本文描述如果通过文件IO sysfs方式控制树莓派 GPIO端口。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。

参考博文：树莓派学习笔记——使用文件IO操作GPIO SysFs方式

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

#define BUFFER_MAX 100
#define DIRECTION_MAX 100
#define IN 0
#define OUT 1
#define LOW 0
#define HIGH 1
#define POUT 21

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);
/*
int main(int argc, char const *argv[])
{
        int i = 0;  
      
    gpio_export(POUT);  
    gpio_direction(POUT, OUT);  
      
    for (i = 0; i < 20; i++) {  
        gpio_write(POUT, i % 2);  
        usleep(500 * 1000);  
    }  
  
    gpio_unexport(POUT);  
    return 0;
}
*/
static int gpio_export(int pin)  
{  
    char buffer[BUFFER_MAX];  
    int len;  
    int fd;  
  
    fd = open("/sys/class/gpio/export", O_WRONLY);  
    if (fd < 0) {  
        fprintf(stderr, "Failed to open export for writing!\n");  
        return(-1);  
    }  
  
    len = snprintf(buffer, BUFFER_MAX, "%d", pin);  
    if (write(fd, buffer, len) < 0) {  
        fprintf(stderr, "Fail to export gpio!");  
        return -1;  
    }  
     
    close(fd);  
    return 0;  
}  

static int gpio_unexport(int pin)  
{  
    char buffer[BUFFER_MAX];  
    int len;  
    int fd;  
  
    fd = open("/sys/class/gpio/unexport", O_WRONLY);  
    if (fd < 0) {  
        fprintf(stderr, "Failed to open unexport for writing!\n");  
        return -1;  
    }  
  
    len = snprintf(buffer, BUFFER_MAX, "%d", pin);  
    if (write(fd, buffer, len) < 0) {  
        fprintf(stderr, "Fail to unexport gpio!");  
        return -1;  
    }  
     
    close(fd);  
    return 0;  
}  

static int gpio_direction(int pin, int dir)
{
    static const char dir_str[] = "in\0out";
    char path[DIRECTION_MAX];
    int fd;

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

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

    close(fd);
    return 0;
}

static int gpio_write(int pin, int value)  
{  
    static const char values_str[] = "01";  
    char path[DIRECTION_MAX];  
    int fd;  
  
    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", pin);  
    fd = open(path, O_WRONLY);  
    if (fd < 0) {  
        fprintf(stderr, "failed to open gpio value for writing!\n");  
        return -1;  
    }  
  
    if (write(fd, &values_str[value == LOW ? 0 : 1], 1) < 0) {  
        fprintf(stderr, "failed to write value!\n");  
        return -1;  
    }  
  
    close(fd);  
    return 0;  
}  

static int gpio_read(int pin)  
{  
    char path[DIRECTION_MAX];  
    char value_str[3];  
    int fd;  
  
    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", pin);  
    fd = open(path, O_RDONLY);  
    if (fd < 0) {  
        fprintf(stderr, "failed to open gpio value for reading!\n");  
        return -1;  
    }  
  
    if (read(fd, value_str, 3) < 0) {  
        fprintf(stderr, "failed to read value!\n");  
        return -1;  
    }  
  
    close(fd);  
    return (atoi(value_str));  
}