glibc库函数，系统调用API

     glibc封装了大部分系统API，我们一般都是使用glibc封装的接口进行系统调用，碰到一些没有封装的接口，可以通过这个

函数syscall 进行系统调用。

 

/* Invoke `system call' number SYSNO, passing it the remaining arguments.
   This is completely system-dependent, and not often useful.

   In Unix, `syscall' sets `errno' for all errors and most calls return -1
   for errors; in many systems you cannot pass arguments or get return
   values for all system calls (`pipe', `fork', and `getppid' typically
   among them).

   In Mach, all system calls take normal arguments and always return an
   error code (zero for success).  */
extern long int syscall (long int __sysno, ...) __THROW;