Unexplained out_of_range in string::substr

Answer 1

Looks like you need an else after

if(bodypos==std::string::npos)
{
    HttpError(...);
}

otherwise you are calling substr with bodypos = npos

Answer 2

You might consider using the (unsigned) type std::string::size_type instead of int.

Why are you casting the result of find to an int here: int(request.find("register"))!=std::string::npos

Answer 3

Are you sure that it's failing on that substr and not on a substr call within the HttpError or StringExplode functions? If you haven't already, you should run this through a debugger so that you can see exactly where it's throwing the exception. Alternatively, you could add a:

std::cout << "calling substr" << std::endl;

line immediately before you call substr, and a similar line immediately afterwards, so that it would look like:

std::cout << "calling substr" << std::endl;
std::string body = request.substr(bodypos);
std::cout << "finished calling substr" << std::endl;

StringExplode(body,"&", "=",keyval);
outRequest = "doing stuff";

If that substr really is throwing the exception, then you'll know because the program will print "calling substr" without a matching "finished calling substr". If it prints the pair of debug messages, though, or none at all, then something else is throwing the exception.

Answer 4

I modified your sample slightly to decrease amount of indentation used.
There are 5 "test cases" and none causes any problem. Could you please provide a sample request to reproduce the problem you're having.

EDIT: Forgot to mention: if this sample as it is (with commented-out bits) doesn't produce that error, your best bet is that you have a bug in your StringExplode function. You could post its source, to get a more helpful advice.

EDIT2: In your StringExplode, change results[tmpKey] = tmpKey.substr(found+1); to results[tmpKey] = tmpResult[i].substr(found+1);. Change int found to size_t found, and remove all of if (found > 0), that will fix your mysterious out_of_range. You were substr-ing a wrong string. Just in case, here's the code with a fix:

void StringExplode(std::string str, std::string objseparator, std::string keyseperator,
                   std::map <std::string, std::string> &results)
{
    size_t found;
    std::vector<std::string> tmpResult;
    found = str.find_first_of(objseparator);
    while(found != std::string::npos)
    {
        tmpResult.push_back(str.substr(0,found));
        str = str.substr(found+1);
        found = str.find_first_of(objseparator);
    }
    if(str.length() > 0)
    {
        tmpResult.push_back(str);
    }

    for(size_t i = 0; i < tmpResult.size(); i++)
    {
        found = tmpResult[i].find_first_of(keyseperator);
        while(found != std::string::npos)
        {
                std::string tmpKey = tmpResult[i].substr(0, found);
                results[tmpKey] = tmpResult[i].substr(found+1);
                found = tmpResult[i].find_first_of(keyseperator, found + results[tmpKey].size());
        }

    }
}

Initial test code:

#include <iostream>
#include <map>
#include <string>

std::string parse(const std::string &request)
{
    std::map<std::string,std::string> keyval;
    std::string outRequest;

    if(request[0] != 'P')
        return outRequest;

    if(request.find("register") == std::string::npos)
        return outRequest;

    //we have a register request
    size_t bodypos = request.find("username");
    if(bodypos==std::string::npos)
    {
        // HttpError(400,"Malformed HTTP POST request. Could not find key username.",request);
        // you said HttpError returns, so here's a return
        return outRequest;
    }

    std::string body = request.substr(bodypos);
    // StringExplode(body,"&", "=",keyval);
    outRequest = "doing stuff";

    return outRequest;
}

int main()
{

    std::string request("P\r\nregister\r\nusername=hello\r\n\r\n");
    std::cout << "[" << parse(request) << "]\n";

    request = "Pregisternusername=hello\r\n\r\n";
    std::cout << "[" << parse(request) << "]\n";

    request = "Pregisternusername=hello";
    std::cout << "[" << parse(request) << "]\n";

    request = "registernusername=hello";
    std::cout << "[" << parse(request) << "]\n";

    request = "";
    std::cout << "[" << parse(request) << "]\n";

    return 0;
}

This outputs, predictably:

[doing stuff]
[doing stuff]
[doing stuff]
[]
[]

Answer 5

One fairly obvious thing wrong with your code:

int k = read(ns, buf, sizeof(buf)-1);
buf[k] = '\0';

You are not checking that read() succeeded - it returns -1 on failure which will cause all sorts of memory corruption problems if it occurs.

Also:

char * buf2 = const_cast<char *>(reply.c_str());
write(ns,buf2,sizeof(buf2));

You are taking the size of the pointer - you want the length of the output string:

write(ns, buf2, reply.size() );

And you should once again test that write succeeded and that it wrote as many bytes as you requested, though this shouldn't directly cause the substr() error.