Cannot save to in-memory stream buffer

[dmikushin]

So strange that save() only works correctly with the output file, but not with an in-memory buffer. When I try to save elf into a stringstream, only first 64 bytes are written:

std::stringstream ss(std::ios::out);
e.save(ss);
std::string elf = ss.str(); // only first 64 bytes always!

Any clue?

[serge1]

Hi,

I didn't check it yet in my environment, but, my guess is that seekp() doesn't work for stringstream.

Yet another idea to check - ss.str() may return the std::string until the first \0 character.

Would you please try to check it with your compiler?

[serge1]

Yes, the first suspicion came true - stringstream is not able to move write pointer over current stream end. When working with files, the operation increases the file size. For stringstream an error condition is returned. This sample code demonstrates the difference:

int main( int argc, char** argv )
{
//    std::ofstream oss("test.out");
    std::stringstream oss("");
    oss << "Hello";
    std::cout <<  oss.tellp() << std::endl;
    oss.seekp(20);
    std::cout <<  oss.tellp() << std::endl;

    return 0;
}

[dmikushin]

Exactly!

% cat test.cpp
#include <iostream>
#include <sstream>

int main( int argc, char** argv )
{
    // std::ofstream oss("test.out");
    std::stringstream oss("");
    oss << "Hello";
    std::cout << oss.tellp() << std::endl;
    oss.seekp(20);
    std::cout << oss.tellp() << std::endl;

    return 0;
}

% g++ test.cpp -o ./test
% ./test                
5
-1

[dmikushin]

So maybe ELFIO should "preallocate" the underlying stream by writing a placeholder buffer, whose size is already the size of the resulting ELF binary? Then seeks should become valid. Could the output ELF size be known beforehand?

[serge1]

Please give a try to commit becd79d. It should fix the problem. Please let me know your results and close the issue in case of success

[serge1]

Please reopen if you still expirience a problem

[dmikushin]

Yes, it works now! Many thanks for your support!