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.

Answer 3

To answer a more generic case, this error is noticed when you pick a function name which is already used in some built in library. For eg, select.

A simple method to know about it is while compiling the file, the compiler will indicate the previous declaration.