Calling a function from another C file

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#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 */


#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!")`


#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

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