This document is a collection of notes regarding tools and techniques for debugging Apache and Apache modules.

Got more tips? Send 'em to docs@httpd.apache.org. Thanks!

1. Using gdb
2. Getting a live backtrace
3. Using 'truss/trace/strace' to trace system calls and signals
4. Getting the server to dump core
5. Solaris 2.7 and coredumps
6. Getting and analyzing a TCP packet trace

If you use the gcc or egcs compilers, it is likely that the best debugger for your system is gdb. This is only a brief summary of how to run gdb on Apache -- you should look at the info and man files for gdb to get more information on gdb commands and common debugging techniques. Before running gdb, be sure that the server is compiled with the -g option in EXTRA_CFLAGS to include the symbol information in the object files.

The only tricky part of running gdb on Apache is forcing the server into a single-process mode so that the parent process being debugged does the request-handling work instead of forking child processes. We have provided the -X option for that purpose, which will work fine for most cases. However, some modules don't like starting up with -X, but are happy if you force only one child to run (using "MaxClients 1"); you can then use gdb's attach command to debug the child server.

The following example, with user input in green, shows the output of gdb run on a server executable (httpd) in the current working directory and using the server root of /usr/local/apache:

    % gdb httpd
    GDB is free software and you are welcome to distribute copies of it
     under certain conditions; type "show copying" to see the conditions.
    There is absolutely no warranty for GDB; type "show warranty" for details.
    GDB 4.16.gnat.1.13 (sparc-sun-solaris2.5), 
    Copyright 1996 Free Software Foundation, Inc...
    (gdb) b ap_process_request
    Breakpoint 1 at 0x49fb4: file http_request.c, line 1164.
    (gdb) run -X -d /usr/local/apache
    Starting program: /usr/local/apache/src/httpd -X -d /usr/local/apache
    
    [at this point I make a request from another window]

    Breakpoint 1, ap_process_request (r=0x95250) at http_request.c:1164
    1164        if (ap_extended_status)
    (gdb) s
    1165            ap_time_process_request(r->connection->child_num, START_PREQUEST);
    (gdb) n
    1167        process_request_internal(r);
    (gdb) s
    process_request_internal (r=0x95250) at http_request.c:1028
    1028        if (!r->proxyreq && r->parsed_uri.path) {
    (gdb) s
    1029            access_status = ap_unescape_url(r->parsed_uri.path);
    (gdb) n
    1030            if (access_status) {
    (gdb) s
    1036        ap_getparents(r->uri);     /* OK --- shrinking transformations... */
    (gdb) n
    1038        if ((access_status = location_walk(r))) {
    (gdb) n
    1043        if ((access_status = ap_translate_name(r))) {
    (gdb) n
    1048        if (!r->proxyreq) {
    (gdb) n
    1053            if (r->method_number == M_TRACE) {
    (gdb) n
    1062        if (r->proto_num > HTTP_VERSION(1,0) && ap_table_get(r->subprocess_env, "downgrade-1.0")) {
    (gdb) n
    1071        if ((access_status = directory_walk(r))) {
    (gdb) s
    directory_walk (r=0x95250) at http_request.c:288
    288         core_server_config *sconf = ap_get_module_config(r->server->module_config,
    (gdb) b ap_send_error_response
    Breakpoint 2 at 0x47dcc: file http_protocol.c, line 2090.
    (gdb) c
    Continuing.
    
    Breakpoint 2, ap_send_error_response (r=0x95250, recursive_error=0)
        at http_protocol.c:2090
    2090        BUFF *fd = r->connection->client;
    (gdb) where
    #0  ap_send_error_response (r=0x95250, recursive_error=0)
        at http_protocol.c:2090
    #1  0x49b10 in ap_die (type=403, r=0x95250) at http_request.c:989
    #2  0x49b60 in decl_die (status=403, phase=0x62db8 "check access", r=0x95250)
        at http_request.c:1000
    #3  0x49f68 in process_request_internal (r=0x95250) at http_request.c:1141
    #4  0x49fe0 in ap_process_request (r=0x95250) at http_request.c:1167
    #5  0x439d8 in child_main (child_num_arg=550608) at http_main.c:3826
    #6  0x43b5c in make_child (s=0x7c3e8, slot=0, now=907958743)
        at http_main.c:3898
    #7  0x43ca8 in startup_children (number_to_start=6) at http_main.c:3972
    #8  0x44260 in standalone_main (argc=392552, argv=0x75800) at http_main.c:4250
    #9  0x449fc in main (argc=4, argv=0xefffee8c) at http_main.c:4534
    (gdb) s
    2091        int status = r->status;
    (gdb) p status
    $1 = 403
    (gdb) 

There are a few things to note about the above example:

1. the "gdb httpd" command does not include any command-line options for httpd: those are provided when the "run" command is done within gdb;
2. I set a breakpoint before starting the run so that execution would stop at the top of ap_process_request();
3. the "s" command steps through the code and into called procedures, whereas the "n" (next) command steps through the code but not into called procedures.
4. additional breakpoints can be set with the "b" command, and the run continued with the "c" command.
5. use the "where" command (a.k.a. "bt") to see a stack backtrace that shows the order of called procedures and their parameter values.
6. use the "p" command to print the value of a variable.

A file in the src/ directory, .gdbinit, provides a useful macro for printing out the contents of a table structure, called dump_table.

If you are debugging a repeatable crash, simply run gdb as above and make the request -- gdb should capture the crash and provide a prompt where it occurs.

If you are debugging an apparent infinite loop, simply run gdb as above and type a Control-C -- gdb will interrupt the process and provide a prompt where it was stopped.

If you are debugging a system crash and you have a core file from the crash, then do the following:

    % gdb httpd -c core
    (gdb) where

and it will (hopefully) print a stack backtrace of where the core dump occurred during processing.

A backtrace will let you know the hierarchy of procedures that were called to get to a particular point in the process. On some platforms you can get a live backtrace of any process.

For SVR4-based variants of Unix, the pstack command for proc can be used to display a a live backtrace. For example, on Solaris it looks like

    % /usr/proc/bin/pstack 10623
    10623:  httpd -d /usr/local/apache
     ef5b68d8 poll     (efffcd08, 0, 3e8)
     ef5d21e0 select   (0, ef612c28, 0, 0, 3e8, efffcd08) + 288
     00042574 wait_or_timeout (0, 75000, 75000, 7c3e8, 60f40, 52c00) + 78
     00044310 standalone_main (5fd68, 75800, 75c00, 75000, 2, 64) + 240
     000449f4 main     (3, efffeee4, efffeef4, 75fe4, 1, 0) + 374
     000162fc _start   (0, 0, 0, 0, 0, 0) + 5c

Another technique is to use gdb to attach to the running process and then using "where" to print the backtrace, as in

    % gdb httpd 10623
    GDB is free software and you are welcome to distribute copies of it
     under certain conditions; type "show copying" to see the conditions.
    There is absolutely no warranty for GDB; type "show warranty" for details.
    GDB 4.16.gnat.1.13 (sparc-sun-solaris2.5), 
    Copyright 1996 Free Software Foundation, Inc...
    
    /usr/local/apache/src/10623: No such file or directory.
    Attaching to program `/usr/local/apache/src/httpd', process 10623
    Reading symbols from /usr/lib/libsocket.so.1...done.
    Reading symbols from /usr/lib/libnsl.so.1...done.
    Reading symbols from /usr/lib/libc.so.1...done.
    Reading symbols from /usr/lib/libdl.so.1...done.
    Reading symbols from /usr/lib/libintl.so.1...done.
    Reading symbols from /usr/lib/libmp.so.1...done.
    Reading symbols from /usr/lib/libw.so.1...done.
    Reading symbols from /usr/platform/SUNW,Ultra-1/lib/libc_psr.so.1...done.
    0xef5b68d8 in   ()
    (gdb) where
    #0  0xef5b68d8 in   ()
    #1  0xef5d21e8 in select ()
    #2  0x4257c in wait_or_timeout (status=0x0) at http_main.c:2357
    #3  0x44318 in standalone_main (argc=392552, argv=0x75800) at http_main.c:4273
    #4  0x449fc in main (argc=3, argv=0xefffeee4) at http_main.c:4534
    (gdb) 

Most Unix-based systems have at least one command for displaying a trace of system calls and signals as they are accessed by a running process. This command is called truss on most SVR4-based systems and either trace or strace on many other systems.

A useful tip for using the truss command on Solaris is the -f option; it tells truss to follow and continue tracing any child processes forked by the main process. The easiest way to get a full trace of a server is to do something like:

    % truss -f httpd -d /usr/local/apache >& outfile

    % egrep '^10698:' outfile

to view just the trace of the process id 10698.

Strangely enough, sometimes you actually want to force the server to crash so that you can get a look at some nutty behavior. Normally this can be done simply by using the gcore command. However, for security reasons, most Unix systems do not allow a setuid process to dump core, since the file contents might reveal something that is supposed to be protected in memory.

Here is one way to get a core file from a setuid Apache httpd process on Solaris, without knowing which httpd child might be the one to die [note: it is probably easier to use the MaxClients trick in the first section above].

    # for pid in `ps -eaf | fgrep httpd | cut -d' ' -f4`
    do
      truss -f -l -t\!all -S SIGSEGV -p $pid 2>&1 | egrep SIGSEGV &
    done

The undocumented '-S' flag to truss will halt the process in place upon receipt of a given signal (SIGSEGV in this case). At this point you can use:

    # gcore PID

and then look at the backtrace as discussed above for gdb.

On Solaris 2.7 use coreadm to make setuid() processes actually dump core. By default an setuid() process does not dump core.

Jens-Uwe Mager wrote:

For example I am using:

	# coreadm
   	  global core file pattern: /var/core/core.%f.%p.u%u
     	    init core file pattern: core
      	         global core dumps: enabled
            per-process core dumps: enabled
           global setid core dumps: enabled
      per-process setid core dumps: enabled
          global core dump logging: disabled

This is more difficult than I have time to describe at the moment. Here are some pointers to useful discussions and tools:

• tools for producing TCP dumps
• tcptrace is a TCP dump file analysis tool
• tcpshow is another

There is also a simple ASCII viewer for TCP dump traces in the Apache repository in the file src/test/tcpdumpscii.txt.