sprintf raw bytes to string in C? - c

Question

I am sending some raw bytes over the wire in C (using HTTP). I'm currently doing it like this:
// response is a large buffer
int n = 0; // response length
int x = 42; // want client to read x
int y = 43; // and y
// write a simple HTTP response containing a 200 status code then x and y in binary format
strcpy(response, "HTTP/1.1 200\r\n\r\n");
n += 16; // status line we just wrote is 16 bytes long
memcpy(response + n, &x, sizeof(x));
n += sizeof(x);
memcpy(response + n, &y, sizeof(y));
n += sizeof(y);
write(client, response, n);
In JavaScript, I then read this data using code like this:
request = new XMLHttpRequest();
request.responseType = "arraybuffer";
request.open("GET", "/test");
request.onreadystatechange = function() { if (this.readyState === XMLHttpRequest.DONE) { console.log(new Int32Array(this.response)) } }
request.send();
which prints [42, 43] as it should.
I'm wondering if there is a more elegant way to do this on the server-side though, e.g.
n += sprintf(response, "HTTP/1.1 200\r\n\r\n%4b%4b", &x, &y);
Where %4b is a made-up format specifier which just says: copy the 4 bytes from that address into the string (which would be "*\0\0\0") Is there a format specifier like the fictional %4b that does something like this?

Answer 1

It is an XY problem, you are asking about how to use sprintf() to solve your problem, rather than simply asking how to solve your problem. YOur actual problem is how to make that code more "elegant".
There is no particular reason to send the data in a single write operation - the network stack buffering will ensure that the data is packetised efficiently:
static const char header[] = "HTTP/1.1 200\r\n\r\n" ;
write( client, header, sizeof(header) - 1 ) ;
write( client, &x, sizeof(x) ) ;
write( client, &y, sizeof(y) ) ;
Note that X and Y will be written in the native machine byte order, which may be incorrect at the receiver. More generically then:
static const char header[] = "HTTP/1.1 200\r\n\r\n" ;
write( client, header, sizeof(header) - 1 ) ;
uint32_t nl = htonl( x ) ;
write( client, &nl, sizeof(nl) ) ;
nl = htonl( y ) ;
write( client, &nl, sizeof(nl) ) ;

Answer 2

Is there a format specifier like the fictional %4b?
No, there is not, and your method is fine. I would suggest using snprintf and some check to avoid buffer overflow, adding ex. static_assert(sizeof(int) == 4, "") checking that the platform uses big endian and similar environment and error handling and avoiding undefined behavior checks.
That said, you can use %c printf specifier multiple times, like "%c%c%c%c", ((char*)&x)[3], ((char*)&x)[2], ((char*)&x)[1], ((char*)&x)[0] to print 4 bytes. You can wrap it in macros and do:
#include <stdio.h>
#define PRI_BYTES_4 "%c%c%c%c"
#define ARG_BYTES_BE_4(var) \
((const char*)&(var))[3], \
((const char*)&(var))[2], \
((const char*)&(var))[1], \
((const char*)&(var))[0]
int main() {
int var =
'l' << 24 |
'a' << 16 |
'm' << 8 |
'e';
printf("Hello, I am " PRI_BYTES_4 ".\n",
ARG_BYTES_BE_4(var));
// will print `Hello, I am lame.`
}