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System Error Text Messages
This article originally appeared in the February 1999 issue of Delphi Informant
Magazine and is being reprinted here with permission from Informant
Communications.
How many times have you seen this reference in the Windows API Help?
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error
information, call GetLastError.
You see� while programmers around the world were sleeping, Microsoft decided
that it was getting too difficult (and inefficient) to return all possible error
values as the result of most function calls. Therefore, they reworked the idea
of obtaining error values through a separate API function, namely GetLastError.
Many API functions however retain some backward compatibility and still return
the error results themselves (in addition to setting the value GetLastError
uses).
However, we need to keep up with the times folks. As time goes on, the function
return values for errors we have come to rely on will quite likely just go away.
A good example of this was the LoadLibrary command. When it succeeds, it returns
the handle of the library (DLL or EXE) specified. However, when it failed, it
used to return a small integer value (between 0 and 32). Now however, if it
fails, it simply returns a zero (NULL) and requires that you go to GetLastError
to get the real error result.
Obviously this is a much more efficient solution to obtaining error values.
After all, mixing error values in with success values as the return result of a
function requires that you know as a programmer which ones are errors and which
ones are good function results.
Everything is wonderful now right? Well� No. If you look at GetLastError, it is
still just returning a numeric result which still needs to be interpreted by
your program. What if you want to get a text message of what the error is?
Microsoft does include another function called FormatMessage which can take the
result of GetLastError and make an intelligible error message out of it. Problem
is that FormatMessage is not the most easy to use function.
Fortunately Delphi comes to the rescue again. In Delphi�s SysUtils unit, there
is a very useful wrapper function around FormatMessage called SysErrorMessage.
Whenever you get an error result from an API function, you simply pass
GetLastError into a call to SysErrorMessage. The result is a string
representation of the error.
For example,
ShowMessage(SysErrorMessage(GetLastError));
will use the result from SysErrorMessage and show it in a dialog box for you.
Pretty handy!
Do keep in mind however that the value stored in GetLastError is very temporary.
If you call a function and get an error result, make sure you call GetLastError
immediately. Its value may change if you calling other API functions before you
go and retrieve its value.
Note also that you can use SetLastError to set your own error values. This is
normally done for functions stored in DLL�s. You can learn more about this in
the Win32 API help under the section on SetLastError.
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