Wine Developer's Guide/Debug Logging

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To better manage the large volume of debugging messages that Wine can generate, we divide the messages on a component basis, and classify them based on the severity of the reported problem. Therefore a message belongs to a channel and a class respectively.

This section will describe the debugging classes, how you can create a new debugging channel, what the debugging API is, and how you can control the debugging output. A picture is worth a thousand words, so here are a few examples of the debugging API in action:

ERR("lock_count == 0 ... please report\n");
FIXME("Unsupported RTL style!\n");
WARN(": file seems to be truncated!\n");
TRACE("[%p]: new horz extent = %d\n", hwnd, extent );
MESSAGE( "Could not create graphics driver '%s'\n", buffer );

1 Debugging classes

A debugging class categorizes a message based on the severity of the reported problem. There is a fixed set of classes, and you must carefully choose the appropriate one for your messages. There are five classes of messages:

Messages in this class are meant to signal unimplemented features, known bugs, etc. They serve as a constant and active reminder of what needs to be done.
Messages in this class indicate serious errors in Wine, such as as conditions that should never happen by design.
These are warning messages. You should report a warning when something unwanted happens, and the function cannot deal with the condition. This is seldom used since proper functions can usually report failures back to the caller. Think twice before making the message a warning.
These are detailed debugging messages that are mainly useful to debug a component. These are turned off unless explicitly enabled.
There messages are intended for the end user. They do not belong to any channel. As with warnings, you will seldom need to output such messages.

2 Debugging channels

Each component is assigned a debugging channel. The identifier of the channel must be a valid C identifier (reserved words like int or static are permitted). To use a new channel, simply use it in your code. It will be picked up automatically by the build process.

Typically, a file contains code pertaining to only one component, and as such, there is only one channel to output to. You can declare a default channel for the file using the WINE_DEFAULT_DEBUG_CHANNEL() macro:

#include "wine/debug.h"


    FIXME("some unimplemented feature", ...);
    if (zero != 0)
        ERR("This should never be non-null: %d", zero);

In rare situations there is a need to output to more than one debug channel per file. In such cases, you need to declare all the additional channels at the top of the file, and use the _-version of the debugging macros:

#include "wine/debug.h"


    FIXME("this one goes to xxx channel");
    FIXME_(yyy)("Some other msg for the yyy channel");
    WARN_(zzz)("And yet another msg on another channel!");

3 Are we debugging?

To test whether the debugging channel xxx is enabled, use the TRACE_ON, WARN_ON, FIXME_ON, or ERR_ON macros. For example:


You should normally need to test only if TRACE_ON, all the others are very seldom used. With careful coding, you can avoid the use of these macros, which is generally desired.

4 Helper functions

Resource identifiers can be either strings or numbers. To make life a bit easier for outputting these beasts (and to help you avoid the need to build the message in memory), I introduced a new function called debugres.

The function is defined in wine/debug.h and has the following prototype:

LPSTR debugres(const void *id);

It takes a pointer to the resource id and returns a nicely formatted string of the identifier (which can be a string or a number, depending on the value of the high word). Numbers are formatted as such:


while strings as:


Simply use it in your code like this:

#include "wine/debug.h"


TRACE("resource is %s", debugres(myresource));

Many times strings need to be massaged before output: they may be NULL, contain control characters, or they may be too long. Similarly, Unicode strings need to be converted to ASCII for usage with the debugging API. For all this, you can use the debugstr_[aw]n? family of functions:

    FIXME("(%s): stub\n", debugstr_a(s));

5 Controlling the debugging output

It is possible to turn on and off debugging output from within the debugger using the set command. Please see the (WineDbg Command Reference section)

You can do the same using the task manager (taskmgr) and selecting your application in the Processes list. Right clicking on the process, and selecting the Edit Debug Channels option in the popup menu, will let you select the modifications you want on the debug channels.

Another way to conditionally log debug output (e.g. in case of very large installers which may create gigabytes of log output) is to create a pipe:

$ mknod /tmp/debug_pipe p

and then to run wine like that:

$ WINEDEBUG=+relay,+snoop wine setup.exe &>/tmp/debug_pipe

Since the pipe is initially blocking (and thus wine as a whole), you have to activate it by doing:

$ cat /tmp/debug_pipe

(press Ctrl-C to stop pasting the pipe content)

Once you are about to approach the problematic part of the program, you just do:

$ cat /tmp/debug_pipe >/tmp/wine.log

to capture specifically the part that interests you from the pipe without wasting excessive amounts of HDD space and slowing down installation considerably.

The WINEDEBUG environment variable controls the output of the debug messages. It has the following syntax: WINEDEBUG=[yyy]#xxx[,[yyy1]#xxx1]*

  • where # is either + or -

  • when the optional class argument (yyy) is not present, then the statement will enable(+)/disable(-) all messages for the given channel (xxx) on all classes. For example:


    enables all messages on the reg channel and disables all messages on the file channel.

  • when the optional class argument (yyy) is present, then the statement will enable (+)/disable(-) messages for the given channel (xxx) only on the given class. For example:


    enables trace messages on the reg channel and disables warning messages on the file channel.

  • also, the pseudo-channel all is also supported and it has the intuitive semantics:

    WINEDEBUG=+all      -- enables all debug messages
    WINEDEBUG=-all      -- disables all debug messages
    WINEDEBUG=yyy+all   -- enables debug messages for class yyy on all channels
    WINEDEBUG=yyy-all   -- disables debug messages for class yyy on all channels

    So, for example:

    WINEDEBUG=warn-all  -- disables all warning messages

Also, note that at the moment:

  • the FIXME and ERR classes are enabled by default
  • the TRACE and WARN classes are disabled by default

6 A Few Notes on Style

This new scheme makes certain things more consistent but there is still room for improvement by using a common style of debug messages. Before I continue, let me note that the output format is the following:

yyy:xxx:fff message


yyy = the class (fixme, err, warn, trace)
xxx = the channel (atom, win, font, etc)
fff = the function name

These fields are output automatically. All you have to provide is the message part.

So here are some ideas:

  • do not include the name of the function: it is included automatically

  • if you want to output the parameters of the function, do it as the first thing and include them in parentheses, like this:

    TRACE("(%d, %p, ...)\n", par1, par2, ...);
  • if you want to name a parameter, use = :

    TRACE("(fd=%d, file=%s): stub\n", fd, name);
  • for stubs, you should output a FIXME message. I suggest this style:

    FIXME("(%x, %d, ...): stub\n", par1, par2, ...);
  • try to output one line per message. That is, the format string should contain only one '\n' and it should always appear at the end of the string.

  • if the output string needs to be dynamically constructed, render it in memory before outputting it:

    char buffer[128] = "";
    if (flags & FLAG_A) strcat(buffer, "FLAG_A ");
    if (flags & FLAG_B) strcat(buffer, "FLAG_B ");
    if (flags & FLAG_C) strcat(buffer, "FLAG_C ");
    TRACE("flags = %s\n", buffer);

    Most of the time however, it is better to create a helper function that renders to a temporary buffer:

    static const char *dbgstr_flags(int flags)
        char buffer[128] = "";
        if (flags & FLAG_A) strcat(buffer, "FLAG_A ");
        if (flags & FLAG_B) strcat(buffer, "FLAG_B ");
        if (flags & FLAG_C) strcat(buffer, "FLAG_C ");
        return wine_dbg_sprintf("flags = %s\n\n", buffer);
    TRACE("flags = %s\n", dbgstr_flags(flags));