Fork–exec
Fork–exec is a commonly used technique in Unix whereby an executing process spawns a new program. Description. codice_1 is the name of the system call that the parent process uses to "divide" itself ("fork") into two identical processes. After calling codice_1, 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 program's executable code. In some cases the two continue to run the same binary, but often one (usually the child) switches to  running another binary executable using the codice_3 system call. When a process forks, a complete copy of the executing program is made into the new process. This new process is a child of the parent process, and has a new process identifier (PID). The codice_1 function returns the child's PID to the parent process. The codice_1 function returns 0 to the child process. This enables the two identical processes to distinguish one another. The parent process can either continue execution or wait for the child process to complete. The child, after discovering that it is the child, replaces itself  completely with another program, so that the code and address space of the original program are lost. If the parent chooses to wait for the child to die, then the parent will receive the exit code of the program that the child executed. To prevent the child becoming a zombie the parent should call wait on its children, either periodically or upon receiving the SIGCHLD signal, which indicates a child process has terminated. One can also asynchronously wait on their children to finish, by using a signal handler for SIGCHLD, if they need to ensure everything is cleaned up. Here 's an example of a signal handler that catches any incoming SIGCHLD signals and handles multiple concurrent signals received. When the child process calls codice_3, 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". This allows for the common practice of the parent creating a pipe prior to calling codice_1 and using it to communicate with the  executed program. Microsoft Windows does not support the fork-exec model, as it does not have a system call analogous to codice_1. The codice_9 family of functions declared in process.h can replace it in cases where the call to codice_1 is followed directly by codice_3.