Unexplained out_of_range in string::substr - c++

Question

I have been getting a really annoying error about an std::out_of_range when calling substr. The exact error is
terminate called after throwing an
instance of 'std::out_of_range'
what(): basic_string::substr
I'm absolutely sure that tmp_request has a length greater then 1. No matter what I pass to substr—1, 2, or bodypos—it always throws that error. I'm using g++ on Unix.
Only interesting thing I can include is the string has multiple "\r\n", including one "\r\n\r\n".
In one cpp file:
std::string tmp_request, outRequest;
tmp_request = SS_Twitter->readData();
outRequest = SS_Twitter->parse(tmp_request);
In another:
std::string parse(const std::string &request)
{
std::map<std::string,std::string> keyval;
std::string outRequest;
if(request[0]=='P')
{
if(request.find("register")!=std::string::npos)
{ //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);
}
else
{
std::string body = request.substr(bodypos);
StringExplode(body,"&", "=",keyval);
outRequest = "doing stuff";
}
}
Update:
std::string request2("P\r\nregister\r\nusername=hello\r\n\r\n");
std::string body = request2.substr(4);
That throws the same error. Now I know this is perfectly valid and correct code, but it's still throwing the error.
//removed source link

Answer 1

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 2

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 3

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.

Answer 4

Looks like you need an else after
if(bodypos==std::string::npos)
{
HttpError(...);
}
otherwise you are calling substr with bodypos = npos

Answer 5

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