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Posted By

moshe on 05/21/12


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Performing sprintf without overflowing the buffer

 / Published in: C++
 

The problem is that we don't know the size of the required output buffer. Providing a static buffer, no matter its size, runs a risk of overrunning the buffer.

This snippet uses vsnprintf() to output an sprintf() string based on a variable length parameter list to a dynamically allocated output buffer. The function is provided with the size of the output buffer, and it returns -1 if the buffer is not large enough. In this case, the code reallocates a larger buffer and attempts once again to output the string. The vsnprintf() will succeed when the buffer is large enough.

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  1. void LogLine (const char *mask, const char* format, ...)
  2. {
  3. 	static bool firstTime = true;
  4. 	static char *outputBuffer = 0;
  5. 	static size_t outputBufferSize = 0;
  6. 	static size_t outputBufferSizeIncrement = (4 * 1024);
  7.  
  8. 	if (firstTime)
  9. 	{
  10. 		firstTime = false;
  11. 		outputBufferSize = (16 * 1024);
  12. 		outputBuffer = new char [outputBufferSize];
  13. 	}
  14.  
  15. 	va_list argptr;
  16. 	va_start (argptr, format);
  17.  
  18. 	int numCharsWritten = 0;
  19.  
  20. 	while ((numCharsWritten = vsnprintf (outputBuffer, outputBufferSize, format, argptr)) < 0)
  21. 	{
  22. 		// <outputBuffer> not large enough to receive the output, increase the buffer
  23. 		outputBufferSize += outputBufferSizeIncrement;
  24. 		delete[] outputBuffer;
  25. 		outputBuffer = new char [outputBufferSize];
  26. 	}
  27.  
  28. 	va_end (argptr);
  29. 	. . .
  30. }

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