Difference between fork() and exec()
Every application(program) comes into execution through means of process, process is a running instance of a program. Processes are created through different system calls, most popular are fork() and exec()
pid_t pid = fork();
fork() creates a new process by duplicating the calling process, The new process, referred to as child, is an exact duplicate of the calling process, referred to as parent, except for the following :
- The child has its own unique process ID, and this PID does not match the ID of any existing process group.
- The child’s parent process ID is the same as the parent’s process ID.
- The child does not inherit its parent’s memory locks and semaphore adjustments.
- The child does not inherit outstanding asynchronous I/O operations from its parent nor does it inherit any asynchronous I/O contexts from its parent.
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.
The exec() family of functions replaces the current process image with a new process image. It loads the program into the current process space and runs it from the entry point. The exec() family consists of following functions, I have implemented execv() in following C program, you can try rest as an exercise
int execl(const char *path, const char *arg, ...); int execlp(const char *file, const char *arg, ...); int execle(const char *path, const char *arg, ..., char * const envp); int execv(const char *path, char *const argv); int execvp(const char *file, char *const argv); int execvpe(const char *file, char *const argv, char *const envp);
fork vs exec
- fork starts a new process which is a copy of the one that calls it, while exec replaces the current process image with another (different) one.
- 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.
parent process, pid = 11523 child process, pid = 14188 Program execution successful
Let us see the differences in a tabular form -:
|1.||It is a system call in the C programming language||It is a system call of operating system|
|2.||It is used to create a new process||exec() runs an executable file|
|3.||Its return value is an integer type||It does not creates new process|
|4.||It does not takes any parameters.||Here the Process identifier does not changes|
|5.||It can return three types of integer values||In exec() the machine code, data, heap, and stack of the process are replaced by the new program.|
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