Unexpected behaviour of printf function in c

Question

I recently encountered with an interview question. I did not understand the behaviour of printf function in this case

 #include <stdio.h>
 int main() {
 int k = printf("String");
 printf("%d",k);
 }

Expected result : Compilation Error

Output : String6

Why is the output String6?

Answer 1

Here is the prototype for printf:

int printf(const char *format, ...);

We can see that printf returns an int.

The documentation indicates that:

Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).

---

You asked why the output is "String6". Well:

printf("String");

This first prints String but does not print a newline character. Since String is 6 characters, printf returns 6, which you store in k:

printf("%d",k);

This then prints 6 (on the same line).

---

Try running this program:

#include <stdio.h>
int main(void)
{
    int bytes_printed = printf("%s\n", "String");
    //              7 =           1  +  6

    printf("printf returned: %d\n", bytes_printed);

    return 0;
}

Output:

String
printf returned: 7

Answer 2

the printf() function returns the number of character it printed. Since you set int k = printf("String");, the print function is executing printing out "String" and setting k equal to 6 since "String" is 6 characters long, then your second call to printf prints the value of k which is 6, resulting in the console displaying "String6".

This is perfectly valid C syntax.