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The first statement in main calls the fork system call to split execution into two processes. The return value of fork is recorded in a variable of type pid_t , which is the POSIX type for process identifiers (PIDs).
The wait system call is commonly invoked in the SIGCHLD handler. POSIX.1-2001 allows a parent process to elect for the kernel to automatically reap child processes that terminate by explicitly setting the disposition of SIGCHLD to SIG_IGN (although ignore is the default, automatic reaping only occurs if the disposition is set to ignore ...
fork() is the name of the system call that the parent process uses to "divide" itself ("fork") into two identical processes. After calling fork(), the created child process is an exact copy of the parent except for the return value of the fork() call. This includes open files, register state, and all memory allocations, which includes the ...
The word "fork" has been used to mean "to divide in branches, go separate ways" as early as the 14th century. [2] In the software environment, the word evokes the fork system call, which causes a running process to split itself into two (almost) identical copies that (typically) diverge to perform different tasks.
In Unix-like operating systems, every process except process 0 (the swapper) is created when another process executes the fork() system call. The process that invoked fork is the parent process and the newly created process is the child process. Every process (except process 0) has one parent process, but can have many child processes.
Copy-on-write finds its main use in operating systems, sharing the physical memory of computers running multiple processes, in the implementation of the fork() system call. Typically, the new process does not modify any memory and immediately executes a new process, replacing the address space entirely.
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The actual system call does transfer control to the kernel (and is more implementation-dependent and platform-dependent than the library call abstracting it). For example, in Unix-like systems, fork and execve are C library functions that in turn execute instructions that invoke the fork and exec system calls.