Missing Include

This is a deliberate experiment, intentionally removing the #include headers and trying to call the printf to see what happens.

#implicit-function-declaration

Here is the exact code what I tried :

Before getting inside, we need to know answer for one question : Why actually the include exists? What it contains? What is it’s significance? (This is one question composed of many question.)

When you ask why?, it will lead you to a wonderfull location - the location is the glibc.so that is a C shared library.

What is #include?

It is nothing but the C Pre Processor (CPP) command or function or whatever I would like to call, which just as the name indicates, does the copy paste mechanism: find the include :

1) <file.h> : First find in the /usr/include directory in the linux file system.

2) “file.h” : First search the current directory ./ for the header file, if not present, then fallback to the /usr/include directory for C header file.

What actually does something.h contains?

Nothing but the function signatures. The shape of the function. What is a function signature? Each function returns a particular type of value and takes a specific type of arguments and some x number of arguments and have a specific name. So : each function can be described with 4 quantites :

  • return type
  • types of arguments
  • number of arguments - directly variadic or non variadic (ie, infinite or non infinte arguments)
  • the name of the function

All these stuffs are designated in the something.h header file.

So? What does the <stdio.h> have to do with printf?

As it’s name suggests, the <stdio.h> (note the < and > opening and closings), it is present in the /usr/include directory, and has the exact function signature (AKA : function declaration) for the printf function.

It is good to note that : cat /usr/include/stdio.h | grep "int printf" will return you the function declaration and not the definition of the printf function. So, more accurately, the stdio.h only tells the compiler that - hey : this is how the printf’s signature looks like. and the linux dynamic linker (ld-linux.so) will search for the printf in the glibc.so (GNU C standard library) and fill the holes.

But which holes?

Not definitely an $$shole, but the functions : as I said earlier, the assembler will produce an object file : that object file will have an unresolved symbol - printf (ie, a function with unknown definition, but has a signature) - this is the hole in object file produced by assembler. Now, the linux dynamic linker (ld-linux.so) will fetch these holes : link the functions to their definitons from the linux standard library - which is a precompiled shared library glibc.so.6 (or whatever depending on your OS).

Without the include?

There will be no copy pasta mechanism and hence no signature for the printf(in our case). This enables the compiler to assume the signature of the printf function - which is known as implicit-function-declaration. (not in C-23, but in old C). The compiler will guess a function which returns int and accept any arguments. This is known as implicit-function-declaration - the compiler invents a fake sign.

So what? Let the compiler guess.

Nope. If that is the case, what would you tell if the compiler guessed a wrong function? What if the signature of the printf function in the glibc.so does not match with whatever the compiler guessed on it’s own?

Boom! You get an UNDEFINED BEHAVIOR - wrong stack layout, wrong functions, crashes.

Is there a way I can use printf without #include <stdio.h>?

Absolutely Possible. Not a surprise because if I am gonna keenly look at the compiler error / warning message (whatever you like to call it as) - say, the compiler’s help message -

note: include '<stdio.h>' or provide a declaration of 'printf'

or provide a declaration of printf. Yes, you should provide a declaration of printf, either : copy pasta from the include headers.

cat /usr/include/stdio.h | grep "int printf"

You get an output like this : (exactly this)

extern int printf (const char *__restrict __format, ...);

And this is what you need to add instead of include .... The below given code resolves the problem :

This is the End of this Error Section.

Good Luck Understanding this.