Question

I am learning linux programming and came across exec function which is kind of very useful. But the problem is exec function arguments are very confusing and I am unable to grasp which argument is for what purpose.. In the following code execl() function is called from a child created through fork(), What is the purpose of the last argument (NULL) in execl()?

execl("/bin/ls","ls","-l",NULL);

If any one can explain what is the purpose of NULL argument and other arguments and the purpose of arguments of exec() family function, It would be a great help to me!

Answer 1

To create undefined behavior. That is not a legal call to execl. A
correct call might be:

execl( "/bin/ls", "ls", "-l", (char*)0 );

The last argument must be (char*)0, or you have undefined behavior.
The first argument is the path of the executable. The following
arguments appear in argv of the executed program. The list of these
arguments is terminated by a (char*)0; that’s how the called function
knows that the last argument has been reached. In the above example,
for example, the executable at "/bin/ls" will replace your code; in
its main, it will have argc equal 2, with argv[0] equal "ls",
and argv[1] equal "-l".

Immediately after this function, you should have the error handling
code. (execl always returns -1, when it returns, so you don’t need to
test it. And it only returns if there was some sort of error.)