C Language Identifier Scope Function Prototype Scope
Example
#include <stdio.h>
/* The parameter name, apple, has function prototype scope. These names
are not significant outside the prototype itself. This is demonstrated
below. */
int test_function(int apple);
int main(void)
{
int orange = 5;
orange = test_function(orange);
printf("%d\r\n", orange); //orange = 6
return 0;
}
int test_function(int fruit)
{
fruit += 1;
return fruit;
}
Note that you get puzzling error messages if you introduce a type name in a prototype:
int function(struct whatever *arg);
struct whatever
{
int a;
// ...
};
int function(struct whatever *arg)
{
return arg->a;
}
With GCC 6.3.0, this code (source file dc11.c) produces:
$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -c dc11.c
dc11.c:1:25: error: ‘struct whatever’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
int function(struct whatever *arg);
^~~~~~~~
dc11.c:9:9: error: conflicting types for ‘function’
int function(struct whatever *arg)
^~~~~~~~
dc11.c:1:9: note: previous declaration of ‘function’ was here
int function(struct whatever *arg);
^~~~~~~~
cc1: all warnings being treated as errors
$
Place the structure definition before the function declaration, or add struct whatever; as a line before the function declaration, and there is no problem. You should not introduce new type names in a function prototype because there's no way to use that type, and hence no way to define or use that function.
