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

close-on-exec flag [see exec(2)]

signal handling settings (i.e., 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

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 (i.e., 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. 

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

Process locks, text locks and data locks are not inherited by the child [see plock(2)]. 

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

The time left until an alarm clock signal is reset to 0. 

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)]. 

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 system wide {PROC_MAX} or by a single user ID {CHILD_MAX} would be exceeded, or the system lacked the necessary resources to create another process."

SEE ALSO

alarm(2), exec(2), fcntl(2), getrlimit(2), nice(2), plock(2), priocntl(2), ptrace(2), semop(2), shmop(2), signal(2), times(2), umask(2), wait(2), system(3S)

DIAGNOSTICS

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.