linux下系统调用小知识

How to define your own sysall?
Three steps:(for arm)
(i)In calls.S：
 /* 256 */
 .long SYMBOL_NAME(sys_readhwproc)
 note：the filename and the syscall_number could be not the same as above in other architectures.
(ii)In unistd.h：
 #define __NR_readhwproc  
   (__NR_SYSCALL_BASE+256)
  note：syscall_number must be the same as above.
(iii)In sys.c：
 add corresponding function:
asmlinkage long sys_readhwproc(void)
{
 current->hwproc=“hello, I am hardwareprocess!”;
 printk(“’%s’\n”,current->hwproc);
 return 0;
}
Before testing this syscall, the following two must be specified:
(1)The relation of C lib and syscall:

(2)Two ways of using syscall directly, not through glibc:
(i) #define __NR_readhwproc  (__NR_SYSCALL_BASE+226)
int main()
{
 …
 syscall(__NR_readhwproc);/**:if there is some params,put them behind the function name and devided by comma.*/
 …
}
(ii)Using _syscalln(n is the number of parameters pssed to syscalls.)
#define __NR_readhwproc  (__NR_SYSCALL_BASE+226)
_syscall0(long,readhwproc);
int main()
{
 …
 readhwproc();
 …
}

pay attention: sys_calls can be used by both user process and kernelprocess. when using it,it's neccessary for you to include some head files, pay attention that the macro "#define __KERNEL_SYSCALLS__" is needed,it is usually definded in the same file as "#define __NR_readhwproc  (__NR_SYSCALL_BASE+226)
_syscall0(long,readhwproc); "