header files and multiple c programs - c

Question

I am trying to use two .c files together. I am lost at how to do this, I have a simple setup for each file but I get a undefined reference to format_lines error when I try to compile. Any help would be muchly appreciated;
formatter.h
#ifndef _FORMATTER_H_
#define _FORMATTER_H_
#include <stdio.h>
char **format_file(FILE *);
char **format_lines(char **, int);
void test();
#endif
formatter.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "formatter.h"
char **format_file(FILE *infile) {
return NULL;
}
char **format_lines(char **lines, int num_lines) {
char **result = NULL;
#ifdef DEBUG
result = (char **)malloc(sizeof(char *) * 2);
if (result == NULL) {
return NULL;
}
result[0] = (char *)malloc(sizeof(char) * 80);
if (result[0] == NULL) {
return NULL;
}
strncpy(result[0], "(machine-like voice) EXTERMINATE THEM!", 79);
result[1] = (char *)malloc(sizeof(char) * 2);
if (result[1] == NULL) {
return NULL;
}
result[1][0] = '\0';
#endif
}
void test(){
print("here");
}
and sengfmt.c
#include <stdio.h>
#include <stdlib.h>
#include "formatter.h"
int main(int argc, char *argv[]) {
test();
#ifdef DEBUG
printf("%s does nothing right now.\n", argv[0]);
#endif
exit(0);
}
When I try to compile, I just type this.
$ gcc sengfmt3.c
/tmp/cc7Ttgne.o: In function `main':
sengfmt3.c:(.text+0x15): undefined reference to `test'
collect2: ld returned 1 exit status

Answer 1

I suspect that your main used to try to call format_lines
You need to do this
gcc formatter.c sendgfmt.c -o myprog
You must list all the c files that you want compiled together

Answer 2

If you have code in multiple source files, then you need to build with all the source files.
There are two ways of doing this:
Compile and link all source files using using one command:
$ gcc sengfmt3.c someOtherSourceFile.c someThirdSourceFile.c
First make object files of all source files, and then link the object files together. This is more work, but if you have a makefile or other build-system it will be better since only the modified source files will be recompiled, and might save you some build-time:
$ gcc -c sengfmt3.c
$ gcc -c someOtherSourceFile.c
$ gcc -c someThirdSourceFile.c
$ gcc sengfmt.o someOtherSourceFile.o someThirdSorceFile.o
Note the command-line option -c for the compilation, this tells GCC to generate object files. Also note that for the linking command (the last one) the file extensions have changed from .c to .o.
The command in point 1 does this internally, using temporary files which are removed when done.