Low-level I/O (File Descriptors)

#include <fcntl.h>    /* open(), O_RDONLY */
#include <unistd.h>   /* read(), write(), close() */
#include <stdio.h>    /* perror() */

int main(void) {
    /* open() returns a file descriptor (integer) or -1 on error */
    int fd = open("hello.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");   /* prints error to stderr */
        return 1;
    }

    char buf[256];
    ssize_t n;

    /* read in a loop — read() may return less than asked */
    while ((n = read(fd, buf, sizeof(buf))) > 0) {
        /* write raw bytes to stdout (fd 1) */
        write(1, buf, n);
    }
    if (n == -1) {
        perror("read");
    }

    close(fd);   /* always close — releases kernel resources */
    return 0;
}

#include <unistd.h>
#include <string.h>   /* strlen() */

int main(void) {
    const char *msg = "Hello from write()\n";

    /* write(fd, buffer, count) — fd=1 is stdout */
    /* Unlike printf, write() does NOT interpret format specifiers */
    /* It sends raw bytes — here the ASCII bytes of the string */
    ssize_t written = write(1, msg, strlen(msg));

    if (written == -1) {
        /* write to stderr (fd=2) for error messages */
        const char *err = "write failed\n";
        write(2, err, strlen(err));
        return 1;
    }
    /* written == number of bytes actually sent */
    return 0;
}

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

int main(void) {
    int fd = open("data.txt", O_RDONLY);
    if (fd == -1) { perror("open"); return 1; }

    char buf[64];
    ssize_t n;

    /* First read — reads from current position (start) */
    n = read(fd, buf, sizeof(buf) - 1);
    if (n > 0) {
        buf[n] = '\0';
        printf("First read: %s\n", buf);
    }

    /* lseek back to the beginning (offset=0 from SEEK_SET) */
    off_t pos = lseek(fd, 0, SEEK_SET);
    if (pos == -1) { perror("lseek"); close(fd); return 1; }

    /* Second read — reads same content again */
    n = read(fd, buf, sizeof(buf) - 1);
    if (n > 0) {
        buf[n] = '\0';
        printf("Second read: %s\n", buf);
    }

    /* Get file size: seek to end, result is size */
    off_t size = lseek(fd, 0, SEEK_END);
    printf("File size: %ld bytes\n", (long)size);

    close(fd);
    return 0;
}

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

int main(void) {
    /* O_WRONLY | O_CREAT | O_TRUNC: open for write, create if missing, truncate if exists */
    /* 0644: permissions — owner rw, group r, others r */
    int fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == -1) { perror("open"); return 1; }

    const char *line1 = "Line 1: written with write()\n";
    const char *line2 = "Line 2: raw bytes, no printf\n";

    write(fd, line1, strlen(line1));
    write(fd, line2, strlen(line2));

    close(fd);
    printf("Wrote to output.txt\n");
    return 0;
}