echo_stdio.c

#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int
main(void)
{
    uint8_t buffer[65536]; // 64KB
    ssize_t bytes_read;
    ssize_t bytes_written;
    ssize_t i;

    do {
        bytes_read = read(STDIN_FILENO, buffer, sizeof(buffer) / sizeof(uint8_t));
        if (bytes_read > 0) {
            bytes_written = 0;
            do {
                if ((i = write(STDOUT_FILENO, buffer + bytes_written, bytes_read - bytes_written)) <= 0) {
                    return 0;
                }
                bytes_written += i;
            } while (bytes_written < bytes_read);
        }
    } while (bytes_read > 0);

    return 0;
}