How can I write to /dev/kmsg?

Question

I want to write to /dev/kmsg so I can compare user space logging in an application to things that are happening in the kernel.

I wrote a simple application that should log a message similar to how you can do this from the command line with echo "foo" > /dev/kmsg

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

int main()
{
  int fdKmsg = -1;
  fdKmsg = open("/dev/kmsg", O_WRONLY);

  if (fdKmsg != -1)
  {
    dprintf(fdKmsg, "-- Hello World --\n");
    close(fd);
  }
  else
  {
    printf("Unable to get file descriptor\n");
  }

  return 0;
}

However, when I run this even as root, I don't see anything either in dmesg or in /proc/kmsg. What am I missing? Even with a call to fsync(), it doesn't appear to write to dmesg.

Answer 1

It appears that dprintf() will not work in this case. However, calling write() will work. I'm not sure why this is, but I would guess that the printf() functions don't call the write method for the exposed /dev interface.

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

int main()
{
  int fdKmsg = -1;
  fdKmsg = open("/dev/kmsg", O_WRONLY);
  char *buf = "Hello World\n";

  if (fdKmsg != -1)
  {
    //dprintf(fdKmsg, "-- Hello World --\n");
    write(fdKmsg, buf, strlen(buf) + 1);
    close(fd);
  }
  else
  {
    printf("Unable to get file descriptor\n");
  }

  return 0;
}