conflicting types error when compiling c program using gcc

Question

I tried to compile following program with gcc.

0 #include  <stdio.h> 1  2 main () 3 4 {     5  char my_string[] = "hello there";       6 7  my_print (my_string);     8  my_print2 (my_string);     9} 10    11 void my_print (char *string) 12 {     13  printf ("The string is %s\n", string);     14 } 15         16 void my_print2 (char *string) 17 { 18  char *string2;     19  int size, i;    20 21  size = strlen (string);     22  string2 = (char *) malloc (size + 1); 23 24  for (i = 0; i < size; i++)     25    string2[size - i] = string[i]; 26 27   string2[size+1] = '\0';     28   printf ("The string printed backward is %s\n", string2);     29 } 

However, it fails and the compiler produces following error log:

1. greeting.c: 11: error:conflicting types for 'my_print'
2. greeting.c: 7: error: previous implicit declaration of 'my_print' was here
3. greeting.c: 16: error:conflicting types for 'my_print2'
4. greeting.c:8: erroro:previous implicit declaration of 'my_print2' was there

And if I move the my_print and my_print2 functions before the main function, everything goes well. So can anyone explain why the problem happens? Thanks!

Answer 1

If you don't declare a function and it only appears after being called, it is automatically assumed to be int, so in your case, you didn't declare

void my_print (char *); void my_print2 (char *); 

before you call it in main, so the compiler assume there are functions which their prototypes are int my_print2 (char *); and int my_print2 (char *); and you can't have two functions with the same prototype except of the return type, so you get the error of conflicting types.

As Brian suggested, declare those two methods before main.

Answer 2

You have to declare your functions before main()

(or declare the function prototypes before main())

As it is, the compiler sees my_print (my_string); in main() as a function declaration.

Move your functions above main() in the file, or put:

void my_print (char *); void my_print2 (char *); 

Above main() in the file.