segfault on memcpy once location approaches 100KB

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I'm attempting to "packetize" a large mmap()d file, like so:

//numBytes is based on user input
data = static_cast<char*>(mmap((caddr_t)0, numBytes, PROT_READ, MAP_SHARED, myFile, 0));

int
Sender::Packetize(char* data, int numBytes)
{
    int seqNum = 1;
    int offset = 0;
    size_t totalPacked = 0;
    unsigned int length = sizeof(struct sockaddr_in);

    bool dataRemaining = true;
    while(dataRemaining)
    {
            //MTU = 1460
        size_t payloadSize;
        (numBytes > MTU) ? payloadSize = MTU : payloadSize = numBytes;
        char* payload = (char*)malloc(payloadSize);

        memcpy(payload, data, payloadSize);
        Packet pac = {seqNum, 0, payloadSize, payload}; //Basic struct

        totalPacked += payloadSize;    
        cout << "Packed Bytes: " << payloadSize << endl;
        cout << "Total Packed: " << totalPacked << endl;

        dataMap.insert(pair<int, struct Packet>(seqNum, pac));

        if(numBytes > MTU)
        {
            offset += MTU;
            data = &data[offset];
        }
        else
            dataRemaining = false;

        numBytes -= MTU;
        seqNum++;
    }

    return 0;
}

I'm working with a 2MB+ file. When I pass in something relatively small (5000), everything appears to work swimmingly. However, if I attempt to pass the entire file (2533431), I get segfaults during the memcpy(). I've noticed that it appears to be an issue at around 100KB:

[.. snip ..]
Packed Bytes: 1460
Total Packed: 99280
Packed Bytes: 1460
Total Packed: 100740
Packed Bytes: 1460
Total Packed: 102200
Segmentation fault (core dumped)

However, if I attempt to go one chunk smaller (100740), I get:

[.. snip ..]
Packed Bytes: 1460
Total Packed: 16060
Packed Bytes: 1460
Total Packed: 17520
Packed Bytes: 1460
Total Packed: 18980
Segmentation fault (core dumped)

Is there some fundamental flaw that I've overlooked, causing my VM to segfault here?

asked 32 secs ago

MrDuk

1,076