rabbitmq-c send nlohmann json

Question

Want to send image through RabbitMQ-C but the image file too big. Receiver cannot retrieve the image. So, I converted image to base64 then put it in JSON.

const char *msg;


FILE *image1;
    if (image1 = fopen(path, "rb")) {
        fseek(image1, 0, SEEK_END); //used to move file pointer to a specific position
                                    // if fseek(0 success, return 0; not successful, return non-zero value
                                    //SEEK_END: end of file
        
        length = ftell(image1); //ftell(): used to get total size of file after moving the file pointer at the end of the file
        sprintf(tmp, "size of file: %d  bytes", length);

//convert image to base64
        std::string line;
        std::ifstream myfile;
        myfile.open(path, std::ifstream::binary);
        std::vector<char> data((std::istreambuf_iterator<char>(myfile)), std::istreambuf_iterator<char>() );
        std::string base64_encode(unsigned char const* bytes_to_encode, unsigned int in_len);
        std::string code = base64_encode((unsigned char*)&data[0], (unsigned int)data.size());
        
//convert std::string to const char 
        const char* base64_Image = code.c_str();
        

        
        json j ;
    
        
        j.push_back("Title");
        j.push_back("content");
        j.push_back(base64_Image);
        
        std::string sa = j.dump();

        msg = sa.c_str();       //convert std::string to const char*
        
    }
    else {
        return;
    }

Using RabbitMQ-C to send the message(msg) to receiver but failed [error point to here]

is const char* cannot use amqp_cstring_bytes(msg) to convert to amqp_bytes_t??

    respo = amqp_basic_publish(conn, 1, amqp_cstring_bytes(exchange), amqp_cstring_bytes(routing_key),0, 0, NULL, amqp_cstring_bytes(msg));

and get this error


If there is a handler for this exception, the program may be safely continued.```


Anyone know how to send image as JSON using RabbitMQ-C & C++ ?

How did you determine the image was "too big" to receive? If that truly was the issue, base64-encoding something increases its size by 33%, so this will only aggravate the problem. — Botje, Jul 19 '21 at 07:19
Thanks @Botje for your reply. Actually I'm not too sure what's the matter makes receiver cannot decode the image i sent. The image I want to send mostly around 400kb but my base64 string more than 50k characters. previously i used RapidJSON to create JSON object ```Write.String(base64,255, true); ``` but the character is more than 255. Now i'm using nlohmann json , it allows me to create and get complete base64 string in JSON but unable to send through RabbitMQ. Do you know any mistake i have done in my code? — nunu, Jul 19 '21 at 08:07

Botje · Accepted Answer · 2021-07-19T09:52:43.210

2

amqp_cstring_bytes expects a C string, which is normally terminated by a NUL byte. Your PNG file is almost guaranteed to contain a NUL byte, so that explains why your message got cut off midway through.

As for the code in your paste: the pointer returned by sa.c_str() is only valid while sa is alive and unmodified. Once you exit the block containing sa's definition, the variable is dead and buried.

Instead, get a buffer of the appropriate size with amqp_bytes_alloc and return that:

amqp_bytes_t bytes = amqp_bytes_malloc(sa.length());
strncpy((char *)bytes.bytes, sa.c_str(), sa.length());

then pass the bytes object to amqp_basic_publish. Don't forget to ampqp_bytes_free it when you're done.

edited Jul 19 '21 at 09:52

answered Jul 19 '21 at 08:18

Botje

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i have tried this and i get this error ```Error C2664 'char *strncpy(char *,const char *,std::size_t)': cannot convert argument 1 from 'void *' to 'char *' ``` when using ```strncpy(bytes.bytes, sa.c_str(), sa.length());``` – nunu Jul 19 '21 at 09:50
You need to add a cast to make that error go away. See edit. – Botje Jul 19 '21 at 09:51

rabbitmq-c send nlohmann json

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