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The child process can then overlay itself with a different program (using exec) as required. [1] Each process may create many child processes but will have at most one parent process; if a process does not have a parent this usually indicates that it was created directly by the kernel.
When the child process calls exec(), all data in the original program is lost, and it is replaced with a running copy of the new program. This is known as overlaying . Although all data are replaced, the file descriptors that were open in the parent are closed only if the program has explicitly marked them close-on-exec .
For a process to start the execution of a different program, it first forks to create a copy of itself. Then, the copy, called the "child process", calls the exec system call to overlay itself with the other program: it ceases execution of its former program in favor of the other. The fork operation creates a separate address space for the ...
The exec calls named ending with an e alter the environment for the new process image by passing a list of environment settings through the envp argument. This argument is an array of character pointers; each element (except for the final element) points to a null-terminated string defining an environment variable .
Suspends parent process until the child process has finished executing (synchronous spawn). P_NOWAIT, P_NOWAITO: Continues to execute calling process concurrently with new process (asynchronous spawn). P_DETACH: the child is run in background without access to the console or keyboard. Calls to _cwait upon the new process will fail (asynchronous ...
Parent is the process that receives the SIGCHLD signal on child's termination, whereas real parent is the thread that actually created this child process in a multithreaded environment. For a normal process, both these two values are same, but for a POSIX thread which acts as a process, these two values may be different. [2]
In the term's metaphor, the child process has "died" but has not yet been "reaped". Unlike normal processes, the kill command has no effect on a zombie process. Zombie processes should not be confused with orphan processes, a process that is still executing, but whose parent has died. When the parent dies, the orphaned child process is adopted ...
It is sometimes possible to bypass the usual cleanup; C99 offers the _exit() function which terminates the current process without any extra program clean-up. This may be used, for example, in a fork-exec routine when the exec call fails to replace the child process; calling atexit routines would erroneously release resources belonging to the ...