Question

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C program without header

I have been studying C for a long time . but one thing that bothering me is that , today I made a C program and forget to include the stdio.h and conio.h header files I saved the file as kc.c ? when I compiled and run this .c file the output was as I was expecting to be.

but how can a C program could run without using the standard header file?

or I am not aware with the concepts that I am missing hear?

EDIT: the program

int main()
{
  int i=12,j=34;
  int *pa=&i,*pb=&j;
  printf("the value of i and j is %d %d respectively ",*pa,*pb);
  getch();
  return 0;
}

because I have used the printf() function of STDIO.H header here ,but without including it how can It got compiled and run successfully?

Answer 1

The compiler is allowed to make things work, but is under no obligation to do so.

You are supposed to declare all variable argument list functions before using them; not declaring printf() properly leads to undefined behaviour (and one valid undefined behaviour is to work as expected).

You should be getting warnings about the undeclared functions if you compile in C99 mode (but Turbo C probably doesn’t have a C99 mode).

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Nit-picking:

[H]ow can a C program could run without using the standard header file?

All programs run without using any headers whatsoever. However, most programs are compiled using the standard headers to declare the standard functions, and the better programs ensure that all functions are declared before they are used (or are defined as static functions before they are used).

C99 requires this, though many compilers will allow errant programs to compile. However, the compilation should produce a diagnostic; the diagnostic may or may not lead to the compilation failing. In practice, it usually doesn’t cause the compilation to fail, but it could and with some compilers (GCC, for example) you can force the compiler’s hand (e.g. with GCC’s -Werror=missing-prototypes -Werror=old-style-definition options).