getline.c

// MSVC apparently doesn't have C's getline
// This is a poor man's implementation of it using fgets.
// See the man page at
// http://pubs.opengroup.org/onlinepubs/9699919799/functions/getdelim.html
size_t getline(char** buf, size_t* bufLen, FILE* f)
{
	if (buf == nullptr || bufLen == nullptr)
	{
		errno = EINVAL;
		return -1;
	}
	
	if (fgets(*buf, *bufLen, f) != *buf)
		return -1;

	while (true)
	{
		const size_t amountRead = strlen(*buf);
		// If the length of the string is less than the whole buffer
		// (minus the last space for \0) or the last character is a newline,
		// we win. Done.
		if (amountRead != *bufLen - 1 || (*buf)[amountRead - 1] == '\n')
			return amountRead;

		// We didn't have enought room, expand with realloc.

		// First make sure we can.

		// If we can't take any more, give up.
		if (*bufLen == SSIZE_MAX)
		{
			errno = EOVERFLOW;
			return -1;
		}

		// If the MSB of bufLen is 1, doubling will overflow.
		const bool willOverflow = (*bufLen >> (sizeof(size_t) * 8 - 1)) == 1;
		const size_t newSize = willOverflow ? SSIZE_MAX : *bufLen * 2;

		char* newBuf = static_cast<char*>(realloc(*buf, newSize));
		if (newBuf == nullptr)
		{
			errno = ENOMEM;
			return -1;
		}
		// We succeeded in expanding.
		*buf = newBuf;
		*bufLen = newSize;
		// Keep reading where we left off
		char* const resumePoint = *buf + amountRead;
		if (fgets(resumePoint, *bufLen - amountRead, f) != resumePoint)
			return -1;
	}
}