C - How to use OpenSSL's BIO_write() correctly

Question

I'm new to OpenSSL. I understand BIO_write(BIO *b, const void *buf, int len) needs to be called in a loop but I'm not entirely sure if I'm using it correctly. I've written a function like this:

int32_t SendPacket(BIO * const pBio, const unsigned char * const pPacket, const int nPacketLength)
{
    int32_t nPos = 0;

    if (!pBio || !pPacket || !nPacketLength)
        return -1;

    while (nPos < nPacketLength)
    {
        int32_t nNumberOfBytesWritten = BIO_write(pBio, &pPacket[nPos], nPacketLength - nPos);
        if (nNumberOfBytesWritten <= 0)
        {
            if (!BIO_should_retry(pBio))
                return -1;
        }
        else
        {
            nPos += nNumberOfBytesWritten;
        }
    }

    return nPos;
}

And I'm thinking of using it like this:

if (SendPacket(pBio, pPacket, nPacketLength) == nPacketLength)
{
    // Packet sent correctly.
}
else
{
    // Error occurred.
}

Does the function look correct? Any feedback is appreciated.

Function looks fine. Did you face any problem? – doptimusprime Mar 31 '13 at 09:16 — doptimusprime, Mar 31 '13 at 09:16
Except that nPacketLength should not be -1. – doptimusprime Mar 31 '13 at 09:17 — doptimusprime, Mar 31 '13 at 09:17

score 6 · Accepted Answer · answered Apr 16 '13 at 11:02

In my original post I wasn't resetting "nNumberOfBytesWritten" to 0 when retrying BIO_write().

I ended up doing something like this:

int32_t SendPacket(BIO * const pBio, const unsigned char * const pPacket, const int32_t nPacketLength)
{
    int32_t nPos = 0;
    int32_t nNumberOfBytesWritten = 0;
    ...
    for (nPos = 0; nPos < nPacketLength; nPos += nNumberOfBytesWritten)
    {
        if ((nNumberOfBytesWritten = BIO_write(pBio, pPacket + nPos, nPacketLength - nPos)) <= 0)
        {
            if (BIO_should_retry(pBio))
            {
                nNumberOfBytesWritten = 0;
                continue;
            }

            return -1;
        }
    }

    return nPos;
}

in my application BIO_write blocks after 1 or 2 mins. Do you have any idea? — Arunraj Shanmugam, Nov 10 '18 at 15:29

C - How to use OpenSSL's BIO_write() correctly

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