Question

I’ve created a node in /dev by following the tutorial here (chardev.c), I tried to access the device in /dev/chardev I created by using the following code :

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h> //perror(), errno
#include <string.h>
#define RSIZE 50 

int main()
{
    int fd,err_save;
    char receive_buff[RSIZE];

    //open device and check for error msg
    fd = open("/dev/chardev", "rw");
    err_save = errno;
    if (fd < 0)
        {
        perror("open perror");
        printf("error opening device, fd = %d, err_save = %d \n", fd,err_save);
        }
    else
        {printf("Device opened\n");}

    //read device and check for error msg
    //memset(receive_buff, 0, sizeof(receive_buff)); //<--- strange
    read(fd, receive_buff, RSIZE);
    err_save = errno;
        if (fd < 0)
        {
        perror("read perror");
        printf("error reading device, fd = %d, err_save = %d \n", fd,err_save);
        }
    else
        {
        printf("Device read successful : %s\n",receive_buff);}

    //close device and check for error msg
    fd = close(fd);
    err_save = errno;
    if (fd < 0)
        {
        perror("close perror");
        printf("error closing device, fd = %d, err_save = %d \n", fd,err_save);
        }
    else
        {printf("Device closed\n");}
    return 0;
}    

successful result:

Device opened
Device read successful : I already told you 7 times Hello world!
w�0 ����
Device closed

However, when memset(receive_buff, 0, sizeof(receive_buff)); is uncommented, I get the following:

open perror: File exists
error opening device, fd = -1, err_save = 17 
read perror: Bad file descriptor
error reading device, fd = -1, err_save = 9 
close perror: Bad file descriptor
error closing device, fd = -1, err_save = 9 

Question: How does the additional memset() causes the open() to fail ?

Answer 1

open takes an integer as the second parameter (you’re getting it confused with fopen). Your open line should be:

fd = open("/dev/chardev", O_RDWR);

The reason it works or fails as code is added and removed has to to with the unpredictable value of the address of "rw", which could happen to be a valid value for open when memset is removed.