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Is there any way to determine how many characters will be written by sprintf?

Asked 2009-02-05T07:06:37.440
16

I'm working in C++.

I want to write a potentially very long formatted string using sprintf (specifically a secure counted version like _snprintf_s, but the idea is the same). The approximate length is unknown at compile time so I'll have to use some dynamically allocated memory rather than relying on a big static buffer. Is there any way to determine how many characters will be needed for a particular sprintf call so I can always be sure I've got a big enough buffer?

My fallback is I'll just take the length of the format string, double it, and try that. If it works, great, if it doesn't I'll just double the size of the buffer and try again. Repeat until it fits. Not exactly the cleverest solution.

It looks like C99 supports passing NULL to snprintf to get the length. I suppose I could create a module to wrap that functionality if nothing else, but I'm not crazy about that idea.

Maybe an fprintf to "/dev/null"/"nul" might work instead? Any other ideas?

EDIT: Alternatively, is there any way to "chunk" the sprintf so it picks up mid-write? If that's possible it could fill the buffer, process it, then start refilling from where it left off.

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2 Answers

0

I've looked for the same functionality you're talking about, but as far as I know, something as simple as the C99 method is not available in C++, because C++ does not currently incorporate the features added in C99 (such as snprintf).

Your best bet is probably to use a stringstream object. It's a bit more cumbersome than a clearly written sprintf call, but it will work.

answered 2009-02-05T07:17:41.677
0

Take a look at CodeProject: CString-clone Using Standard C++. It uses solution you suggested with enlarging buffer size.

// -------------------------------------------------------------------------
    // FUNCTION:  FormatV
    //      void FormatV(PCSTR szFormat, va_list, argList);
    //

    // DESCRIPTION:
    //      This function formats the string with sprintf style format-specs. 
    //      It makes a general guess at required buffer size and then tries
    //      successively larger buffers until it finds one big enough or a
    //      threshold (MAX_FMT_TRIES) is exceeded.
    //
    // PARAMETERS: 
    //      szFormat - a PCSTR holding the format of the output
    //      argList - a Microsoft specific va_list for variable argument lists
    //
    // RETURN VALUE: 
    // -------------------------------------------------------------------------



void FormatV(const CT* szFormat, va_list argList)
{
#ifdef SS_ANSI

    int nLen    = sslen(szFormat) + STD_BUF_SIZE;
    ssvsprintf(GetBuffer(nLen), nLen-1, szFormat, argList);
    ReleaseBuffer();
#else
    CT* pBuf            = NULL;
    int nChars          = 1;
    int nUsed           = 0;
    size_type nActual   = 0;
    int nTry            = 0;

    do  
    {
        // Grow more than linearly (e.g. 512, 1536, 3072, etc)

        nChars          += ((nTry+1) * FMT_BLOCK_SIZE);
        pBuf            = reinterpret_cast<CT*>(_alloca(sizeof(CT)*nChars));
        nUsed           = ssnprintf(pBuf, nChars-1, szFormat, argList);

        // Ensure proper NULL termination.
        nActual         = nUsed == -1 ? nChars-1 : SSMIN(nUsed, nChars-1);
        pBuf[nActual+1]= '\0';


    } while ( nUsed < 0 && nTry++ < MAX_FMT_TRIES );

    // assign whatever we managed to format

    this->assign(pBuf, nActual);
#endif
}

answered 2009-02-05T07:19:25.417

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