Solved

System Call Returns Gibberish

Posted on 2006-11-27
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Last Modified: 2010-04-22
I attempting to add a system call to 2.4.18-3, but I keep getting garbage on the return from the system call instead of the correct and much friendlier "Howdy" that's supposed to show up.  Here's the code I've used in each of the directories:

/linux/arch/i386/kernel/entry.S:

.long SYMBOL_NAME(sys_hello_world)

/linux/include/asm/unistd.h:

#define __NR_hello_world

linux/include/linux/hello_world.h:

#ifndef __LINUX_HELLO_WORLD_H
#define __LINUX_HELLO_WORLD_H
#include <linux/linkage.h>
#endif

usr/include/sys/hello_world-usr.h:
#include <unistd.h>
_syscall1(void, hello_world, char*, buf);

linux/ipc/hello_world.c:
#include <linux/kernel.h>
  #include <asm/uaccess.h>
  #include <linux/hello_world.h>

  asmlinkage void sys_hello_world(char *buf)
  {
          int num_left;
          static char *hello = "Hello World";

          printk("Copy hello world to user space.\n");
          num_left = copy_to_user(buf, hello, 12);
          if (num_left)
                  printk("Number of bytes left, %d\n", num_left);
  }

I added the following line to the Makefile in the Linux-2.4 directory:

obj-y   := util.o hello_world.o

This is my test file:

#include <stdio.h>
#include <linux/hello_world.h>

main()
  {
          char buf[25];
          hello_world(buf);
          puts(buf);
  }

The output is some ASCII gibberish and I can't figure out why it would be ASCII (gibberish I can understand, but ASCII...no).  Any ideas?

Jason
0
Comment
Question by:cdromes
2 Comments
 
LVL 43

Accepted Solution

by:
ravenpl earned 200 total points
ID: 18026004
You don't include /usr/include/sys/hello_world-usr.h - suppose just a typo, since it's compiling.
the problem might be that buf variable is not null delimited string. Try
#include <string.h>
int main()
{
 char buf[25];
 bzero(buf, 25);
 hello_world(buf);
 puts(buf);
 return 0;    
}
0
 

Author Comment

by:cdromes
ID: 18040931
I was forgetting to do a 'make install' in order to ensure I was running the modified kernel after every change.  

Yes, that's right, I AM a genius, thanks.  

Jason
0

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