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Valgrind 3.3 - Advanced Debugging and Profiling for GNU/Linux applications
by J. Seward, N. Nethercote, J. Weidendorfer and the Valgrind Development Team
Paperback (6"x9"), 164 pages
ISBN 0954612051
RRP £12.95 ($19.95)

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3.3 The Commentary

Valgrind tools write a commentary, a stream of text, detailing error reports and other significant events. All lines in the commentary have following form:

==12345== some-message-from-Valgrind

The ‘12345’ is the process ID. This scheme makes it easy to distinguish program output from Valgrind commentary, and also easy to differentiate commentaries from different processes which have become merged together, for whatever reason.

By default, Valgrind tools write only essential messages to the commentary, so as to avoid flooding you with information of secondary importance. If you want more information about what is happening, re-run, passing the -v flag to Valgrind. A second -v gives yet more detail.

You can direct the commentary to three different places:

  1. The default: send it to a file descriptor, which is by default 2 (stderr). So, if you give the core no options, it will write commentary to the standard error stream. If you want to send it to some other file descriptor, for example number 9, you can specify --log-fd=9. This is the simplest and most common arrangement, but can cause problems when Valgrinding entire trees of processes which expect specific file descriptors, particularly stdin/stdout/stderr, to be available for their own use.
  2. A less intrusive option is to write the commentary to a file, which you specify by --log-file=filename. There are special format specifiers that can be used to use a process ID or an environment variable name in the log file name. These are useful/necessary if your program invokes multiple processes (especially for MPI programs). See the basic options section (see 3.6.2) for more details.
  3. The least intrusive option is to send the commentary to a network socket. The socket is specified as an IP address and port number pair, like this: --log-socket=192.168.0.1:12345 if you want to send the output to host IP 192.168.0.1 port 12345 (note: we have no idea if 12345 is a port of pre-existing significance). You can also omit the port number: --log-socket=192.168.0.1, in which case a default port of 1500 is used. This default is defined by the constant ‘VG_CLO_DEFAULT_LOGPORT’ in the sources. Note, unfortunately, that you have to use an IP address here, rather than a hostname. Writing to a network socket is pointless if you don't have something listening at the other end. We provide a simple listener program, ‘valgrind-listener’, which accepts connections on the specified port and copies whatever it is sent to stdout. Probably someone will tell us this is a horrible security risk. It seems likely that people will write more sophisticated listeners in the fullness of time. valgrind-listener can accept simultaneous connections from up to 50 Valgrinded processes. In front of each line of output it prints the current number of active connections in round brackets. valgrind-listener accepts two command-line flags:
    • -e or --exit-at-zero: when the number of connected processes falls back to zero, exit. Without this, it will run forever, that is, until you send it Control-C.
    • portnumber: changes the port it listens on from the default (1500). The specified port must be in the range 1024 to 65535. The same restriction applies to port numbers specified by a --log-socket to Valgrind itself.
    If a Valgrinded process fails to connect to a listener, for whatever reason (the listener isn't running, invalid or unreachable host or port, etc), Valgrind switches back to writing the commentary to stderr. The same goes for any process which loses an established connection to a listener. In other words, killing the listener doesn't kill the processes sending data to it.

Here is an important point about the relationship between the commentary and profiling output from tools. The commentary contains a mix of messages from the Valgrind core and the selected tool. If the tool reports errors, it will report them to the commentary. However, if the tool does profiling, the profile data will be written to a file of some kind, depending on the tool, and independent of what --log-* options are in force. The commentary is intended to be a low-bandwidth, human-readable channel. Profiling data, on the other hand, is usually voluminous and not meaningful without further processing, which is why we have chosen this arrangement.

ISBN 0954612051Valgrind 3.3 - Advanced Debugging and Profiling for GNU/Linux applicationsSee the print edition