pipe and stringstream - memory leak when writing in the stringstream

Question

I have an memory issue with my code when I try to write binary data in a stringstream object. Valgrind log (first spotted with system monitor):

==23562== 16,368 bytes in 1 blocks are possibly lost in loss record 1,612 of 1,612
==23562==    at 0x402A6DC: operator new(unsigned int) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==23562==    by 0x4C72213: std::string::_Rep::_S_create(unsigned int, unsigned int, std::allocator<char> const&) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==23562==    by 0x4C73332: std::string::_Rep::_M_clone(std::allocator<char> const&, unsigned int) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==23562==    by 0x4C733D1: std::string::reserve(unsigned int) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==23562==    by 0x4C4E1FF: std::basic_stringbuf<char, std::char_traits<char>, std::allocator<char> >::overflow(int) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==23562==    by 0x4C52AEC: std::basic_streambuf<char, std::char_traits<char> >::xsputn(char const*, int) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==23562==    by 0x4C48E8D: std::ostream::write(char const*, int) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
**==23562==    by 0x8140EAC: MsgMgt::update_stream() (MsgMgt.cpp:524)**
==23562==    by 0x814192E: MsgMgt::thread() (MsgMgt.cpp:727)
==23562==    by 0x807614E: dlib::threaded_object::thread_helper() (threaded_object_extension.cpp:256)
==23562==    by 0x80EBEC2: void dlib::dlib_create_new_thread_helper<dlib::threaded_object, &dlib::threaded_object::thread_helper>(void*) (create_new_thread_extension.h:24)
==23562==    by 0x80768C3: dlib::threads_kernel_shared::thread_starter(void*) (threads_kernel_shared.cpp:272)

The following function (udpate stream) checks a pipe between another process and the current one and writes the pipe content inside the stringstream object, which leads to the memory leak:

(I removed non necessary code for the sake of clarity)

std::stringstream _outputstream;

unsigned int MsgMgt::update_stream()
{
fd_set set;

struct timeval timeout;
//Number of bytes read on the pipe
ssize_t read_bytes=0;

/* Initialize the file descriptor set. */
FD_ZERO(&set);
FD_SET(_fd[0], &set);
FD_SET(_fd_stop[0], &set); //allowing to stop the select

/* Initialize the timeout data structure. */
timeout.tv_sec = _timeout;
timeout.tv_usec = 0;

int ret;
errno=0;
if(!should_stop()){

    ret = select(FD_SETSIZE, &set, NULL, NULL, &timeout);

    if (ret > 0){

        if(FD_ISSET(_fd[0],&set)){
            //ACTIVITY ON THE PIPE
            char* buffer=new char[DATA_MAX_LENGTH];
            errno=0;
            //Data transfer process pipe to _outputstream
            read_bytes = read(_fd[0],buffer,DATA_MAX_LENGTH);  //None blocking read

            if(read_bytes<0 && errno !=EAGAIN){
            //Error in read
            }else{
            //write the data in the stringstream
            //supposed memory leak
                _outputstream.write(buffer,read_bytes); 
            }
        delete[] buffer;


        }
    }

}

return read_bytes;
}

I don't understand what's going on. Any ideas ?

Thank you,

Pierre.

Answer 1

Your buffer will be leaked, if _outputstream.write throws an exception.

You could use a smart pointer to store the pointer to your buffer or use a vector as buffer. Both solutions would automatically delete it in case of an exception.