C program will not compile

student04 · 03-26-2007, 12:15 AM

Hello, if you have a C compiler in your grasp, could you see if this compiles on your system without errors or warnings?

Code:

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>

int value = 5;

int main()
{
  pid_t pid;
  pid = fork();
  
  if (pid == 0) {
    value += 15;
  } else if (pid > 0) {
    wait(NULL);
    printf("PARENT: value = %d", value);
    exit(0);
  }
}

This is what I get:

Code:

[amm1747@enterprise homework]$ gcc-4.1 -Wall ch3ex4.c -o ch3ex4
ch3ex4.c: In function ‘main’:
ch3ex4.c:15: warning: implicit declaration of function ‘wait’
ch3ex4.c:17: warning: implicit declaration of function ‘exit’
ch3ex4.c:17: warning: incompatible implicit declaration of built-in function ‘exit’
ch3ex4.c:19: warning: control reaches end of non-void function
[amm1747@enterprise homework]$ ll
total 392K
-rwxr-xr-x 1 amm1747 amm1747 6.9K 2007-03-26 01:12 ch3ex4
-rw-r--r-- 1 amm1747 amm1747  259 2007-03-26 00:49 ch3ex4.c
-rw------- 1 amm1747 amm1747  17K 2007-03-21 02:25 chapter2.odt
-rw------- 1 amm1747 amm1747 342K 2007-03-21 02:25 chapter2.pdf
-rw-r--r-- 1 amm1747 amm1747 9.0K 2007-03-26 00:06 chapter3.odt
[amm1747@enterprise homework]$ ./ch3ex4 
[amm1747@enterprise homework]$ prise homework]$

It doesn't print anything.. Instead it messes up the prompt at the end.

I'd appreciate any help! By the way, this is code copied directly out of my OS Concepts book, so it's not something I wrote.

Thank you,
AM

student04 · 03-26-2007, 12:34 AM

Well, so I add a \n to the end of the string in the printf statement, and it prints something out. But that still doesn't explain why I am getting those errors/warnings. Any ideas?

Thanks.

graemef · 03-26-2007, 12:42 AM

You may want to try and #include <sys/wait.h>
Chect out man 2 wait

student04 · 03-26-2007, 12:48 AM

Hi, that did get ride of my wait error messages (but leaving exit still). I do not have a man page entry for wait, for some reason. I appreciate the feedback

-AM

graemef · 03-26-2007, 12:57 AM

either use the keyword return or _exit(int)

student04 · 03-26-2007, 01:01 AM

_exit(int) works! Thanks a lot, I'll keep this in mind.

-AM

Quigi · 03-26-2007, 08:59 AM

You probably still get this warning,

Code:

ch3ex4.c:19: warning: control reaches end of non-void function

Because you define main as returning an int, but then there is no return statement. You should add return 0; just before the final closing brace. Alternatively, make sure both branches of the if statement return a value. BTW, in this situation I'd prefer return over exit_(...).

osor · 03-26-2007, 10:02 AM

Quote:

Originally Posted by student04

_exit(int) works! Thanks a lot, I'll keep this in mind.

-AM

_exit() is declared in unistd.h

but

exit() is declared in stdlib.h

What’s going on (or what was going on in the original question) is very simple: you were trying to use functions that you hadn’t declared (in this case, wait() and exit()). Normally, I believe that such a function is assumed to return an int and take as it’s parameters whatever types you passed. Some functions, however, are so commonly used that gcc has them built-in, and this is where the problem with exit() was — the implicit declaration (which returns an int) did not match the built-in declaration (of void).