based on https://gist.github.com/mattn/966e1cb039ee32291f52a8b22ebaf5ec

code.c

#include <windows.h>
#include <stdio.h>

void readfile(HANDLE h, char *buf, int size) {
    BOOL r;
    DWORD nr;
    int i;
    r = ReadFile(h, buf, size, &nr, NULL);
    printf("r=%d nr=%d", r, nr);
    if (r != 0) {
        printf(" :");
        for (i = 0; i < nr; ++i) {
            printf(" %02x", (int)buf[i] & 0xff);
        }
    }
    printf("\n");
}

int
main(int argc, char* argv[]) {
    HANDLE h = GetStdHandle(STD_INPUT_HANDLE);
    char buf[7];
    readfile(h, buf, sizeof(buf)-1);
    readfile(h, buf, sizeof(buf)-1);
    return 0;
}

result.txt

D:\home\koron\tmp>type test.txt
aあいうえおかきくけこ

D:\home\koron\tmp>type test.txt | a
r=1 nr=6 : 61 82 a0 82 a2 82
r=1 nr=6 : a4 82 a6 82 a8 82

D:\home\koron\tmp>a
aあいうえおかきくけこ
r=1 nr=7 : 61 82 a0 82 a2 82 00
r=1 nr=6 : 82 a9 82 ab 82 ad