k.c

#include <stdio.h>
#include <inttypes.h>

void print_bit(uint8_t c, FILE* s) {
    uint_fast8_t i = 8;

AGAIN:
    if (i > 0) {
        putc(((c & 0x80) ? '1' : '0'), s);
        c <<= 1;
        i -= 1;
        goto AGAIN;
    }
}

void pipe_stream(FILE *s1, FILE *s2) {
    uint8_t buf[4096];
    while (!feof(s1)) {
        size_t r = fread(buf, 1, sizeof buf, s1);
        fwrite(buf, 1, r, s2);
    }
}

int main(void) {
    char buf[512];
    FILE *bufs = fmemopen(buf, sizeof buf, "w+");
    if (bufs != NULL) {
        print_bit(0x46, bufs);
        putc('\n', bufs);
    }

    fseek(bufs, 0, SEEK_SET);
    pipe_stream(bufs, stderr);
    fclose(bufs);

    return 0;
}