Tag Archives: GCC

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);


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

Great new GCC option – Effective-C++ Warnings

Did you know about -Weffc++ in GCC? Neither did I, until I upgraded to new Code::Blocks from Jens’ unofficial Debian repository for Code::Blocks.

Here’s a few sample warnings, with sample lines and commentary:

  • /home/ivucica/Development/project/src/types.h|12|warning: ‘operator=’ should return a reference to ‘*this’|

Position &operator=(const Position& other) { x=other.x; y=other.y; z=other.z; return *this;}

What’s the big deal here? By leaving out the ampersand in this operator overloading, I accidentally returned a copy of entire Position class. (On the other hand, I had no reason to actually overload this operator so perhaps real fix is to erase entire line. Still…)

  • /home/ivucica/Development/project/src/mat.h|10|warning: ‘class Mat’ has pointer data members|
    /home/ivucica/Development/project/src/mat.h|10|warning: but does not override ‘Mat(const Mat&)’|
    /home/ivucica/Development/project/src/mat.h|10|warning: or ‘operator=(const Mat&)’|

No sample needed.

What’s the problem here? Well, we have a Mat class containing pointer. So what happens when someone does this: Mat x = y; or this: Mat x(y);? We have another instance of a class with copies of all the data, except the data pointed to by the pointer. Instead the pointer itself gets copied.

So if we keep a C-style string in there, we’re not really copying the string … we’re copying the pointer to it, and modifying the string in the new instance still modifies the string in the original ocpy.

While not applicable everywhere, still a good and useful warning.

  • /home/ivucica/Development/project/src/obj.h||In constructor ‘Obj::Obj()’:|
    /home/ivucica/Development/project/src/obj.h|15|warning: ‘Obj::m_radius’ should be initialized in the member initialization list|

Simple. Instead of initializing (or forgetting to initialize, or intentionally doing so) a variable in the constructor:
Obj() {m_classname=”Obj”; m_usecount = 0; }
why not do it in a “safe” and readable way (and perhaps more optimal?)
Obj() : m_classname(“Obj”), m_usecount(0) { }

While these warnings are certainly not the Universal Elixir to cure all your troubles, they will certainly prove useful in furthering your 1337 skills.