illumos: manual page: fork.2

FORK(2) System Calls FORK(2)

NAME

fork, fork1, forkall, forkx, forkallx - create a new process

SYNOPSIS

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

pid_t fork(void);

pid_t fork1(void);

pid_t forkall(void);

#include <sys/fork.h>

pid_t forkx(int flags);

pid_t forkallx(int flags);

DESCRIPTION

The fork(), fork1(), forkall(), forkx(), and forkallx() functions create
a new process. The address space of the new process (child process) is an
exact copy of the address space of the calling process (parent process).
The child process inherits the following attributes from the parent
process:

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

o environment

o open file descriptors

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

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

o supplementary group IDs

o set-user-ID mode bit

o set-group-ID mode bit

o profiling on/off status

o nice value (see nice(2))

o scheduler class (see priocntl(2))

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

o process group ID -- memory mappings (see mmap(2))

o session ID (see exit(2))

o current working directory

o root directory

o file mode creation mask (see umask(2))

o resource limits (see getrlimit(2))

o controlling terminal

o saved user ID and group ID

o task ID and project ID

o processor bindings (see processor_bind(2))

o processor set bindings (see pset_bind(2))

o process privilege sets (see getppriv(2))

o process flags (see getpflags(2))

o active contract templates (see contract(5))

Scheduling priority and any per-process scheduling parameters that are
specific to a given scheduling class might or might not be inherited
according to the policy of that particular class (see priocntl(2)). The
child process might or might not be in the same process contract as the
parent (see process(5)). The child process differs from the parent
process in the following ways:

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

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

o 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.

o Each shared memory segment remains attached and the value of
shm_nattach is incremented by 1.

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

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

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

o 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).

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

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

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

o Any preferred hardware address translation sizes (see
memcntl(2)) are inherited by the child.

o The child process holds no contracts (see contract(5)).

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

Although any open door descriptors in the parent are shared by the child,
only the parent will receive a door invocation from clients even if the
door descriptor is open in the child. If a descriptor is closed in the
parent, attempts to operate on the door descriptor will fail even if it
is still open in the child.

Threads

A call to forkall() or forkallx() replicates in the child process all of
the threads (see thr_create(3C) and pthread_create(3C)) in the parent
process. A call to fork1() or forkx() replicates only the calling thread
in the child process.

A call to fork() is identical to a call to fork1(); only the calling
thread is replicated in the child process. This is the POSIX-specified
behavior for fork().

In releases of Solaris prior to Solaris 10, the behavior of fork()
depended on whether or not the application was linked with the POSIX
threads library. When linked with -lthread (Solaris Threads) but not
linked with -lpthread (POSIX Threads), fork() was the same as forkall().
When linked with -lpthread, whether or not also linked with -lthread,
fork() was the same as fork1().

Prior to Solaris 10, either -lthread or -lpthread was required for
multithreaded applications. This is no longer the case. The standard C
library provides all threading support for both sets of application
programming interfaces. Applications that require replicate-all fork
semantics must call forkall() or forkallx().

Fork Extensions

The forkx() and forkallx() functions accept a flags argument consisting
of a bitwise inclusive-OR of zero or more of the following flags, which
are defined in the header <sys/fork.h>:

FORK_NOSIGCHLD

Do not post a SIGCHLD signal to the parent process when the child
process terminates, regardless of the disposition of the SIGCHLD
signal in the parent. SIGCHLD signals are still possible for job
control stop and continue actions if the parent has requested them.

FORK_WAITPID

Do not allow wait-for-multiple-pids by the parent, as in wait(),
waitid(P_ALL), or waitid(P_PGID), to reap the child and do not allow
the child to be reaped automatically due the disposition of the
SIGCHLD signal being set to be ignored in the parent. Only a
specific wait for the child, as in waitid(P_PID, pid), is allowed and
it is required, else when the child exits it will remain a zombie
until the parent exits.

If the flags argument is 0 forkx() is identical to fork() and forkallx()
is identical to forkall().

fork() Safety
If a multithreaded application calls fork(), fork1(), or forkx(), and the
child does more than simply call one of the exec(2) functions, there is a
possibility of deadlock occurring in the child. The application should
use pthread_atfork(3C) to ensure safety with respect to this deadlock.
Should there be any outstanding mutexes throughout the process, the
application should call pthread_atfork() to wait for and acquire those
mutexes prior to calling fork(), fork1(), or forkx(). See "MT-Level" on
the attributes(7) manual page.

The pthread_atfork() mechanism is used to protect the locks that
libc(3LIB) uses to implement interfaces such as malloc(3C). All
interfaces provided by libc are safe to use in a child process following
a fork(), except when fork() is executed within a signal handler.

The POSIX standard (see standards(7)) requires fork to be Async-Signal-
Safe (see attributes(7)). This cannot be made to happen with fork
handlers in place, because they acquire locks. To be in nominal
compliance, no fork handlers are called when fork() is executed within a
signal context. This leaves the child process in a questionable state
with respect to its locks, but at least the calling thread will not
deadlock itself attempting to acquire a lock that it already owns. In
this situation, the application should strictly adhere to the advice
given in the POSIX specification: "To avoid errors, the child process may
only execute Async-Signal-Safe operations until such time as one of the
exec(2) functions is called."

RETURN VALUES

Upon successful completion, fork(), fork1(), forkall(), forkx(), and
forkallx() return 0 to the child process and return the process ID of the
child process to the parent process. Otherwise, (pid_t)-1 is returned to
the parent process, no child process is created, and errno is set to
indicate the error.

ERRORS

The fork(), fork1(), forkall(), forkx(), and forkallx() functions will
fail if:

EAGAIN
A resource control or limit on the total number of processes,
tasks or LWPs under execution by a single user, task, project,
or zone has been exceeded, or the total amount of system memory
available is temporarily insufficient to duplicate this
process.

ENOMEM
There is not enough swap space.

EPERM
The {PRIV_PROC_FORK} privilege is not asserted in the effective
set of the calling process.

The forkx() and forkallx() functions will fail if:

EINVAL
The flags argument is invalid.

ATTRIBUTES

NOTES

An application should call _exit() rather than exit(3C) if it cannot
execve(), since exit() will flush and close standard I/O channels and
thereby corrupt the parent process's standard I/O data structures. Using
exit(3C) will flush buffered data twice. See exit(2).

The thread in the child that calls fork(), fork1(), or fork1x() must not
depend on any resources held by threads that no longer exist in the
child. In particular, locks held by these threads will not be released.

In a multithreaded process, forkall() in one thread can cause blocking
system calls to be interrupted and return with an EINTR error.

August 18, 2019 FORK(2)