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0-read_textfile.c
0-read_textfile.c
Oct 31, 2022
1-create_file.c
1-create_file.c
Oct 31, 2022
100-elf_header.c
100-elf_header.c
Oct 31, 2022
2-append_text_to_file.c
2-append_text_to_file.c
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3-cp.c
3-cp.c
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README.md
README.md
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_putchar.c
_putchar.c
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main.h
main.h
Oct 31, 2022

README.md

0x15. C - File I/O

Resource

Tasks

  1. Tread lightly, she is near : A function that reads a text file and prints it to the POSIX standard output.
    • Prototype: ssize_t read_textfile(const char *filename, size_t letters);
    • Where letters is the number of letters it should read and print.
    • Returns the actual number of letters it could read and print.
    • If the file can not be opened or read, return 0.
    • If filename is NULL return 0
    • If write fails or does not write the expected amount of bytes, return 0.
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/0-main.c 0-read_textfile.c -o a
    • Run this way: ./a main/Requiescat
julien@ubuntu:~/0x15. File descriptors and permissions$ cat Requiescat 
Requiescat
by Oscar Wilde

Tread lightly, she is near
Under the snow,
Speak gently, she can hear
The daisies grow.

All her bright golden hair
Tarnished with rust,
She that was young and fair
Fallen to dust.

Lily-like, white as snow,
She hardly knew
She was a woman, so
Sweetly she grew.

Coffin-board, heavy stone,
Lie on her breast,
I vex my heart alone,
She is at rest.

Peace, Peace, she cannot hear
Lyre or sonnet,
All my life's buried here,
Heap earth upon it.
julien@ubuntu:~/0x15. File descriptors and permissions$ cat 0-main.c
#include <stdio.h>
#include <stdlib.h>
#include "main.h"

/**
* main - check the code
*
* Return: Always 0.
*/
int main(int ac, char **av)
{
  ssize_t n;

  if (ac != 2)
  {
      dprintf(2, "Usage: %s filename\n", av[0]);
      exit(1);
  }
  n = read_textfile(av[1], 114);
  printf("\n(printed chars: %li)\n", n);
  n = read_textfile(av[1], 1024);
  printf("\n(printed chars: %li)\n", n);
  return (0);
}
julien@ubuntu:~/0x15. File descriptors and permissions$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 0-main.c 0-read_textfile.c -o a
julien@ubuntu:~/0x15. File descriptors and permissions$ ./a Requiescat 
Requiescat
by Oscar Wilde

Tread lightly, she is near
Under the snow,
Speak gently, she can hear
The daisies grow.
(printed chars: 114)
Requiescat
by Oscar Wilde

Tread lightly, she is near
Under the snow,
Speak gently, she can hear
The daisies grow.

All her bright golden hair
Tarnished with rust,
She that was young and fair
Fallen to dust.

Lily-like, white as snow,
She hardly knew
She was a woman, so
Sweetly she grew.

Coffin-board, heavy stone,
Lie on her breast,
I vex my heart alone,
She is at rest.

Peace, Peace, she cannot hear
Lyre or sonnet,
All my life's buried here,
Heap earth upon it.

(printed chars: 468)
julien@ubuntu:~/0x15. File descriptors and permissions$
  1. Under the snow : A function that creates a file.
    • Prototype: int create_file(const char *filename, char *text_content);
    • Where filename is the name of the file to create and text_content is a NULL terminated string to write to the file.
    • Returns: 1 on success, -1 on faliure (file can not be created, file can not be written, write "fails", etc)
    • The created file must have those permissions: rw-------. If the file already exists, do not change the permissions.
    • If the file already exists, truncate it.
    • If filename is NULL return -1
    • If text_content is NULL create an empty file.
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/1-main.c 1-create_file.c -o b
    • Run this way: ./b hello world OR ./b hello "you own this world"
      • Confirm file permissions: ls -l hello
      • Confirm file contents: cat hello
      • You can try to run this way ./b hello "Is the previous data present" to confirm the truncate bit.
julien@ubuntu:~/0x15. File descriptors and permissions$ cat 1-main.c
#include <stdio.h>
#include <stdlib.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(int ac, char **av)
{
    int res;

    if (ac != 3)
    {
        dprintf(2, "Usage: %s filename text\n", av[0]);
        exit(1);
    }
    res = create_file(av[1], av[2]);
    printf("-> %i)\n", res);
    return (0);
}
julien@ubuntu:~/0x15. File descriptors and permissions$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 1-main.c 1-create_file.c -o b
julien@ubuntu:~/0x15. File descriptors and permissions$ ./b hello world
-> 1)
julien@ubuntu:~/0x15. File descriptors and permissions$ ls -l hello
-rw------- 1 julien julien 5 Dec  3 14:28 hello
julien@ubuntu:~/0x15. File descriptors and permissions$ cat hello 
worldjulien@ubuntu:~/0x15. File descriptors and permis$
  1. Speak gently, she can hear : A function that appends text at the end of a file.
    • Prototype: int append_text_to_file(const char *filename, char *text_content);
    • Where filename is the name of the file and text_content is the NULL terminated string to add at the end of the file.
    • Return: 1 on success and -1 on faliure.
    • Do not create the file if it does not exist.
    • If filename is NULL return -1
    • If text_content is NULL`, do not add anything to the file.
    • Return 1 if the file exists and -1 if the file does not exit or if you do not have the required permissions to write the file.
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 main/2-main.c 2-append\_text\_to\_file.c -o c
    • Run the following to text the program:
      • Create hello file: echo -n Hello > hello
      • Confirm file was created: ls -l hello
      • View contents of our hello file: cat hello
      • Run this way: ./c hello " World! "
      • View contents again: cat hello
      • On the same file you can try and see if we pass no text if the file will change: ./c hello ""
julien@ubuntu:~/0x15. File descriptors and permissions$ cat 2-main.c
#include <stdio.h>
#include <stdlib.h>
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(int ac, char **av)
{
    int res;

    if (ac != 3)
    {
        dprintf(2, "Usage: %s filename text\n", av[0]);
        exit(1);
    }
    res = append_text_to_file(av[1], av[2]);
    printf("-> %i)\n", res);
    return (0);
}
julien@ubuntu:~/0x15. File descriptors and permissions$ echo -n Hello > hello
julien@ubuntu:~/0x15. File descriptors and permissions$ ls -l hello
-rw-rw-r-- 1 julien julien 5 Dec  3 14:48 hello
julien@ubuntu:~/0x15. File descriptors and permissions$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 2-main.c 2-append_text_to_file.c -o c
julien@ubuntu:~/0x15. File descriptors and permissions$ ./c hello " World!
> "
-> 1)
julien@ubuntu:~/0x15. File descriptors and permissions$ cat hello 
Hello World!
julien@ubuntu:~/0x15. File descriptors and permissions$
  1. cp A program that copies the content of a file to another file.
    • Usage: cp file_from file_to
    • If the number of argument is not the correct one, exit with code 97 and print Usage: cp file_from file_to, followed by a new line, on the POSIX standard error.
    • If file_to already exists, truncate it.
    • If file_from does not exist, or if you can not read it, exit with code 98 and print Error: Can't read from file NAME_OF_THE_FILE, followed by a new line, on the POSIX standard error.
      • Where NAME_OF_THE_FILE is the second argument passed to your program.
    • If you can not create or if write to file_to fails, exit with code 99 and print Error: Can't write to NAME_OF_THE_FILE, followed by a new line, on the POSIX standard error.
      • Where the NAME_OF_THE_FILE is the second argument passed to your program.
    • If you can not close a file descriptor , exit with code 100 and print Error: Can't close fd FD_VALUE, followed by a new line, on the POSIX standard error.
      • Where FD_VALUE is the value of the file descriptor.
    • Permissions of the created file: rw-rw-r--. If the file already exists, do not change the permissions.
    • You must read 1,024 bytes at a time from the file_from to make less system calls. Use a buffer.
    • You are allowed to use dprintf.
    • Compile the code this way: gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-cp.c -o cp
    • To run the program, follow below steps:
      • View contents of file to copy from: cat main/incitatous
      • Copy file to new file: ./cp main/incitatous Incitatous
      • Confirm if file has been created: ls -l Incitatous
      • Confirm contents: cat Incitatous
julien@ubuntu:~/0x15. File descriptors and permissions$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 3-cp.c -o cp
julien@ubuntu:~/0x15. File descriptors and permissions$ cat incitatous 
Why you should think twice before putting pictures on social media.
(What you always wanted to know about @Incitatous)
#PrivacyAware
http://imgur.com/a/Mq1tc
julien@ubuntu:~/0x15. File descriptors and permissions$ ./cp incitatous Incitatous
julien@ubuntu:~/0x15. File descriptors and permissions$ ls -l Incitatous 
-rw-rw-r-- 1 julien julien 158 Dec  3 15:39 Incitatous
julien@ubuntu:~/0x15. File descriptors and permissions$ cat Incitatous 
Why you should think twice before putting pictures on social media.
(What you always wanted to know about @Incitatous)
#PrivacyAware
http://imgur.com/a/Mq1tc
julien@ubuntu:~/0x15. File descriptors and permissions$
  1. elf : A program that displays the information contained in the ELF header at the start of an ELF file.
    • Usage: elf_header elf_filename
    • Displays the following information: (no less, no more, do not include trailing whitespace)
      • Magic
      • Class
      • Data
      • Version
      • OS/ABI
      • ABI Version
      • Type
      • Entry point address
    • Format: the same as readelf -h (version 2.26.1)
    • If the file is not an ELF file, or on error, exit with status code 98 and display a comprehensive error message to stderr
    • You are allowed to use lseek once
    • You are allowed to use read a maximum of 2 times at runtime
    • You are allowed to to have as many functions as you want in your source file.
    • You are allowed to use printf
    • man elf, readelf will give you details on the functions they use and struct type.
    • Compile the code this way: ``
    • Confirm if the code is running by following the below steps:
      • Run readelf first with the --header / -h option:
julien@ubuntu:~/0x15. File descriptors and permissions$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 100-elf_header.c -o elf_header
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header ubuntu64 
ELF Header:
Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class:                             ELF64
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              EXEC (Executable file)
Entry point address:               0x400600
julien@ubuntu:~/0x15. File descriptors and permissions$ readelf --version
GNU readelf (GNU Binutils for Ubuntu) 2.26.1
Copyright (C) 2015 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) any later version.
This program has absolutely no warranty.
julien@ubuntu:~/0x15. File descriptors and permissions$ readelf -h ubuntu64 
ELF Header:
Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
Class:                             ELF64
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              EXEC (Executable file)
Machine:                           Advanced Micro Devices X86-64
Version:                           0x1
Entry point address:               0x400600
Start of program headers:          64 (bytes into file)
Start of section headers:          6936 (bytes into file)
Flags:                             0x0
Size of this header:               64 (bytes)
Size of program headers:           56 (bytes)
Number of program headers:         9
Size of section headers:           64 (bytes)
Number of section headers:         31
Section header string table index: 28
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header /lib/ld-linux.so.2
ELF Header:
Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class:                             ELF32
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              DYN (Shared object file)
Entry point address:               0xac0
julien@ubuntu:~/0x15. File descriptors and permissions$ readelf -h /lib/ld-linux.so.2
ELF Header:
Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
Class:                             ELF32
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              DYN (Shared object file)
Machine:                           Intel 80386
Version:                           0x1
Entry point address:               0xac0
Start of program headers:          52 (bytes into file)
Start of section headers:          145756 (bytes into file)
Flags:                             0x0
Size of this header:               52 (bytes)
Size of program headers:           32 (bytes)
Number of program headers:         7
Size of section headers:           40 (bytes)
Number of section headers:         24
Section header string table index: 23
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header netbsd32 
ELF Header:
Magic:   7f 45 4c 46 01 01 01 02 00 00 00 00 00 00 00 00
Class:                             ELF32
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - NetBSD
ABI Version:                       0
Type:                              EXEC (Executable file)
Entry point address:               0x80484c0
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header sortix32 
ELF Header:
Magic:   7f 45 4c 46 01 01 01 53 00 00 00 00 00 00 00 00
Class:                             ELF32
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            <unknown: 53>
ABI Version:                       0
Type:                              EXEC (Executable file)
Entry point address:               0x80484c0
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header solaris32 
ELF Header:
Magic:   7f 45 4c 46 01 01 01 06 01 00 00 00 00 00 00 00
Class:                             ELF32
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - Solaris
ABI Version:                       1
Type:                              EXEC (Executable file)
Entry point address:               0x8052400
julien@ubuntu:~/0x15. File descriptors and permissions$ ./elf_header sparc32 
ELF Header:
Magic:   7f 45 4c 46 01 02 01 00 00 00 00 00 00 00 00 00
Class:                             ELF32
Data:                              2's complement, big endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              EXEC (Executable file)
Entry point address:               0x10d20
julien@ubuntu:~/0x15. File descriptors and permissions$