Why is this working? I can't understand why my code works in C

Question

I am currently learning C and I have come across this strange behaviour (based on my understanding):

I have 2 files:

file1.c

#include <stdio.h>

int main()
{
    printNumber(2);
    return 0;
}

file2.c

void printNumber(int number)
{
    printf("Number %d is printed.", number);
}

output:

Number 2 is printed.

Why am I not getting a declaration error? I thought you needed to declare a function somewhere in a file (or better a header file)

I've searched the forums trying to get an answer but I've seen people with very similar code who get the error but somehow I am not getting it...

Btw, I am using GCC and C11. Thanks.

Check https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html especially about the prototype — Martheen, Jun 02 '20 at 03:54
Does this answer your question? [Warning: implicit declaration of function — why does my code work anyway?](https://stackoverflow.com/questions/21169545/warning-implicit-declaration-of-function-why-does-my-code-work-anyway) — kaylum, Jun 02 '20 at 03:54

score 3 · Accepted Answer · answered Jun 02 '20 at 03:54

In general, if you called a function not defined anywhere, you will see error by linker, not compiler. Compiler may give you warning for implicit declaration but usually let it go.

An example, using gcc 8.3 to compile your file1.c:

$ gcc -o test file1.c
file1.c: In function ‘main’:
file1.c:5:5: warning: implicit declaration of function ‘printNumber’ [-Wimplicit-function-declaration]
     printNumber(2);
     ^~~~~~~~~~~
/usr/bin/ld: /tmp/ccs0wv7L.o: in function `main':
file1.c:(.text+0xf): undefined reference to `printNumber'
collect2: error: ld returned 1 exit status

Note the warning is from compiler but only the ld (linker) give you error for undefined reference to the function.

So I suspect you compile the program like the following, which the linker is smart enough to relate the object codes:

$ gcc -o test file1.c file2.c
file1.c: In function ‘main’:
file1.c:5:5: warning: implicit declaration of function ‘printNumber’ [-Wimplicit-function-declaration]
     printNumber(2);
     ^~~~~~~~~~~
file2.c: In function ‘printNumber’:
file2.c:3:5: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
     printf("Number %d is printed.", number);
     ^~~~~~
file2.c:3:5: warning: incompatible implicit declaration of built-in function ‘printf’
file2.c:3:5: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
file2.c:1:1:
+#include <stdio.h>
 void printNumber(int number)
file2.c:3:5:
     printf("Number %d is printed.", number);
     ^~~~~~

You see warnings above but it actually compiles and the linker does not complain. If you run this in your IDE, probably you don't even see the warning messages. So you have the illusion that your code works even you think it should have an error

Why is this working? I can't understand why my code works in C

1 Answers1