Bug 10515 - cgi lock when writing to stderr
Summary: cgi lock when writing to stderr
Status: CLOSED DUPLICATE of bug 22030
Alias: None
Product: Apache httpd-2
Classification: Unclassified
Component: mod_cgi (show other bugs)
Version: 2.0.39
Hardware: PC All
: P3 major with 7 votes (vote)
Target Milestone: ---
Assignee: Apache HTTPD Bugs Mailing List
Depends on:
Reported: 2002-07-05 17:35 UTC by Peter Whiting
Modified: 2004-11-16 19:05 UTC (History)
3 users (show)


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Description Peter Whiting 2002-07-05 17:35:27 UTC
I just upgraded to 2.0 today - all of my cgi-scripts that write debug to stderr
now hang. Prior to the upgrade, the output from these scripts showed up in the
error log. I've verified that the error log exists and can be written to by the
httpd process owner. Removing the debug statements from the CGI scripts fixes
the problem.
Comment 1 Joshua Slive 2002-07-09 17:19:38 UTC
Are you writing a very large quantity of stuff to stderr?
Comment 2 William A. Rowe Jr. 2002-07-09 17:40:23 UTC
  This is not a new issue.  Linux has a 64kb pipe limit by default.  This means
  that once our stdout or stderr have in excess of 64kb written, before Apache
  has finished reading the request body and piping it to your CGI's stdin, you
  will deadlock.

  Try deferring your stderr logging until the entire body is read and the entire
  response is written.
Comment 3 Brian Nelson 2002-08-22 01:31:54 UTC
This is an apache-2.0 only issue.  1.3 does not exhibit this problem, and is
seriously hampering development for us.  

"Deferring your stderr logging until the entire body is read and the entire
response is written." doesn't work for large scripts, sites, development
environments, etc.  

Also, this system will hang with as little as 4k of std_err action at leasr on
RH 6.soemthing, apache 2.0.40...  FreeBSD seems immune to this wackyness....

for example:
// Credit to: K.C. Wong
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <fcntl.h>

#define SIZE 4075

void out_err()
	char buffer[SIZE];
	int i = 0;

	for (i = 0; i < SIZE - 1; ++i)
		buffer[i] = 'a' + (char )(i % 26);

	buffer[SIZE - 1] = '\0';

fcntl(2, F_SETFL, fcntl(2, F_GETFL) | O_NONBLOCK);

	fprintf(stderr, "short test\n");

	fprintf(stderr, "test error=%s\n", buffer);
} // out_err()

int main(int argc, char ** argv)
	fprintf(stdout, "Context-Type: text/html\r\n");
	fprintf(stdout, "\r\n\r\n");
	fprintf(stdout, "<HTML>\n");
	fprintf(stdout, "<body>\n");
	fprintf(stdout, "<h1>hello world</h1>\n");
	fprintf(stdout, "</body>\n");
	fprintf(stdout, "</HTML>\n");
} // main()
Comment 4 William A. Rowe Jr. 2002-08-22 04:01:15 UTC
  I will preface this with a single observation, we will need to expand the
  apr_poll api to include other 'objects', notably httpd-2.0 filters.  The
  only way to accomplish this is to provide a set of added callbacks to
  query the filter chain for a data-ready state, and then add the filter
  endpoints (e.g. sockets or pipes) to the pollset if there is no data 
  ready from any filter [or other extended] source or ready to send to any 
  destination sink.

  It is doable, but it is dirty.

  It's my impression that the default pipe size on Linux had been 64kb.
  Did this change somewhere along the way?  Is it a kernel tuning option
  or can it be controlled with something like ioctl?

  If we can make the CGI pipes accept 64kb, at least, this remains a problem
  for both Win32 and Linux, but at least it's a sane boundry instead of 4kb!!!

Comment 5 Brian Nelson 2002-08-27 04:50:09 UTC
Hello.  I need to give a status update on this issue...  is there anyone 
activly working on/assigned to a resolution for this issue?  If not, is there 
perhaps a way I can escalate the issue in any way?  I would offer to help, but 
I am not much of a developer, but this is a major stopping block for us at this 

Thanks in advance.  If I can help in any way, please let me know.
Comment 6 Mark Cox 2002-09-26 20:30:13 UTC
Someone asked me if this was an Apache or OS issue; I asked our kernel guys who

"if nothing is reading the pipe (or the reader is held off from reading by your
writer user space locking) then 4K is the internal buffering on a linux pipe. 
You can't change that.

Ok my assessment is that you either hit a weird glibc bug in stdio with pipes or
apache 2.x has a deadlock in its own pipe handling"
Comment 7 Michael G. Martin 2002-10-08 21:59:36 UTC
Ran into the same behavior today.  After upgrading to Redhat 8, and apache
2.0.40.  I just installed 2.0.43 and it exhibits the same lock.  Both PERL and C
++ programs exhibit the locking.  Locking only happens when writing to STDERR. 
This is using a stock Redhat 8 install.  Can reproduce on apache 2.0.40-8 (
redhat default) and fresh build of 2.0.43.

Sample C++ CGI Program:
#include <iostream>
#include <sys/types.h>
#include <unistd.h>

using namespace std;

int main() {
  cout << "Content-Type:text/plain\n\n";
  cout << getpid() << endl;
  sleep (20); // allow me to hook an strace to the pid

  for (int i=0; i < 1000; i++) {
    cout << "Here " << i << endl;
    cerr << "Here " << i << endl;
  return 0;

Here is the last of an strace up to hang:

write(1, "Here 466\n", 9)               = 9
write(2, "H", 1)                        = 1
write(2, "e", 1)                        = 1
write(2, "r", 1)                        = 1
write(2, "e", 1)                        = 1
write(2, " ", 1)                        = 1
write(2, "4", 1)                        = 1
write(2, "6", 1)                        = 1
write(2, "6", 1)                        = 1
write(2, "\n", 1)                       = 1
write(1, "Here 467\n", 9)               = 9
write(2, "H", 1)                        = 1
write(2, "e", 1)                        = 1
write(2, "r", 1)                        = 1
write(2, "e", 1

Comment 8 Michael G. Martin 2002-10-08 22:18:59 UTC
I just installed a copy of apache 1.3.27.  The same sample program runs fine. 
Always completes.
Comment 9 Christian Lizell 2003-09-22 10:50:50 UTC
On standard RH8 and RH9 installations this script causes locking;


print "Content-type: text/html\n\n";

print STDERR "This will lock cgi\n" x 220; ## Slightly more than 4k

There are no problems on the same installations with apache 1.3.

(More information on the same issue:
Comment 10 Jeff Trawick 2003-09-22 11:21:55 UTC
the newer PR #22030 has more info, so leave that one open

*** This bug has been marked as a duplicate of 22030 ***