C Language Declarations Calling a function from another C file
Example
foo.h
#ifndef FOO_DOT_H /* This is an "include guard" */
#define FOO_DOT_H /* prevents the file from being included twice. */
/* Including a header file twice causes all kinds */
/* of interesting problems.*/
/**
* This is a function declaration.
* It tells the compiler that the function exists somewhere.
*/
void foo(int id, char *name);
#endif /* FOO_DOT_H */
foo.c
#include "foo.h" /* Always include the header file that declares something
* in the C file that defines it. This makes sure that the
* declaration and definition are always in-sync. Put this
* header first in foo.c to ensure the header is self-contained.
*/
#include <stdio.h>
/**
* This is the function definition.
* It is the actual body of the function which was declared elsewhere.
*/
void foo(int id, char *name)
{
fprintf(stderr, "foo(%d, \"%s\");\n", id, name);
/* This will print how foo was called to stderr - standard error.
* e.g., foo(42, "Hi!") will print `foo(42, "Hi!")`
*/
}
main.c
#include "foo.h"
int main(void)
{
foo(42, "bar");
return 0;
}
Compile and Link
First, we compile both foo.c and main.c to object files. Here we use the gcc compiler, your compiler may have a different name and need other options.
$ gcc -Wall -c foo.c
$ gcc -Wall -c main.c
Now we link them together to produce our final executable:
$ gcc -o testprogram foo.o main.o
