Active Oldest Votes 24 Man pages are usually terse reference documents. Wikipedia is a better place to turn to for conceptual explanations. Fork duplicates a process: it creates a child process which is almost identical to the parent process the most obvious difference is that the new process has a different process ID. As this is rather costly, vfork was invented to handle a common special case where the copy is not necessary.
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Most of these concepts are explained using programming examples. In this article, I will be covering what are fork, vfork, exec and wait system calls, their distinguishing characters and how they can be better used. Resource utilization is set to zero.
File locks and pending signals are not inherited. Following is the c-programming example which explains how fork system call works. First of all context switch happens from user mode to kernel system mode. The following table explains context switching very clearly. Note:- In vfork, signal handlers are inherited but not shared. Note: vfork is sometimes referred to as special case of clone. Following is the C programming example for vfork how it works. Aborted Note:— As explained earlier, many a times the behaviour of the vfork system call is not predictable.
It is better to use fork system call unless otherwise and avoid using vfork as much as possible. Differences between fork and vfork Vfork behaviour explained in more details in the below program.
Aborted Note: Again if you observe the outcome of vfork is not defined. But the next time in the parent process it has printed some garbage value. There are functions like execl, execlp,execle,execv, execvp and execvpe are used to execute a file. These functions are combinations of array of pointers to null terminated strings that represent the argument list , this will have path variable with some environment variable combinations.
The status of the process is captured for future reference. One has to use these functions cautiously keeping in mind the outcome and behaviour.
Learn and use fork(), vfork(), wait() and exec() system calls across Linux Systems
Fork : The fork call basically makes a duplicate of the current process, identical in almost every way not everything is copied over, for example, resource limits in some implementations but the idea is to create as close a copy as possible. Because the two processes are now running exactly the same code, they can tell which is which by the return code of fork - the child gets 0, the parent gets the PID of the child. This is all, of course, assuming the fork call works - if not, no child is created and the parent gets an error code. This strange state of affairs continues until the child process either exits, or calls execve , at which point the parent process continues. This means that the child process of a vfork must be careful to avoid unexpectedly modifying variables of the parent process. Exec : The exec call is a way to basically replace the entire current process with a new program.
vfork() - Unix, Linux System Call
For details and return value and errors, see fork 2. It is used to create new processes without copying the page tables of the parent process. It may be useful in performance sensitive applications where a child will be created which then immediately issues an execve. The child shares all memory with its parent, including the stack, until execve is issued by the child.
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