Question

The library header files contain only the declarations of functions. So, how exactly does the function run when we call the library functions?

Ex: int strcmp ( const char * str1, const char * str2 ); is the declaration for strcmp function in C.

When we call strcmp function in a program, how exactly does the strcmp function get executed if the function body is not present in the header files?

Answer 1

The code that you want to use like printf is already compiled and stored somewhere. Your final executable is built when the linker links your object code with the standard library.

try gcc -c somefile.c and see what you get.

Also try gcc -v option and learn the output that it gives.

Find out what ld command does and you shall get your answer.

Also this what I think should help you : All about compilation/linking/related

UPDATE :

Also Imagine if you defined printf for every program in stdio.h and you included stdio.h

program.c :

#include <stdio.h>  //suppose this contains definition of printf     

int main()
{            

/*Call printf()  somewhere or maybe not */

}

gcc program.c -o output

So here all your definitions that are there in stdio.h would be replaced in the program.c file and your executable output will become pretty HUGE. Instead It would be a better idea to call functions that you use frequently like printf in stdio.h from one place whenever they are needed