Linux下用文件IO的方式操作GPIO（/sys/class/gpio）

通过sysfs方式控制GPIO，先访问/sys/class/gpio目录，向export文件写入GPIO编号，使得该GPIO的操作接口从内核空间暴露到用户空间，GPIO的操作接口包括direction和value等，direction控制GPIO方向，而value可控制GPIO输出或获得GPIO输入。文件IO方式操作GPIO，使用到了4个函数open、close、read、write。

首先，看看系统中有没有“/sys/class/gpio”这个文件夹。如果没有请在编译内核的时候加入 Device Drivers-> GPIO Support ->/sys/class/gpio/… (sysfs interface)。

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另外参考网上一个网友的程序，这里做了验证，并实现中断检测函数。如下：
#include stdlib.h
#include stdio.h
#include string.h
#include unistd.h
#include fcntl.h
#include poll.h

#define MSG(args...) printf(args)

//函数声明
static int gpio_export(int pin);
static int gpio_unexport(int pin);
static int gpio_direction(int pin, int dir);
static int gpio_write(int pin, int value);
static int gpio_read(int pin);

static int gpio_export(int pin)
{
char buffer[64];
int len;
int fd;

fd = open("/sys/class/gpio/export", O_WRONLY);
if (fd < 0) {
MSG("Failed to open export for writing!\n");
return(-1);
}

len = snprintf(buffer, sizeof(buffer), "%d", pin);
if (write(fd, buffer, len) < 0) {
MSG("Failed to export gpio!");
return -1;
}

close(fd);
return 0;
}

static int gpio_unexport(int pin)
{
char buffer[64];
int len;
int fd;

fd = open("/sys/class/gpio/unexport", O_WRONLY);
if (fd < 0) {
MSG("Failed to open unexport for writing!\n");
return -1;
}

len = snprintf(buffer, sizeof(buffer), "%d", pin);
if (write(fd, buffer, len) < 0) {
MSG("Failed to unexport gpio!");
return -1;
}

close(fd);
return 0;
}

//dir: 0-->IN, 1-->OUT
static int gpio_direction(int pin, int dir)
{
static const char dir_str[] = "in\0out";
char path[64];
int fd;

snprintf(path, sizeof(path), "/sys/class/gpio/gpio%d/direction", pin);
fd = open(path, O_WRONLY);
if (fd < 0) {
MSG("Failed to open gpio direction for writing!\n");
return -1;
}

if (write(fd, &dir_str[dir == 0 ? 0 : 3], dir == 0 ? 2 : 3) < 0) {
MSG("Failed to set direction!\n");
return -1;
}

close(fd);
return 0;
}

//value: 0-->LOW, 1-->HIGH
static int gpio_write(int pin, int value)
{
static const char values_str[] = "01";
char path[64];
int fd;

snprintf(path, sizeof(path), "/sys/class/gpio/gpio%d/value", pin);
fd = open(path, O_WRONLY);
if (fd < 0) {
MSG("Failed to open gpio value for writing!\n");
return -1;
}

if (write(fd, &values_str[value == 0 ? 0 : 1], 1) < 0) {
MSG("Failed to write value!\n");
return -1;
}

close(fd);
return 0;
}

static int gpio_read(int pin)
{
char path[64];
char value_str[3];
int fd;

snprintf(path, sizeof(path), "/sys/class/gpio/gpio%d/value", pin);
fd = open(path, O_RDONLY);
if (fd < 0) {
MSG("Failed to open gpio value for reading!\n");
return -1;
}

if (read(fd, value_str, 3) < 0) {
MSG("Failed to read value!\n");
return -1;
}

close(fd);
return (atoi(value_str));
}

// none表示引脚为输入，不是中断引脚
// rising表示引脚为中断输入，上升沿触发
// falling表示引脚为中断输入，下降沿触发
// both表示引脚为中断输入，边沿触发
// 0-->none, 1-->rising, 2-->falling, 3-->both
static int gpio_edge(int pin, int edge)
{
const char dir_str[] = "none\0rising\0falling\0both";
char ptr;
char path[64];
int fd;
switch(edge){
case 0:
ptr = 0;
break;
case 1:
ptr = 5;
break;
case 2:
ptr = 12;
break;
case 3:
ptr = 20;
break;
default:
ptr = 0;
}

snprintf(path, sizeof(path), "/sys/class/gpio/gpio%d/edge", pin);
fd = open(path, O_WRONLY);
if (fd < 0) {
MSG("Failed to open gpio edge for writing!\n");
return -1;
}

if (write(fd, &dir_str[ptr], strlen(&dir_str[ptr])) < 0) {
MSG("Failed to set edge!\n");
return -1;
}

close(fd);
return 0;
}

//GPIO1_17
int main()
{
int gpio_fd, ret;
struct pollfd fds[1];
char buff[10];
unsigned char cnt = 0;
//LED引脚初始化
gpio_export(115);
gpio_direction(115, 1);
gpio_write(115, 0);
//按键引脚初始化
gpio_export(49);
gpio_direction(49, 0);
gpio_edge(49,1);
gpio_fd = open("/sys/class/gpio/gpio49/value",O_RDONLY);
if(gpio_fd < 0){
MSG("Failed to open value!\n");
return -1;
}
fds[0].fd = gpio_fd;
fds[0].events = POLLPRI;
ret = read(gpio_fd,buff,10);
if( ret == -1 )
MSG("read\n");
while(1){
ret = poll(fds,1,0);
if( ret == -1 )
MSG("poll\n");
if( fds[0].revents & POLLPRI){
ret = lseek(gpio_fd,0,SEEK_SET);
if( ret == -1 )
MSG("lseek\n");
ret = read(gpio_fd,buff,10);
if( ret == -1 )
MSG("read\n");
gpio_write(115, cnt++%2);
}
usleep(100000);
}
return 0;
}