Problems calling syscall function in C - c

Question

For a homework assignment, I have to modify the linux kernel.
I am working on a virtual machine, and I added a system call to the kernel, which I called get_unique_id. Here is the code for get_unique_id.c :
#include <linux/linkage.h>
#include <asm/uaccess.h>
asmlinkage long sys_get_unique_id(int * uuid)
{
// static because we want its state to persist between calls
static int uid = 0;
++uid;
// assign new uid value to user-provided mem location
// returns non-zero if success or -EFAULT otherwise
int ret = put_user(uid, uuid);
return ret;
}
I also added this line to syscalls.h :
asmlinkage long sys_get_unique_id(int * uuid);
This line to syscall_32.tbl :
383 i386 get_unique_id sys_get_unique_id
And finally this line to syscall_64.tbl :
548 common get_unique_id sys_get_unique_id
After recompiling and reloading the kernel, I wrote a little C program to test my system call, here is the code for the C test file :
// get_unique_id_test.c
#include <stdio.h>
#include <limits.h>
#include "syscalls_test.h"
int main(void)
{
// initialize the ints we want
int id1;
int id2;
// check the id's are unique and that no error occured
for (int i = INT_MIN; i < INT_MAX - 1; i += 2) {
long ret1 = get_unique_id(&id1);
long ret2 = get_unique_id(&id2);
if (ret1 != 0)
printf("ERROR: get_unique_id returned: %ld\n", ret1);
if (ret2 != 0)
printf("ERROR: get_unique_id returned: %ld\n", ret2);
if (id2 != id1 + 1)
printf("ERROR: successive id's did not increment properly: id1 = %d, id2 = %d\n", id1, id2);
}
return 0;
}
And its header file :
// syscalls_test.h
#include <errno.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#define __NR_get_unique_id 383
inline long get_unique_id(int * uuid)
{
return syscall(__NR_get_unique_id, uuid) ? errno : 0;
}
Unfortunately, while trying to compile the C test file with the following command : gcc -std=c99 get_unique_id_test.c -o get_unique_id_test, I get the following error :
In file included from get_unique_id_test.c:4:0:
syscalls_test.h: In function ‘get_unique_id’:
syscalls_test.h:10:5: warning: implicit declaration of function ‘syscall’ [-Wimplicit-function-declaration]
return syscall(__NR_get_unique_id, uuid) ? errno : 0;
^
syscalls_test.h: In function ‘get_unique_id’:
syscalls_test.h:10:5: warning: implicit declaration of function ‘syscall’ [-Wimplicit-function-declaration]
return syscall(__NR_get_unique_id, uuid) ? errno : 0;
^
/tmp/cc1euZ3r.o: In function `main':
get_unique_id_test.c:(.text+0x22): undefined reference to `get_unique_id'
get_unique_id_test.c:(.text+0x34): undefined reference to `get_unique_id'
collect2: error: ld returned 1 exit status
It appears gcc cannot find the function get_unique_id(int * uuid), which is declared in syscalls_test.h, and the syscall function, which should be declared, I believe, in syscall.h, right ?
I don't understand why this happens. Does anybody have an idea ?
EDIT : my problems were solved using a3f's solution (see below) PLUS moving the #include "syscalls_test.h" at the very top of the file, as he said in the comments. Thank you very much.

Answer 1

#define _GNU_SOURCE before including unistd.h or any other header as syscall(2) is not POSIX.
Use static inline instead of plain inline. Plain inline supplies an inline definition, but the compiler is free to ignore it and use the external definition instead, which you aren't providing.

Answer 2

Try the following one:
#include <unistd.h>