fork , 122but it is async-signal safe. 123.Fn _Fork 124does not call atfork handlers, and does not create a cancellation point. 125It can be used safely from signal handlers, but then no userspace 126services ( 127.Xr malloc 3 128or 129.Xr rtld 1 ) 130are available in the child if forked from multi-threaded parent. 131In particular, if using dynamic linking, all dynamic symbols used by the 132child after 133.Fn _Fork 134must be pre-resolved. 135Note: resolving can be done globally by specifying the 136.Ev LD_BIND_NOW 137environment variable to the dynamic linker, or per-binary by passing the 138.Fl z Ar now 139option to the static linker 140.Xr ld 1 , 141or by using each symbol before the 142.Fn _Fork 143call to force the binding. 144.Sh RETURN VALUES 145Upon successful completion, 146.Fn fork 147and 148.Fn _Fork 149return a value 150of 0 to the child process and return the process ID of the child 151process to the parent process. 152Otherwise, a value of -1 is returned 153to the parent process, no child process is created, and the global 154variable 155.Va errno 156is set to indicate the error. 157.Sh EXAMPLES 158The following example shows a common pattern of how 159.Fn fork 160is used in practice. 161.Bd -literal -offset indent 162#include <err.h> 163#include <stdio.h> 164#include <stdlib.h> 165#include <unistd.h> 166 167int 168main(void) 169{ 170 pid_t pid; 171 172 /* 173 * If child is expected to use stdio(3), state of 174 * the reused io streams must be synchronized between 175 * parent and child, to avoid double output and other 176 * possible issues. 177 */ 178 fflush(stdout); 179 180 switch (pid = fork()) { 181 case -1: 182 err(1, "Failed to fork"); 183 case 0: 184 printf("Hello from child process!\en"); 185 186 /* 187 * Since we wrote into stdout, child needs to use 188 * exit(3) and not _exit(2). This causes handlers 189 * registered with atexit(3) to be called twice, 190 * once in parent, and once in the child. If such 191 * behavior is undesirable, consider 192 * terminating child with _exit(2) or _Exit(3). 193 */ 194 exit(0); 195 default: 196 break; 197 } 198 199 printf("Hello from parent process (child's PID: %d)!\en", pid); 200 201 return (0); 202} 203.Ed 204.Pp 205The output of such a program is along the lines of: 206.Bd -literal -offset indent 207Hello from parent process (child's PID: 27804)! 208Hello from child process! 209.Ed 210.Sh ERRORS 211The 212.Fn fork 213system call will fail and no child process will be created if: 214.Bl -tag -width Er 215.It Bq Er EAGAIN 216The system-imposed limit on the total 217number of processes under execution would be exceeded. 218The limit is given by the 219.Xr sysctl 3 220MIB variable 221.Dv KERN_MAXPROC . 222(The limit is actually ten less than this 223except for the super user). 224.It Bq Er EAGAIN 225The user is not the super user, and 226the system-imposed limit 227on the total number of 228processes under execution by a single user would be exceeded. 229The limit is given by the 230.Xr sysctl 3 231MIB variable 232.Dv KERN_MAXPROCPERUID . 233.It Bq Er EAGAIN 234The user is not the super user, and 235the soft resource limit corresponding to the 236.Fa resource 237argument 238.Dv RLIMIT_NPROC 239would be exceeded (see 240.Xr getrlimit 2 ) . 241.It Bq Er ENOMEM 242There is insufficient swap space for the new process. 243.El 244.Sh SEE ALSO 245.Xr execve 2 , 246.Xr rfork 2 , 247.Xr setitimer 2 , 248.Xr setrlimit 2 , 249.Xr sigaction 2 , 250.Xr vfork 2 , 251.Xr wait 2 , 252.Xr pthread_atfork 3 253.Sh STANDARDS 254The 255.Fn fork 256and 257.Fn _Fork 258functions conform to 259.St -p1003.1-2024 . 260.Sh HISTORY 261The 262.Fn fork 263function appeared in 264.At v1 . 265The 266.Fn _Fork 267function appeared in 268.Fx 13.1 . 269