read.cpp

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

#include <iostream>
#include <vector>

struct header {
    int n;
};

int main()
{
    int fd = open("foo.dat", O_RDONLY, 0644);
    if (fd == -1) {
        perror("Failed to open file");
        return -1;
    }

    struct header h;
    read(fd, &h, sizeof(h));

    std::vector<double> buf(h.n);

    char *ptr = reinterpret_cast<char *>(buf.data());
    ssize_t bytes_remaining = h.n * sizeof(double);

    while (bytes_remaining > 0) {
        ssize_t bytes_read = read(fd, ptr, bytes_remaining);

        ptr += bytes_read;
        bytes_remaining -= bytes_read;
    }

    for (int i = 0; i < h.n; i++) {
        std::cout << buf[i] << std::endl;
    }

    close(fd);

    return 0;
}

write.cpp

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

#include <vector>

struct header {
    int n;
};

int main()
{
    size_t N = 1000;

    std::vector<double> buf(N);

    for (size_t i = 0; i < N; i++) {
        buf[i] = i;
    }

    int fd = open("foo.dat", O_CREAT | O_WRONLY, 0644);
    if (fd == -1) {
        perror("Failed to open file");
        return -1;
    }

    struct header h;
    h.n = N;

    write(fd, &h, sizeof(h));

    char *ptr = reinterpret_cast<char *>(buf.data());
    ssize_t bytes_remaining = h.n * sizeof(double);

    while (bytes_remaining > 0) {
        ssize_t bytes_written = write(fd, ptr, bytes_remaining);

        ptr += bytes_written;
        bytes_remaining -= bytes_written;
    }

    close(fd);

    return 0;
}