Need of stdio.h header file

We include <stdio.h> (standard input/output header) because it contains the declarations (prototypes) of the standard I/O functions such as printf(), scanf(), getchar(), putchar(), gets() and the file functions, as well as macros such as NULL and EOF. Without it the compiler does not know these functions and gives an error or warning.

Can we write a C program without any header file? Yes, if the program does not use any library function. For example:

int main(void)
{
    int a = 2, b = 3;
    return a + b;     /* no library function used, so no header needed */
}

This compiles and runs; it just returns 5 to the operating system. It is also possible to use printf without the header by declaring its prototype ourselves:

int printf(const char *format, ...);   /* declare printf ourselves */
int main(void)
{
    printf("Works without stdio.h\n");
    return 0;
}

Output:

Works without stdio.h

In practice, headers are included so that functions are declared correctly.