For some data transfer via UDP I am using the async_receive_from
-function from boost. My receiving function is
udp::socket socket_;
udp::endpoint remote_endpoint_;
boost::array<char, 200> recv_buffer_;
void start_receive()
{
std::fill(recv_buffer_.begin(), recv_buffer_.end(), '\0');
socket_.async_receive_from(boost::asio::buffer(recv_buffer_), remote_endpoint_, boost::bind(&udp_server_ping::handle_receive, this, boost::asio::placeholders::error));
}
void handle_receive(const boost::system::error_code& error)
{
if (!error || error == boost::asio::error::message_size)
{
for(int i = 0; i < 200; i++)
std::cout << recv_buffer_.c_array()[i];
std::cout << '\n';
for(auto it = boost::begin(recv_buffer_); it != boost::end(recv_buffer_); ++it)
std::cout << *it;
std::cout << '\n';
switch(recv_buffer_.c_array()[0])
{
case '#':
{
got_ping = true;
std::cout << "Gotcha!\n";
break;
}
case '$':
{
std::vector<char> char_buf(boost::begin(recv_buffer_), boost::end(recv_buffer_));
std::stringstream ss(std::string(char_buf.begin(), char_buf.end()));
std::vector<std::string> ip_list;
std::string ip;
std::cout << "Char_buf is: ";
for(auto it = boost::begin(recv_buffer_); it != boost::end(recv_buffer_); ++it)
std::cout << *it;
std::cout << "\nStringstream is: " << ss << '\n';
while(std::getline(ss, ip, '$'))
{
ip_list.push_back(ip);
};
ip_adr_ccd = ip_list[0];
ip_adr_daisy = ip_list[1];
std::cout << "ip_adr_ccd is: " << ip_list[1] << " and ip_adr_daisy is: " << ip_list[2] << '\n';
ip_adr_display.push_back(ip_list[3]);
break;
}
default:
break;
start_receive();
}
while my transmitting function is
DLL void transmit_ip(void)
{
WORD wVersionRequested;
WSADATA wsaData;
int err;
wVersionRequested = MAKEWORD( 2, 2 );
err = WSAStartup( wVersionRequested, &wsaData );
if ( err != 0 ) {
/* Tell the user that we could not find a usable */
/* WinSock DLL. */
return;
}
struct sockaddr_in sa;
struct hostent *hp;
SOCKET s;
std::string ip_adr;
if(Use == 'T')
ip_adr = ip_adr_ccd;
else
{
if(Use == 'S')
ip_adr = ip_adr_daisy;
else
ip_adr = ip_adr_display[0];
};
//Debug
//std::cout << "Pinging ip: " << ip_adr << '\n';
hp = gethostbyname(ip_adr.c_str());
if (hp == NULL) /* we don't know who this host is */
return;
memset(&sa,0,sizeof(sa));
memcpy((char *)&sa.sin_addr, hp->h_addr, hp->h_length); /* set address */
sa.sin_family = hp->h_addrtype;
sa.sin_port = htons((u_short)PORTNUM_UDP_OUT);
s = socket(hp->h_addrtype, SOCK_DGRAM, 0);
if (s == INVALID_SOCKET)
return;
std::string tx_str = '$' + ip_adr_ccd + '$' + ip_adr_daisy + '$' + retLocalIP() + '$';
//char str[] = "$127.0.0.1$128.0.0.1$129.0.0.1$";
std::cout << "tx_str is: " << tx_str << '\n';
char * buffer = new char[tx_str.length() + 1];
std::strcpy(buffer, tx_str.c_str());
int ret;
ret = sendto( s, buffer, sizeof(buffer), 0, (struct sockaddr *)&sa, sizeof(sa));
delete buffer;
}
When I am using the str[] for transmitting, everything is fine, but when I want to transmitt my tx_str, the receiver crashes at once and only shows $192 as received data. What am I doing wrong for creating a buffer overflow?
When buffer
is a char*
, sizeof(buffer)
returns the size of a pointer, rather than the length of the pointed to string. It appears as though the system on which it is compiled uses 4 bytes for a pointer, hence only 4 characters are transmitted. Upon receiving the 4 bytes, handle_receive()
invokes undefined behavior when it attempts to access an invalid index in ip_list
, as the code assumes 3 strings are always extracted from the receive message.
To resolve the problem, explicitly provide the buffer size to sendto()
rather than using sizeof()
. Change:
ret = sendto( s, buffer, sizeof(buffer), 0, ...)
to:
ret = sendto( s, buffer, tx_str.length() + 1, 0, ...)
It may also be worth considering checking input and verifying ip_list
is the expected size before indexing into it.
char[]
and char*
are different types. In the case of char*
, sizeof()
will return the size of a pointer on the given system and not the length of the string pointed to by the pointer. On the other hand, for char[]
, sizeof()
will return the size of the array. For example:
#include <iostream>
int main()
{
char str[] = "123456789ABCDEF";
char* buffer = new char[100];
std::cout << "char[] size = " << sizeof(str) << "\n"
"char* size = " << sizeof(buffer) << std::endl;
}
Results in:
char[] size = 16
char* size = 8
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