curl_easy_perform() crashing on Ubuntu Server 14

Question

Here's my upload function.

The goal is: to upload only a chunk of a file depending on the offset and chunk size

In this function, if p_offset is not zero, I call fseek()by myself and then I let libcurl read the contents of the file using fread().

The caller of the function is responsible for giving a correct and valid size of chunk, making sure that p_offset + p_sizeOfChunk <= ACTUAL_SIZE_OF_FILE

The answer from the server is supposed to be a string. I get it via my callback writeToString()

The code works fine on Windows and OS X. But curl_easy_perform() crashes on sometimes on Ubuntu 14.

Is there anything in my code that I am missing that could cause this crash?

void uploadFile( const string & p_filename, const string & p_url, size_t p_offset, size_t p_sizeOfChunk )
{
    FILE * file( fopen( p_filename.c_str(), "rb" ) );

    if ( !file )
    {
        throw Exception( __FILE__, __LINE__ ) << "Could not open file " << p_filename << " when posting to " << p_url;
    }

    if ( p_offset )
    {
        if ( fseek( file, (long)p_offset, SEEK_SET ) )
        {
            throw Exception( __FILE__, __LINE__ ) << "Could not seek in file " << p_filename << " when posting to " << p_url;
        }
    }

    CURL * curl( curl_easy_init() );

    if ( !curl )
    {
        throw Exception( __FILE__, __LINE__ ) << "Could not initialize cURL when posting " << p_filename << " to " << p_url;
    }

    // URL
    curl_easy_setopt( curl, CURLOPT_URL, p_url.c_str() );

    // PUT HTTP method
    string answer;
    curl_easy_setopt( curl, CURLOPT_UPLOAD, 1L );
    curl_easy_setopt( curl, CURLOPT_READFUNCTION, fread );
    curl_easy_setopt( curl, CURLOPT_READDATA, file );
    curl_easy_setopt( curl, CURLOPT_WRITEFUNCTION, writeToString );
    curl_easy_setopt( curl, CURLOPT_WRITEDATA, &answer );
    curl_easy_setopt( curl, CURLOPT_INFILESIZE_LARGE, (curl_off_t)p_sizeOfChunk );

    char errorBuffer[ CURL_ERROR_SIZE + 1 ];
    curl_easy_setopt( curl, CURLOPT_ERRORBUFFER, errorBuffer );

    // No signal handlers...
    curl_easy_setopt( curl, CURLOPT_NOSIGNAL, 1 );
    curl_easy_setopt( curl, CURLOPT_TIMEOUT_MS, 120000 );

    // HEADER
    char contentLength[ 512 ];
    snprintf( contentLength, sizeof( contentLength ), "Content-Length: %zu", p_sizeOfChunk );

    struct curl_slist * headers( nullptr );
    headers = curl_slist_append( headers, contentLength );
    curl_easy_setopt( curl, CURLOPT_HTTPHEADER, headers );

    // SSL
    curl_easy_setopt( curl, CURLOPT_CAINFO, "path/to/cacert.pem" );

    CURLcode res( curl_easy_perform( curl ) );

    fclose( file );

    if ( res != CURLE_OK && res != CURLE_SEND_ERROR )
    {
        curl_easy_cleanup( curl );
        throw Exception( __FILE__, __LINE__ ) << "cURL error when posting " << p_filename << " to " << p_url << ": " << errorBuffer;
    }

    long httpResponseCode( 0 );
    curl_easy_getinfo( curl, CURLINFO_RESPONSE_CODE, &httpResponseCode );
    curl_easy_cleanup( curl );

    if ( ( httpResponseCode / 100 ) != 2 )
    {
        cout << answer << endl;
        throw Exception( __FILE__, __LINE__ ) << "HTTP error " << httpResponseCode << " when posting " << p_filename;
    }
}

I get the answer and record it on a std::string with writeToString(). It's for sure not the reason for the crash. I tested it just returning the size * count and the crash still happens.

static size_t writeToString( const void * ptr, size_t size, size_t count, FILE * stream )
{
    string & retContent( *( reinterpret_cast< string * >( stream ) ) );

    if ( !retContent.length() )
    {
        int skipBOM( ( reinterpret_cast< const unsigned char * >( ptr )[ 0 ] == 0xEF && reinterpret_cast< const unsigned char * >( ptr )[ 1 ] == 0xBB && reinterpret_cast< const unsigned char * >( ptr )[ 2 ] == 0xBF ) ? 3 : 0 );
        retContent += string( static_cast< const char * >( ptr ) + skipBOM, static_cast< int >( size * count ) - skipBOM );
    }
    else
    {
        retContent += string( static_cast< const char * >( ptr ), size * count );
    }

    return size * count;
}

Here is the stack in the moment of crash! It seems to be related to OpenSSL.

#0  0x00007ffff65ad35d in write () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ffff73187a6 in ?? () from /lib/x86_64-linux-gnu/libcrypto.so.1.0.0
#2  0x00007ffff731684b in BIO_write () from /lib/x86_64-linux-gnu/libcrypto.so.1.0.0
#3  0x00007ffff6ffcb72 in ?? () from /lib/x86_64-linux-gnu/libssl.so.1.0.0
#4  0x00007ffff6ffd273 in ?? () from /lib/x86_64-linux-gnu/libssl.so.1.0.0
#5  0x00007ffff76873e1 in ossl_send (conn=0x7ffef8013b28, sockindex=0, mem=0x7ffef8005379, len=16384, curlcode=0x7fff127fa5c0) at vtls/openssl.c:2720
#6  0x00007ffff762fe0f in Curl_write (conn=0x7ffef8013b28, sockfd=64, mem=0x7ffef8005379, len=16384, written=0x7fff127fa608) at sendf.c:233
#7  0x00007ffff764fb01 in readwrite_upload (data=0x7ffef8000a78, conn=0x7ffef8013b28, k=0x7ffef8000af0, didwhat=0x7fff127fa664) at transfer.c:954
#8  0x00007ffff764fdd9 in Curl_readwrite (conn=0x7ffef8013b28, done=0x7fff127fa6dc) at transfer.c:1059
#9  0x00007ffff765ced7 in multi_runsingle (multi=0x7ffef800a668, now=..., data=0x7ffef8000a78) at multi.c:1484
#10 0x00007ffff765d60c in curl_multi_perform (multi_handle=0x7ffef800a668, running_handles=0x7fff127fa870) at multi.c:1759
#11 0x00007ffff7652103 in easy_transfer (multi=0x7ffef800a668) at easy.c:705
#12 0x00007ffff7652311 in easy_perform (data=0x7ffef8000a78, events=false) at easy.c:793
#13 0x00007ffff7652364 in curl_easy_perform (easy=0x7ffef8000a78) at easy.c:812
#14 ...

Answer 1

The problem was in a detail that I've completely misunderstood.

curl_easy_setopt( curl, CURLOPT_NOSIGNAL, 1 );

The effect of CURLOPT_NOSIGNAL prevents libcurl from handling any signals! My original interpretation was completely the inverse. The mistake happened because on OS X it's working [and the pipe handling also exists], so I assumed that the "signal management" was correct. I don't know why on OS X the same bug didn't show up. Anyways, I removed this line and it's working perfectly.

And, of course, another possible solution is to explicit set ir to zero:

curl_easy_setopt( curl, CURLOPT_NOSIGNAL, 0 );

Answer 2

First thing... run it with the debugger (or get a proper core dump) to find out where is really failing.

Not sure if this might be the problem... but looking at the documentation, the function you pass in CURLOPT_WRITEFUNCTION must have the following signature.

size_t function( char *ptr, size_t size, size_t nmemb, void *userdata);

However in your case:

size_t writeToString(const void * ptr, size_t size, size_t count, FILE * stream);

Apparently the default implementation for CURLOPT_WRITEFUNCTION expects a FILE * there... but in void * form