fork, vfork - Creates a new process
pid_t fork (void);
pid_t vfork (void);
Application developers may want to specify an #include statement for
<<sys/types.h>> before the one for <<unistd.h>> if programs are being developed
for multiple platforms. The additional #include statement is not required
on Tru64 UNIX systems or by ISO or X/Open standards, but may be required on
other vendors' systems that conform to these standards.
Interfaces documented on this reference page conform to industry standards
Refer to the standards(5) reference page for more information about indus-
try standards and associated tags.
The fork() and vfork() functions create a new process (child process) that
is identical to the calling process (parent process).
The child process inherits the following attributes from the parent pro-
+ Close-on-exec flags
+ Signal handling settings
+ Set user ID mode bit
+ Set group ID mode bit
+ Trusted state
+ Profiling on/off status
+ Nice value
+ All attached shared libraries
+ Process group ID
+ tty group ID
+ Current directory
+ Root directory
+ File mode creation mask
+ File size limit
+ Attached shared memory segments
+ Attached mapped file segments
+ All mapped regions with the same protection and sharing mode as in the
+ Its own copy of the parent's open directory streams
The child process differs from the parent process in the following ways:
+ The child process has a unique process ID that does not match any
active process group ID.
+ The parent process ID of the child process matches the process ID of
+ The child process has its own copy of the parent process's file
descriptors. Each of the child's file descriptors refers to the same
open file description with the corresponding file descriptor of the
+ The child process has its own copy of the parent's open directory
streams. Each open directory stream in the child process may share
directory stream positioning with the corresponding directory stream
of the parent.
+ All semadj values are cleared.
+ Process locks, text locks, and data locks are not inherited by the
+ The child process' values of tms_utime, tms_stime, tms_cutime, and
tms_cstime are set to 0 (zero).
+ Any pending alarms are cleared in the child process.
+ [Tru64 UNIX] Any interval timers enabled by the parent process are
reset in the child process.
+ Any signals pending for the parent process are cleared for the child
If a multithreaded process forks a child process, the new process contains
a replica of the calling thread and its entire address space, possibly
including the states of mutexes and other resources. Consequently, to
avoid errors, the child process should only execute operations it knows
will not cause deadlock.
The fork() and vfork() functions have essentially the same implementation
at the level of the operating system kernel but may differ in how they are
supported through different libraries. Some libraries, such as libpthread
and libc, support fork handler routines that can acquire and release
resources that are critical to the child process. Fork handlers therefore
allow an application to manage potential deadlock situations that might
occur between the parent and child processes. Fork handlers do not work
correctly if the application calls vfork() to create the child process.
Therefore, applications using libpthread and libc should call fork() to
create a child process.
For more information about fork handler routines, see pthread_atfork(3).
For a complete list of system calls that are reentrant with respect to sig-
nals, see signal(4).
Upon successful completion, the fork() and vfork() functions return a value
of 0 (zero) to the child process and return the process ID of the child
process to the parent process. Otherwise, a value of -1 is returned to the
parent, no child process is created, and errno is set to indicate the
The fork() and vfork() functions set errno to the specified values for the
[EAGAIN] The limit on the total number of processes executing for a single
user would be exceeded. This limit can be exceeded by a process
with superuser privilege.
[ENOMEM] There is not enough space left for this process.
Functions: exec(2), exit(2), getpriority(2), getrusage(2), plock(2),
ptrace(2), semop(2), shmat(2), sigaction(2), sigvec(2), umask(2), wait(2)
Routines: nice(3), pthread_atfork(3), raise(3), times(3), ulimit(3)