System Calls fork(2)
fork, fork1 - create a new process
The fork() and fork1() functions create a new process. The
new process (child process) is an exact copy 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, effec-
tive group ID
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))
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System Calls fork(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))
Scheduling priority and any per-process scheduling parame-
ters that are specific to a given scheduling class may or
may not be inherited according to the policy of that partic-
ular class (see priocntl(2)). 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
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
o The set of signals pending for the child process is
initialized to the empty set.
o Timers created by timer_create(3RT) are not inherited
by the child process.
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System Calls fork(2)
o No asynchronous input or asynchronous output opera-
tions are inherited by the child.
o Any preferred hardware address tranlsation sizes (see
memcntl(2)) are inherited by the child.
Record locks set by the parent process are not inherited by
the child process (see fcntl(2)).
In applications that use the Solaris threads API rather than
the POSIX threads API (applications linked with -lthread but
not -lpthread),fork() duplicates in the child process all
threads (see thr_create(3THR)) and LWPs in the parent pro-
cess. The fork1() function duplicates only the calling
thread (LWP) in the child process.
In applications that use the POSIX threads API rather than
the Solaris threads API ( applications linked with
-lpthread, whether or not linked with -lthread), a call to
fork() is like a call to fork1(), which replicates only the
calling thread. There is no call that forks a child with all
threads and LWPs duplicated in the child.
Note that if a program is linked with both libraries (-
lthread and -lpthread), the POSIX semantic of fork() pre-
If a Solaris threads application calls fork1() or a POSIX
threads application calls fork(), and the child does more
than simply call exec(), 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()
or fork1(). See "MT-Level of Libraries" on the attri-
butes(5) manual page.
Upon successful completion, fork() and fork1() 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.
The fork() function will fail if:
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System Calls fork(2)
The system-imposed limit on the total number of
processes under execution by a single user has been
exceeded; or the total amount of system memory avail-
able is temporarily insufficient to duplicate this
There is not enough swap space.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | fork() is Async-Signal-Safe |
alarm(2), exec(2), exit(2), fcntl(2), getitimer(2),
getrlimit(2), memcntl(2), mmap(2), nice(2), priocntl(2),
ptrace(2), semop(2), shmop(2), times(2), umask(2), wait(2),
exit(3C), plock(3C), pthread_atfork(3C), signal(3C),
system(3C), thr_create(3THR) timer_create(3RT), attri-
An applications should call _exit() rather than exit(3C) if
it cannot execve(), since exit() will flush and close stan-
dard I/O channels and thereby corrupt the parent process's
standard I/O data structures. Using exit(3C) will flush buf-
fered data twice. See exit(2).
The thread (or LWP) in the child that calls fork1() must not
depend on any resources 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 multithreaded process, fork() or fork1() can cause
blocking system calls to be interrupted and return with an
The fork() and fork1() functions suspend all threads in the
process before proceeding. Threads that are executing in
the kernel and are in an uninterruptible wait cannot be
suspended immediately and therefore cause a delay before
fork() and fork1() can complete. During this delay, since
all other threads will have already been suspended, the pro-
cess will appear "hung."
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