Re: Problems with read() on /proc/devices with x86_64 system

[linux-os (Dick Johnson)]

On Wed, 22 Feb 2006, Martin Schlemmer wrote:

> Hi,
>
> Not sure when it started, but 2.6.16-rc[1234] at least have problems
> with unbuffered read() and /proc/devices on my x86_64 box.  I first
> picked it up with dmsetup that did not want to work properly built
> against klibc (glibc with fread() worked fine though, as it mmap()'d the
> file).
>
> Following code (from HPA and klibc mailing lists), when compiled and run
> with /proc/devices only reads the first two lines and then exits
> normally, where with any other file works as expected.
>
> -----
> #include <stdlib.h>
> #include <stdio.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <sys/stat.h>
>
> int main(int argc, char *argv[])
> {
>  char c;
>  int i, fd, rv;
>
>  for ( i = 1 ; i < argc ; i++ ) {
>    fd = open(argv[i], O_RDONLY);
>    if ( fd < 0 ) {
>      perror(argv[i]);
>      exit(1);
>    }
>
>    while ( (rv = read(fd, &c, 1)) ) {
>      if ( rv == -1 ) {
>        if ( errno == EINTR || errno == EAGAIN )
>          continue;
>
>        perror(argv[i]);
>        exit(1);
>      }
>      putchar(c);
>    }
>
>    close(fd);
>  }
>  return 0;
> }
> -----
>
> Output over here:
>
> -----
> # ./readbychar.klibc /proc/devices
> Character devices:
>  1 mem
> #
> -----
> Thanks,
> Martin Schlemmer

If your code ever worked, it's probably because of some
fortuitous buffering in the 'C' runtime library. Most
of the 'read' code in drivers that have a /proc interface
is not designed for 1-character-at-a-time I/O. It's expected
that it will be accessed like `cat` or `more` or other
such tools access it, -- one read with 4096-byte buffer --

read(3, "MemTotal:       773860 kB\nMemFre"..., 4096) = 670
write(1, "MemTotal:       773860 kB\nMemFre"..., 670) = 670

The read code uses sprintf to write all the parameters to
a buffer, then it copies the parameters to the user. The
next read will return 0 for EOF and reset the interface
for the next access.

If your code read /proc without any help from the 'C' runtime
library, you would read the same first character, every time
you attempted to read a character. Don't do that! Your code
should do (with some error-checking):

         fd = open(argv[1], O_RDONLY);
 	buffer = malloc(LEN);
         read(fd, buffer, len);
         puts(buffer);

Also, something seems somewhat strange because it is not
commonplace to provide a mmap() interface to /proc file-system
capability in drivers and the /proc base code doesn't
provide memory-map capability at least on 2.6.15.4. So,
your reference to memory-mapping the file seems to be
incorrect.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction.
_


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