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Monday, August 14, 2006
 
Solaris: Workaround to stdio's 255 open file descriptors limitation

A quick web search with key words Solaris stdio open file descriptors results in numerous references to stdio's limitation of 255 open file descriptors on Solaris.

#1085341: 32-bit stdio routines should support file descriptors >255, a 14 year old RFE explains the problem and the bug report links to handful of other bugs which are some how related to stdio's open file descriptors limitation.

Now the good news: the wait is finally over. Sun Microsystems finally made a fix/workaround available to the community in the form of a new library called extendedFILE. If you are running Solaris Express (SX) 06/06 or later builds, your system already has the workaround. You just need to enable it to get around the 255 open file descriptors problem with stdio's API. This workaround will be part of the Update 3 release of Solaris 10, which is due in October 2006. There are some plans to backport this workaround to Solaris 9 and 10. However there is no clear timeline for completion of this backport, at the moment.

The workaround does not require any source code changes or re-compilation of the objects. You just need to increase the file descriptor limit using limit or ulimit commands; and pre-load /usr/lib/extendedFILE.so.1 before running the application.

However applications fiddling with _file field of FILE structure may not work. This is because when extendedFILE library is pre-loaded, descriptors > 255 will be stored in an auxiliary location and a fake descriptor will be stored in the FILE structure's _file field. In fact, accessing _file field was long discouraged; and to discourage non-standard practices even further _file has been renamed to _magic starting with SX 06/06. So, applications which access _file directly rather than with fileno() function, may encounter compilation errors starting with S10 U3. This step is necessary to ensure that the source code is in a clean state, so the resulting object code is not vulnerable to data corruption during run-time.

The following example shows the failure and the steps to workaround the issue. Note that with the extendedfile library pre-loaded, the process can open upto 65532 files excluding stdin, out and err.

* Test case (simple C program tries to open 65536 files):

 % cat files.c
 #include <stdio.h>
 #define NoOfFILES 65536

 int main()
 {
  char filename[10];
  FILE *fds[NoOfFILES];
  int i;
 
  for (i = 0; i < NoOfFILES; ++i)
  {
  sprintf (filename, "%d.log", i);
  fds[i] = fopen(filename, "w");
 
  if (fds[i] == NULL)
  {
  printf("\n** Number of open files = %d. fopen() failed with error:  ", i);
  perror("");
  exit(1);
  }
  else
  {
  fprintf (fds[i], "some string");
  }
 
  }
  return (0);
 }

 % cc -o files files.c


* Re-producing the failure:

 % limit
 cputime unlimited
 filesize unlimited
 datasize unlimited
 stacksize 8192 kbytes
 coredumpsize unlimited
 descriptors 256
 memorysize unlimited

 % uname -a
 SunOS sunfire4 5.11 snv_42 sun4u sparc SUNW,Sun-Fire-280R

 %./files

 ** Number of open files = 253. fopen() failed with error: Too many open files


* Showcasing the Solaris workaround:

 % limit descriptors 5000

 % limit
 cputime unlimited
 filesize unlimited
 datasize unlimited
 stacksize 8192 kbytes
 coredumpsize unlimited
 descriptors 5000
 memorysize unlimited

 % setenv LD_PRELOAD_32 /usr/lib/extendedFILE.so.1

 % ./files

 ** Number of open files = 4996. fopen() failed with error: Too many open files

 % limit descriptors 65536

 % limit
 cputime unlimited
 filesize unlimited
 datasize unlimited
 stacksize 8192 kbytes
 coredumpsize unlimited
 descriptors 65536
 memorysize unlimited

 %./files

 ** Number of open files = 65532. fopen() failed with error: Too many open files

(Note that descriptor 0, 1 and 2 will be used by stdin, stdout and stderr)

 % ls -l 65531.log
 -rw-rw-r-- 1 gmandali ccuser 11 Aug 9 12:35 65531.log


For further information about the extendedFILE library and for the extensions to fopen, fdopen, popen, .. please have a look at the new and updated man pages:

extendedFILE
enable_extended_FILE_stdio
fopen
fdopen

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