fork(2)

NAME

fork − create a new process

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

pid_t fork(void);

DESCRIPTION

fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process). This means the child process inherits the following attributes from the parent process:

real user ID, real group ID, effective user ID, effective group ID

environment

open file descriptors

close-on-exec flags (see exec(2))

signal handling settings (that is, SIG_DFL, SIG_IGN, SIG_HOLD, function address)

supplementary group IDs

set-user-ID mode bit

set-group-ID mode bit

profiling on/off status

nice value (see nice(2))

scheduler class (see priocntl(2))

all attached shared memory segments (see shmop(2))

process group ID

session ID (see exit(2))

current working directory

root directory

file mode creation mask (see umask(2))

resource limits (see getrlimit(2))

controlling terminal

saved user ID and group ID

Scheduling priority and any per-process scheduling parameters that are specific to a given scheduling class may or may not be inherited according to the policy of that particular class (see priocntl(2)).

The child process differs from the parent process in the following ways:

The child process has a unique process ID which does not match any active process group ID.

The child process has a different parent process ID (that is, the process ID of the parent process).

The child process has its own copy of the parent’s file descriptors and directory streams. Each of the child’s file descriptors shares a common file pointer with the corresponding file descriptor of the parent.

Each attached shared memory segment is detached and the value of shm_nattach in the data structure associated with its shared memory identifier is incremented by 1.

All semadj values are cleared (see semop(2)).

Process locks, text locks, data locks, and other memory locks are not inherited by the child (see plock(2) and memcntl(3)).

The child process’s tms structure is cleared: tms_utime, stime, cutime, and cstime are set to 0 (see times(2)).

The child processes resource utilizations are set to 0; see getrlimit(2). The it_value and it_interval values for the ITIMER_REAL timer are reset to 0; see getitimer(2).

The set of signals pending for the child process is initialized to the empty set.

Record locks set by the parent process are not inherited by the child process (see fcntl(2)).

RETURN VALUES

Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of (pid_t)−1 is returned to the parent process, no child process is created, and errno is set to indicate the error.

ERRORS

fork() will fail and no child process will be created if one or more of the following are true:

EAGAIN The system-imposed limit on the total number of processes under execution by a single user would be exceeded.

EAGAIN Total amount of system memory available when reading via raw I/O is temporarily insufficient.

ENOMEM There is not enough swap space.

NOTES

Be careful to call _exit() rather than exit(3C) if you cannot execve(), since exit(3C) will flush and close standard I/O channels, and thereby corrupt the parent processes standard I/O data structures. Using exit(3C) will flush buffered data twice. See exit(2). ptrace(2), semop(2), shmop(2), signal(2), times(2), umask(2), wait(2), system(3S)

SunOS 5.1/x86 — Last change: 17 Dec 1991

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