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;