Fixing “warning: missing sentinel in function call”

I got it for the folowing line:

execl("/bin/sh", "sh", "-c", sessioncommand.c_str(), 0);

Quite easy to fix:

execl("/bin/sh", "sh", "-c", sessioncommand.c_str(), (char*)0);

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Alright. Since I’m getting “lots” of hits on this, let’s see if we can improve this post. (June 28th 2010, more examples added July 22nd 2010, style but not content updated April 19 2014)

Generally, you’re missing a sentinel if you don’t add a “NULL” in C/C++, or a “nil” in Objective-C. In above example in C, I presume the compiler did not notice the zero (a valid sentinel) and threw a warning. That’s what this post was mostly about.

However, it could also happen if you really did forget to add the sentinel. What is the purpose of this “sentinel”? When iterating through a varlist**, the function needs to know where to stop. In printf() and scanf(), the format string specifies this number of arguments. Functions could also accept the number of arguments as one of the arguments. Third option is this — specifying a sentinel, such as NULL or nil, as something that will stop further iteration.

Examples of sentinels in C:

execl("/bin/sh", "sh", "-c", sessioncommand.c_str(), NULL);

and in Objective-C:

NSArray *names = [NSArray arrayWithObjects: @"Ivan", @"Ana", @"Marko", @"Petar", nil];

** Iterating through a varlist – which is how you iterate through arguments in variadic functions, those with variable number of arguments

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