fork(2)fork(2)NAME
fork, vfork - create a new process
C SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t fork (void);
#if _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED==1
pid_t vfork (void);
#endif
Only when in the X/Open XPG4 Extended Unix name space are vfork and fork
exactly the same in this system and any reference to fork in this
document also applies to vfork. It should also be noted that this vfork
is NOT and DOES NOT have the same semantics as the original BSD vfork.
DESCRIPTIONfork 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/mode status [see sprofil(2)]
debugger tracing status [see proc(4)]
nice value [see nice(2)]
scheduler class [see proc(4) and schedctl(2)]
all attached shared memory segments [see shmop(2)]
all mapped files [see mmap(2)]
non-degrading priority [see schedctl(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
all machine register state including floating point
control register(s)
The child process differs from the parent process in the following ways:
Page 1
fork(2)fork(2)
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.
File locks previously set by the parent are not inherited by the
child [see fcntl(2)].
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 set of signals pending to the parent is not inherited by the
child.
Page locks are not inherited [see mpin(2)].
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 time left until an itimer signal is reset to 0.
The child will not inherit the ability to make graphics calls. The
child process may receive a segmentation fault upon attempting to
make a graphics call, unless it initializes itself as a graphics
process via winopen() or ginit(). NOTE: Currently, if the parent is
a graphics process, the child's attempt to become a graphics process
will fail.
The share mask is set to 0 [see sproc(2)].
If a fork is performed by a process that is part of a share group [see
sproc(2)]) then the new child process will automatically be added to the
parallel C library I/O arena. This can result in additional ways that
fork can fail.
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, {NPROC} [see intro(2)], would be exceeded.
Page 2
fork(2)fork(2)
[EAGAIN] The system-imposed limit on the total number of processes
under execution by a single user {CHILD_MAX} [see intro(2)],
would be exceeded.
[EAGAIN] Amount of system memory required is temporarily unavailable.
When the caller is a member of a share group in addition to the above
errors fork will fail and no new process will be created if one or more
of the following are true:
[ENOSPC] If the total number of share group members plus children
exceeds the maximum number of users specified via
usconfig(3P) (8 by default). Any changes via usconfig(3P)
must be done BEFORE the first sproc is performed.
[ENOLCK] There are not enough file locks in the system.
New process pid # could not join I/O arena:<..>
if the new process could not properly join the parallel C
library I/O arena. The new process in this case exits with a
-1.
SEE ALSOexec(2), intro(2), mmap(2), nice(2), plock(2), ptrace(2), schedctl(2),
semop(2), shmop(2), signal(2), sigset(2), sigaction(2), schedctl(2),
sproc(2), sprofil(2), times(2), ulimit(2), umask(2), wait(2),
pcreate(3C), signal(3B), sigvec(3B), usinit(3P), usconfig(3P), proc(4).
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.
Page 3
