how to send a large file to browser using socket in C?

Question

I'm working on writing a simple web server in C,and I want to send a image~17KB to the browser. I'm trying to do it with send() function,but the buffer I defined is only 4096 and I don't want to change it.So I want to divide the image into byte chunks to send,but the browser just seem to receive the response header and the image can't load.And if I change the BUF_SIZE to 1024*32 and send the all the image,it will work.

What should I do to send the image?I would appreciate it if you could give me a suggestion. My code as follow:

#define BUF_SIZE 4096
char buf[BUF_SIZE];
readret=sprintf(buf,"HTTP/1.1 200 OK\r\n"
        "Content-Length: %ld\r\n"
        "Content-type: image/png"
        "Accept-Ranges: bytes\r\n\r\n");

FILE *fp=fopen(file,"rb");
if (send(client_sock, buf, readret, 0) != readret){
    close_socket(client_sock);
    close_socket(sock);
    fprintf(stderr, "Error sending to client.\n");
    return EXIT_FAILURE;
}
if(fp){
    while((readret=fread(buf,sizeof(unsigned char),BUF_SIZE,fp))>0){
            if (send(client_sock, buf, readret, 0) != readret){
            close_socket(client_sock);
            close_socket(sock);
            fprintf(stderr, "Error sending to client.\n");
            return EXIT_FAILURE;
        }
    }
    fclose(fp);
}

The browser as follow:

The response header of the image

the image can't load

Edit 2022/3/25

I delete parameter to the %ld by mistake,sorry

Answer 1

I got the reason:

I add a blank to the end of every chunk carelessly. In another words, the image's data is an error. Thank you every one!