Why doesn't this simple program compile?

SparceMatrix · 08-26-2002, 05:50 PM

This is a demonstration of multiple source code compilation in the book "Linux Programming Unleashed 2nd Edition" from SAMS publishing page 48:

There are three seperate files in the same directory,

helper.c

Code:

/*
 * helper.c - Helper code for howdy.c
 */

#include <stdio.h>
void msg(void)
{
 printf("This message sent from Jupiter.\n");
}

helper.h

Code:

/*
 * helper.h - Header for helper.c
 */

void msg(void)

howdy.c

Code:

/*
 * * howdy.c - Canonical "Hello, world!" program
 * */
#include <stdio.h>
#include "helper.h"

int main(void)
{
 printf("Hello, Linux programming world!\n");
 return 0;
}

This is compiled, according to the book,

Code:

gcc howdy.c helper.c -o howdy

But, I get the following errors:

Code:

howdy.c: In function `msg':
howdy.c:8: parse error before `{'
howdy.c:8: declaration for parameter `main' but no such parameter
howdy.c:10: number of arguments doesn't match prototype
cc1: prototype declaration

I have been through numerous C++ texts and this is the first time I have seen anything like an elementary example and explanation of how to use the preprocessor directives. And even here there is very little encouragement. For example, what is the difference between a library enclosed in '<>' and one enclosed in '""'? What is the purpose of the code in the helper.h?

rshaw · 08-26-2002, 07:25 PM

the <> is compiler supplied, the "" are things you've written. in helper.h you need a semicolon, it's a function call.

it's not a function call, it's a prototype, either way it needed a semicolon. sorry if i caused any confusion.

Malicious · 08-26-2002, 07:56 PM

I can't help but believe there is a typo somewhere.

One cause of the errors (howdy.c:8) is the missing semicolon in helper.h:

Code:

/*
 * helper.h - Header for helper.c
 */
void msg(void); /* semicolon added */

After the precompiler reads in all the files to compile howdy.c you get:

Code:

(cruft from stdio.h)
void msg(void)
int main(void)
{
printf("Hello, Linux programming world!\n");
return 0;
}

Without the semicolon, the next thing the compiler expects is the open brace for the code in the msg function which accounts for part of the errors (howdy.c:8). The second part is because "main" is prototyped as "main(int, char**)" and the declaration of main() doesn't match the prototype (howdy.c:10). Older non-ansi C compilers did not require prototypes and main() worked fine. Add the semicolon and fix the "main" declaration and it should compile fine.

I don't have a clue why helper.h was included in howdy.c since the msg function was not called from howdy.c. Like I said, must be a typo somewhere. Look for an errata sheet for the book.

The use of quotes and <> symbols for include files determines the directory search order for that include file. "man cpp" and look for the -I and -I- directives for an explanation.

SparceMatrix · 08-27-2002, 12:20 AM

Thanks so much all. The semicolon seems to be all that was needed. It is not there in the text and no errata either. There is something I missed, but it was functional:

Code:

/*
 * * hello2.c - Canonical "Hello, world!" program
 * */
#include <stdio.h>
#include "helper.h"

int main(void)
{
 printf("Hello, Linux programming world!\n");
 msg(); 
 return 0;
}

And so the whole point of the example is fulfilled.

Thanks again for your assistance. If I had not seen the same kind of errors in post grad mathematics text books, I would be more irate.

tundra · 08-27-2002, 03:37 AM

i think that's why the jedi philosophy works in learning programming. and books are not enuff.
one master, one apprentice.