在C中调用syscall函数时出现问题

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

请尝试以下方法：

 #include <unistd.h>