A confusion about invoking my syscall

[Jeff Haran]

From: 王哲 [mailto:wangzhe5004 at gmail.com]
Sent: Wednesday, June 20, 2012 1:16 AM
To: Jeff Haran
Cc: kernelnewbies
Subject: Re: A confusion about invoking my syscall


2012/6/20 Jeff Haran <jharan at bytemobile.com<mailto:jharan at bytemobile.com>>


From: 王哲 [mailto:wangzhe5004 at gmail.com<mailto:wangzhe5004 at gmail.com>]
Sent: Monday, June 18, 2012 9:32 PM
To: Jeff Haran
Cc: kernelnewbies
Subject: Re: A confusion about invoking my syscall


2012/6/19 Jeff Haran <jharan at bytemobile.com<mailto:jharan at bytemobile.com>>


From: kernelnewbies-bounces at kernelnewbies.org<mailto:kernelnewbies-bounces at kernelnewbies.org> [mailto:kernelnewbies-bounces at kernelnewbies.org<mailto:kernelnewbies-bounces at kernelnewbies.org>] On Behalf Of ??
Sent: Monday, June 18, 2012 6:40 PM
To: kernelnewbies
Subject: A confusion about invoking my syscall

Hello everyone:

         I append a simple syscall in kernel. and the function is as follows:

  asmlinkage  long sys_mysyscall(long data)
 {
          printk("This is my syscall!\n");
          return data;
  }

and i test it sucessfully in user space . and the test program:

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



   int main(void)
   {
   long n = 0,m = 0,pid1,pid2;
   n = syscall(345,190);// #define __NR_mysyscall          345
   printf("n = %ld\n",n);
   pid1 = syscall(SYS_getpid);  //getpid
   printf("pid = %ld\n",pid1);
   pid2 = syscall(20);  //getpid
   printf("pid = %ld\n",pid2);
   return 0;
  }
and the result:
n = 190
pid = 4097
pid = 4097

but if the test program is:
#include <linux/unistd.h>
#include <syscall.h>
#include <sys/types.h>
#include <stdio.h>



int main(void)
{
 long n = 0,m = 0,pid1,pid2;
 n = syscall(345,190);// #define __NR_mysyscall          345
 printf("n = %ld\n",n);
 m = syscall(SYS_mysyscall,190);
 printf("m = %ld\n",m);
 pid1 = syscall(SYS_getpid);  //getpid
 printf("pid = %ld\n",pid1);
 pid2 = syscall(20);  //getpid
 printf("pid = %ld\n",pid2);
 return 0;
}
and the result:
wanny at wanny-C-Notebook-XXXX:~/syscall/src$ gcc test1.c
test1.c: In function ‘main’:
test1.c:13:14: error: ‘SYS_mysyscall’ undeclared (first use in this function)
test1.c:13:14: note: each undeclared identifier is reported only once for each function it appears in


why i can't invoke my syscall with "SYS_mysyscall"?

Thanks in advance!
Because it appears you never defined the symbol SYS_mysyscall.
 I think so,but where shoud i defne the  symbol SYS_mysyscall ?
  and where is the symbol SYS_getpid defined?
On my system /usr/include/bits/syscall.h, which is being included in your program because it includes syscall.h.
           83 #define SYS_getpid __NR_getpid  ，so SYS_getpid is replaced by __NR_getpid. and __NR_getpid was defined in the kernel(arch/x86/include/asm/unistd_32.h). and my syscall was also defined there.#define SYS_mysyscall __NR_mysyscall, i don't kown why it doesn't works.

My sources contain no reference to SYS_mysyscall nor __NR_mysyscall, so I assume you’ve added them to the Linux include files that you built your module from.

User space programs like your main() program above generally aren’t going to include Linux source tree include files. When you include <syscall.h> from a user space program in a typical development environment, the compiler is by default going to look for syscall.h in /usr/include, not in the Linux source tree where presumably you’ve made your modifications. Of course you can always tell the compiler to look there using the -I command line option to gcc, if you want to. The usual practice however is to keep kernel code and user code include files completely separate. That means some duplication of effort, like having to define SYS_mysyscall in two different places, but that’s the usual practice because most people aren’t building kernels and thus haven’t installed the kernel source include files.

Jeff Haran


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