关于syscall()函数的疑惑,该怎么处理

关于syscall()函数的疑惑
syscall(sys_gpioCtrl,GPIO_OP_OUTPUT,gpio,value)
其中sys_gpioCtrl = 347， GPIO_OP_OUTPUT = 0x00,  
gpio = 35, value = 1

这句什么意思啊  
顺便求syscall（）函数的详细用法，网上关于这个的资料好像很少，有一些也看不大明白

------解决方案--------------------
SYNOPSIS
#include <sys/syscall.h>
#include <unistd.h>

int
syscall(int number, ...);

DESCRIPTION
Syscall() performs the system call whose assembly language interface has
the specified number with the specified arguments. Symbolic constants
for system calls can be found in the header file 〈sys/syscall.h〉.

RETURN VALUES
The return value is defined by the system call being invoked. In gen-
eral, a 0 return value indicates success. A -1 return value indicates an
error, and an error code is stored in errno.

HISTORY
The syscall() function call appeared in 4.0BSD.