NAME

fork − create a new process

SYNOPSIS

#include <sys/types.h>
 
pid_t fork ()

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:

environment

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

signal handling settings (i.e., SIG_DFL, SIG_IGN, function address)

set-user-ID mode bit

set-group-ID mode bit

profiling on/off status

nice value (see nice(2))

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

process group ID

tty group ID (see exit(2) and signal(2))

current universe (see setuniverse(2))

current working directory

root directory

file mode creation mask (see umask(2))

file size limit (see ulimit(2))

time quantum (see getquantum(2))

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

The child process has a unique process 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.  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)). 

Memory locks established by the parent process via calls to mlockall() or mlock() are not established for the child process. 

The child process’s utime, stime, cutime, and cstime are set to 0.  The time left until an alarm clock signal is reset to 0.  The child’s resource utilizations are set to 0 (see setrlimit(2)). 

If the parent is currently being traced by a debugging process that issued a ptrace(2) request PT_TRACE_CHILD:

the child process is also attached to the debugging process;

the child process is marked as currently being traced;

a SIGTRAP signal is sent to the child process. 

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 would be exceeded. 

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

RETURN VALUE

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 −1 is returned to the parent process, no child process is created, and errno is set to indicate the error. 

SEE ALSO

exec(2), getquantum(2), nice(2), plock(2), ptrace(2), semop(2), shmop(2), signal(2), times(2), ulimit(2), umask(2), wait(2), setuniverse(2). 

CX/UX Programmer’s Reference Manual