### 27.4 Creating a Process ¶

The `fork` function is the primitive for creating a process. It is declared in the header file `unistd.h`.

Function: `pid_t` fork `(void)` ¶

  Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | See POSIX Safety Concepts.

  The `fork` function creates a new process.

  If the operation is successful, there are then both parent and child processes and both see `fork` return, but with different values: it returns a value of `0` in the child process and returns the child’s process ID in the parent process.

  If process creation failed, `fork` returns a value of `-1` in the parent process. The following `errno` error conditions are defined for `fork`:

  `EAGAIN`

    There aren’t enough system resources to create another process, or the user already has too many processes running. This means exceeding the `RLIMIT_NPROC` resource limit, which can usually be increased; see Limiting Resource Usage.

  `ENOMEM`

    The process requires more space than the system can supply.

The specific attributes of the child process that differ from the parent process are:

- The child process has its own unique process ID.
- The parent process ID of the child process is the process ID of its parent process.
- The child process gets its own copies of the parent process’s open file descriptors. Subsequently changing attributes of the file descriptors in the parent process won’t affect the file descriptors in the child, and vice versa. See Control Operations on Files. However, the file position associated with each descriptor is shared by both processes; see File Position.
- The elapsed processor times for the child process are set to zero; see Processor Time Inquiry.
- The child doesn’t inherit file locks set by the parent process. See Control Operations on Files.
- The child doesn’t inherit alarms set by the parent process. See Setting an Alarm.
- The set of pending signals (see How Signals Are Delivered) for the child process is cleared. (The child process inherits its mask of blocked signals and signal actions from the parent process.)

Function: `pid_t` _Fork `(void)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `_Fork` function is similar to `fork`, but it does not invoke any callbacks registered with `pthread_atfork`, nor does it reset any internal state or locks (such as the `malloc` locks). In the new subprocess, only async-signal-safe functions may be called, such as `dup2` or `execve`.

  The `_Fork` function is an async-signal-safe replacement of `fork`.

  This function was originally a GNU extension, but was added in POSIX.1-2024.

Function: `pid_t` vfork `(void)` ¶

  Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | See POSIX Safety Concepts.

  The `vfork` function is similar to `fork` but on some systems it is more efficient; however, there are restrictions you must follow to use it safely.

  While `fork` makes a complete copy of the calling process’s address space and allows both the parent and child to execute independently, `vfork` does not make this copy. Instead, the child process created with `vfork` shares its parent’s address space until it calls `_exit` or one of the `exec` functions. In the meantime, the parent process suspends execution.

  You must be very careful not to allow the child process created with `vfork` to modify any global data or even local variables shared with the parent. Furthermore, the child process cannot return from (or do a long jump out of) the function that called `vfork`! This would leave the parent process’s control information very confused. If in doubt, use `fork` instead.

  Some operating systems don’t really implement `vfork`. The GNU C Library permits you to use `vfork` on all systems, but actually executes `fork` if `vfork` isn’t available. If you follow the proper precautions for using `vfork`, your program will still work even if the system uses `fork` instead.