Why is GCC not seeing functions in header file? - header-files

Question

When I try to compile I get this:
aero$ gcc hostinfo.c
/tmp/cc2RfYB2.o: In function `main':
hostinfo.c:(.text+0x72): undefined reference to `Gethostbyaddr'
hostinfo.c:(.text+0x8b): undefined reference to `Gethostbyname'
collect2: ld returned 1 exit status
hostinfo.c looks like this:
/* $begin hostinfo */
#include "csapp.h"
int main(int argc, char **argv)
{
...
if (inet_aton(argv[1], &addr) != 0)
hostp = Gethostbyaddr((const char *)&addr, sizeof(addr), AF_INET);
else
hostp = Gethostbyname(argv[1]);
...
}
/* $end hostinfo */
And csapp.h looks like this:
/* $begin csapp.h */
#ifndef __CSAPP_H__
#define __CSAPP_H__
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <setjmp.h>
#include <signal.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <errno.h>
#include <math.h>
#include <pthread.h>
#include <semaphore.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
...
/* DNS wrappers */
struct hostent *Gethostbyname(const char *name);
struct hostent *Gethostbyaddr(const char *addr, int len, int type);
...
#endif /* __CSAPP_H__ */
/* $end csapp.h */
Both hostinfo.c and csapp.h are in the same directory. I'm new to Unix and gcc so I'm sure it's something simple.

Answer 1

You need to compile the file containing the implementation of these functions, and then link it with the calling file:
gcc -c csapp.c # This creates csapp.o
gcc -c hostinfo.c # This creates hostinfo.o
gcc -o hostinfo hostinfo.o csapp.o # Link them together, creating executable hostinfo

Answer 2

I was basically having the error of undefined reference to `function_name'
It was finally resolved by combining both the c files in the executable
For example, I had csv.c and columns.c with columns.h, so instead of
gcc csv.c -o csv
I used
gcc csv.c columns.c -o csv