Logging Support for DLLs using C

Writing Flexible Debug Logs in a C DLL

Lately, I was working on a DLL written in C that is called from .NET Web Services, and I ran into the challenge of how to debug the DLL effectively.

I found it more convenient to write debug output to a file. However, I needed the logging mechanism to be flexible enough to accept a variable number of arguments. Using a quick and practical approach, I ended up leveraging va_list along with vfprintf / vfprintf_s.

Below is the implementation. Enjoy 🙂

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Variadic Logging Function in C

#include <stdio.h>
#include <stdarg.h>

int WriteLOG(char *format, ...)
{
    FILE *logFile;
    int status = 0;
    va_list arguments;

    va_start(arguments, format);

    fopen_s(&logFile, "D:\\logfile.log", "a+");
    if (logFile != NULL)
    {
        status = vfprintf_s(logFile, format, arguments);
        fclose(logFile);
    }

    va_end(arguments);
    return status;
}

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How It Works

• Uses variadic arguments (...) to accept a flexible number of parameters
• va_list, va_start, and va_end manage the argument list
• vfprintf_s safely writes formatted output to a log file
• Logs are appended to D:\logfile.log

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Example Usage

WriteLOG("Error code: %d, Message: %s\n", errorCode, errorMessage);

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This approach provides a simple and effective way to debug native DLLs invoked from managed environments like .NET.