C program to read information from sysfs file in Linux

In this post, we will write a simple C program which can read any sysfs file from Linux and display the output to console.

 $ vim readsysfs.c 

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define print_error \
	do { \
		fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", \
		__LINE__, __FILE__, errno, strerror(errno)); exit(1); \
	} while(0)

int main(int argc, char **argv)
{
        char buf[1024];
        ssize_t len;
        int fd;
	char *filename;

	if(argc < 2) {
		fprintf(stderr, "\nUsage:\t%s { sysfs file }\n", argv[0]);
		exit(1);
	}

	filename = argv[1]; /* accept argument 1 as filename */

        fd = open(filename, O_RDONLY);
        if (fd < 0) {
		fprintf(stderr, "open %s: %s\n", filename, strerror(errno));
		print_error;
        }

        len = read(fd, buf, sizeof(buf)-1);
        if (len < 0) {
                fprintf(stderr, "read %s: %s", filename, strerror(errno));
		print_error;
        }

        buf[len] = 0;
	
	printf("%s : %s\n", filename, buf);

#if 0
	printf("bufferlen is %d\n", strlen(buf));
#endif
        
	close(fd);
	return 0;
}

 $ vim Makefile 

CC=gcc
#CC=arm-linux-gcc
CFLAGS=-Wall -g

main: sysfsread

clean:
	rm -f *.o *~

Above makefile is using GCC so it can be compiled for Desktop / Laptop but if you want to cross compile for some embedded target like ARM boards, you can export the toolchain and change CC to your toolchain specific. like as we did in commented code above i.e. CC=arm-linux-gcc

$ make

After we compile using make, we can get the binary sysfsread, now we can execute this binary by passing the absolute path of the filename in sysfs file system.

$ ./sysfsread /sys/bus/usb/devices/usb1/manufacturer