NAME

fork, fork1 - create a new process

SYNOPSIS

#include <sys/types.h>

#include <unistd.h>
pid_t fork(void);
pid_t fork1(void);

DESCRIPTION

fork( ) and fork1( ) cause 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 -- memory mappings (see mmap(2))

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 shared memory segment remains attached and shm_nattach 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(3C) and memcntl(2)).

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.

Timers created by timer_create(3R) are not inherited by the child process.

No asynchronous input or asynchronous output operations are inherited by the child.

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

fork( ) duplicates all the threads (see thr_create(3T)) and LWPs in the parent process in the child process. fork1( ) duplicates only the calling thread (LWP) in the child process.

RETURN VALUES

Upon successful completion, fork( ) and fork1( ) 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

There are two conditions that will cause an EAGAIN error.

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

The total amount of system memory available is temporarily insufficient to duplicate this process.

ENOMEM

There is not enough swap space.

SEE ALSO

alarm(2), exec(2), exit(2), fcntl(2), getitimer(2), getrlimit(2), mmap(2), nice(2), plock(3C), priocntl(2), ptrace(2), semop (2),shmop(2), times(2), umask(2), wait(2), memcntl(2), exit(3C), signal(3C), timer_create(3R), system(3S), thr_create(3T)

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).

When calling fork1( ) the thread (or LWP) in the child must not depend on any resources that are held by threads (or LWPs) that no longer exist in the child. In particular, locks held by these threads (or LWPs) will not be released.

In a multi-threaded process, fork( ) can cause blocking system calls to be interrupted and return with an error of EINTR.