What is the execl() function in C.?
The execl() is a function that belongs to the exec family of functions and is provided by the unistd.h library. This function can only be used in the C programming language as it is widely used in operating system programming, which is the intent of this function.
How does the function work?
The execl() function is used to replace the currently running process with another process. When the running process calls the execl() function, its process image gets overwritten with the process specified within the execl() function.
The diagram below further explains how the function works.
In the diagram above, we see that when we call execl(Run Y), the current process X is replaced by the process Y specified in the execl() function. Also, we see that the process id (PID) will remain the same; hence a new process will not be created.
Syntax
Below, we can see the syntax for the function.
int execl (const char *Path, const char *Argument0, const char *Argument1, ... , NULL);
In the syntax, we observe three things: the Path, the Argument list (Argument0, Argument1), and the ending NULL pointer. Let's discuss what they represent below:
Path: This represents the path to the binary file or program we want to run instead of the current process. The path to the program must be an absolute path instead of a relative one.Argument list: Any arguments we want to pass on to the program should be included in the function as char pointers separated by commas. In the syntax, we only passed two arguments,Argument0andArgument1. However, we can pass as many arguments as we deem necessary.NULL: At the end of the function, we will always useNULLto represent the end of the arguments.
Return value
On a successful function call, it does not return anything as the current process gets entirely replaced with the process specified in the function. So, any lines that are written after the execl() function would not get executed.
If the execl() function fails, it will return an integer value of -1, which can be used to handle any errors in the program.
Code example
Now that we have gone through the syntax for the execl() function, let's look at the C code below to show how to use it in C programs.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main(){
// display current process id
printf("\nCurrently running process has PID: %d\n", getpid());
// exec to run ps instead of execl
int error = execl("/usr/bin/ps", "ps", "-l", NULL);
// for error handling
printf("\nThere was an error during the execl function!\n");
exit(-1);
}When we run the above code, we see that first, we get an output of the program that shows the process ID of the current process that is named execl. We also see a table that contains details about the different running processes. However, in the table, there is no entry for the execl process, only ps and bash.
When we compare the process id of the ps process and the process ID of the execl process, we see that the process ID are the same, which shows that after the execl() function call, the image of the execl process was replaced by the ps process.
Code explanation
Line 7: We print the process ID of the current process by using the function
getpid().Line 10: We then call the
execl()function to run the processps -linstead of the current process. We first pass the path where the file for the programpsis located. In Linux-based systems, it can be found in the location/usr/bin/ps.Then, we pass the additional arguments to the program that consists of the program's nameps, and the-loption to list the processes in a long list format. Finally, we passNULLto the function that shows the end of the arguments that we have provided.Lines 13–14: These lines will be executed if the
execl()function fails for some reason. Here we handle errors by exiting the program.
Practice quiz
Now, let's try to solve the practice quiz below to see how much we have understood about the execl() function.
Which C library to include when using the execl() function?
stdio.h
stdlib.h
unistd.h
fcntl.h
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