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。
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;