man pages section 2: System Calls

fork1(2)

Name

fork, fork1, forkall– create a new process

Synopsis

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

pid_t fork(void);

pid_t fork1(void);

pid_t forkall(void);

Description

The fork(), fork1(), and forkall() 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:

real user ID, real group ID, effective user ID, effective group ID
environment
open file descriptors
close-on-exec flags (see exec(2))
signal handling settings (that is, 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 -- memory mappings (see mmap(2))
session ID (see exit(2))
current working directory
root directory
file mode creation mask (see umask(2))
resource limits (see getrlimit(2))
controlling terminal
saved user ID and group ID
task ID and project ID
processor bindings (see processor_bind(2))
processor set bindings (see pset_bind(2))
process privilege sets (see getppriv(2))
process flags (see getpflags(2))
active contract templates (see contract(4))

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(4)). 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 (that is, 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.
Each shared memory segment remains attached and the value of shm_nattach is incremented by 1.
All semadj values are cleared (see semop(2)).
Process locks, text locks, data locks, and other memory locks are not inherited by the child (see plock(3C) and memcntl(2)).
The child process's tms structure is cleared: tms_utime, stime, cutime, and cstime are set to 0 (see times(2)).
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).
The set of signals pending for the child process is initialized to the empty set.
Timers created by timer_create(3RT) are not inherited by the child process.
No asynchronous input or asynchronous output operations are inherited by the child.
Any preferred hardware address tranlsation sizes (see memcntl(2)) are inherited by the child.
The child process holds no contracts (see contract(4)).

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() 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() replicates only the calling thread in the child process.

In Solaris 10, 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 previous releases of Solaris, 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().

In Solaris 10, neither -lthread nor -lpthread is required for multithreaded applications. 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().

`fork()` Safety

If a multithreaded application calls fork() or fork1(), 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() or fork1(). See "MT-Level of Libraries" on the attributes(5) manual page.

Return Values

Upon successful completion, fork(), fork1(), and forkall() 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(), and forkall()function 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.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	`fork()` is Standard. `fork1()` and `forkall()` are Stable.
MT-Level	Async-Signal-Safe.

Notes

An applications 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() or fork1() 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.

SunOS 5.10 Last Revised 19 Jul 2004