How to read and transfer PDF files in C with fread()-CodePudding

I'm writing TFTP client in C. So far I had no problem with reading file into buffers(512 B) and transfer it to the server. However I'm struggling to read files like PDF that contains binary data.

        size_t nmemb = fread(data_buf, sizeof(char), 512, fp);
        ...
        /* Create DATA packet and send it to TFTP server */ 
        count = sprintf(message, "%c%c%c%c%s", 0, DATA, 0, block_number, data_buf); 
        sendto(sock, message, count, 0, (const struct sockaddr *)&server, sizeof(server));

I tested this with printing every buffer separately: printf("%d: %s\n", nmemb, data_buf); Looks like nmemb is always 512, which is correct, but printf() stops printing data when it gets to a first NULL byte, which I'm sure represents the end of a given string(buffer).

Buffer data within DATA packet that is sent through a TFTP protocol are also cut off at first occurrence of NULL byte - I tested this in Wireshark.

Is there a way to fix this problem by using some alternative to fread() or changing a way packet is sent through socket? Thanks for any answer.

CodePudding user response：

You can't use the printf family of functions for this since they stop when \0 is encountered.

First approach: Use memcpy:

size_t nmemb = fread(data_buf, 1, sizeof data_buf, fp);

char message[sizeof data_buf   4]; // presumably

count = sprintf(message, "%c%c%c%c", 0, DATA, 0, block_number);
memcpy(message   count, data_buf, nmemb);

Second approach (the preferred way): Avoid the copying and fread directly into the buffer you'll later send:

char message[512   4] = {0, DATA, 0, block_number};
size_t nmemb = fread(message   4, 1, 512, fp);
sendto(sock, message, nmemb   4, 0, (const struct sockaddr *)&server, sizeof(server))

^{Note: sizeof(char) is always 1. It can't be anything else.}