Why fork and exec

There is a family of exec functions, all of which have slightly different characteristics:. Each system call is the word exec followed by either l or v and then possibly followed by either e or p. The first three have are of the form execl and accept a variable number of arguments. Please see the example stdarg. Check out our Data Structures in C course to start learning today.

Return value of fork On success, the PID of the child process is returned in the parent, and 0 is returned in the child.

On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately. Detailed article on fork system call. It loads the program into the current process space and runs it from the entry point. Both parent and child processes are executed simultaneously in case of fork while Control never returns to the original program unless there is an exec error.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Skip to content. Forking provides a way for an existing process to start a new one, but what about the case where the new process is not part of the same program as parent process?

This is the case in the shell; when a user starts a command it needs to run in a new process, but it is unrelated to the shell.

This is where the exec system call comes into play. Thus the process the shell follows when launching a new program is to firstly fork , creating a new process, and then exec i. In the kernel, fork is actually implemented by a clone system call. This clone interfaces effectively provides a level of abstraction in how the Linux kernel can create processes.

This may seem a bit strange at first, but allows us to easily implement threads with one very simple interface. While fork copies all of the attributes we mentioned above, imagine if everything was copied for the new process except for the memory. This means the parent and child share the same memory, which includes program code and data. This hybrid child is called a thread.

Threads have a number of advantages over where you might use fork. Separate processes can not see each others memory. They can only communicate with each other via other system calls. Threads however, share the same memory. So you have the advantage of multiple processes, with the expense of having to use system calls to communicate between them. The problem that this raises is that threads can very easily step on each others toes. One thread might increment a variable, and another may decrease it without informing the first thread.

These type of problems are called concurrency problems and they are many and varied. To help with this, there are userspace libraries that help programmers work with threads properly.

Switching processes is quite expensive, and one of the major expenses is keeping track of what memory each process is using. By sharing the memory this overhead is avoided and performance can be significantly increased.

There are many different ways to implement threads.