manual-append.cpp

#include <fcntl.h>
#include <unistd.h>
#include <cerrno>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#define ERROR(fmt, ...)                                         \
    do {                                                        \
        fprintf(stderr, "%s:%d: " fmt "\n", __FILE__, __LINE__, \
                ##__VA_ARGS__);                                 \
        exit(1);                                                \
    } while (false)

// 128 MiB buffer
constexpr size_t BUF_SIZE = 128 * 1024;
char buf[BUF_SIZE] = {};

// Cannot open file with O_APPEND; cannot use lseek.
void manual_append(int fd, char* msg, size_t msg_len) {
    while (true) {
        int read_n = read(fd, buf, BUF_SIZE);

        if (read_n < 0) {
            ERROR("%s", strerror(errno));
        }

        if (size_t(read_n) < BUF_SIZE) {
            break;
        }
    }

    if (write(fd, msg, msg_len) < 0) {
        ERROR("%s", strerror(errno));
    }
}

int main(int argc, char** argv) {
    if (argc != 2) {
        ERROR("Provide file to append to");
    }

    int fd_1 =
        open(argv[1], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
    if (fd_1 < 0) {
        ERROR("%s: %s", argv[1], strerror(errno));
    }
    int fd_2 = dup(fd_1);
    if (fd_2 < 0) {
        ERROR("dup failed: %s", strerror(errno));
    }

    char first_line[] = "first line\n";
    char second_line[] = "second line\n";
    manual_append(fd_1, first_line, sizeof(first_line) - 1);
    manual_append(fd_2, second_line, sizeof(second_line) - 1);

    close(fd_1);  // ignore errors
    close(fd_2);
}