glibc封裝了大部分系統API，我們一般都是使用glibc封裝的接口進行系統調用，碰到一些沒有封裝的接口，可以通過這個

函數syscall 進行系統調用。

 1 /* Invoke `system call‘ number SYSNO, passing it the remaining arguments.
 2    This is completely system-dependent, and not often useful.
 3 
 4    In Unix, `syscall‘ sets `errno‘ for all errors and most calls return -1
 
 5    for errors; in many systems you cannot pass arguments or get return
 6    values for all system calls (`pipe‘, `fork‘, and `getppid‘ typically
 7    among them).
 8 
 9    In Mach, all system calls take normal arguments and always return an
10    error code (zero for success).  */
11 extern long
 
 int syscall (long int __sysno, ...) __THROW;

glibc庫函數，系統調用API