When and why can sprintf fail?

Question

I'm using swprintf to build a string into a buffer (using a loop among other things).

const int MaxStringLengthPerCharacter = 10 + 1;
wchar_t* pTmp = pBuffer;
for ( size_t i = 0; i < nNumPlayers ; ++i)
{
    const int nPlayerId = GetPlayer(i);
    const int nWritten = swprintf(pTmp, MaxStringLengthPerCharacter, TEXT("%d,"), nPlayerId);
    assert(nWritten >= 0 );
    pTmp += nWritten;
}

*pTaskPlayers = '\0';

If during testing the assert never hits, can I be sure that it will never hit in live code? That is, do I need to check if nWritten < 0 and handle that, or can I safely assume that there won't be a problem?

Under which circumstances can it return -1? The documentation more or less just states "If the function fails". In one place I've read that it will fail if it can't match the arguments (i.e. the formatting string to the varargs) but that doesn't worry me.

I'm also not worried about buffer overrun in this case - I know the buffer is big enough.

Answer 1

From the c99 standard:

The sprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

This generally happens only with the multi-byte and wide character character set functions.