Resources

Read or watch:

File descriptors
C Programming in Linux Tutorial #024 - open() read() write() Functions

man or help:

open
close
read
write
dprintf

Learning Objectives At the end of this project, you are expected to be able to explain to anyone, without the help of Google:

General

Look for the right source of information online
How to create, open, close, read and write files
What are file descriptors
What are the 3 standard file descriptors, what are their purpose and what are their POSIX names
How to use the I/O system calls open, close, read and write
What are and how to use the flags O_RDONLY, O_WRONLY, O_RDWR
What are file permissions, and how to set them when creating a file with the open system call
What is a system call
What is the difference between a function and a system call

Requirements

General

Allowed editors: vi, vim, emacs
All your files will be compiled on Ubuntu 20.04 LTS using gcc, using the options -Wall -Werror -Wextra -pedantic -std=gnu89
All your files should end with a new line
A README.md file, at the root of the folder of the project is mandatory
Your code should use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
You are not allowed to use global variables
No more than 5 functions per file
The only C standard library functions allowed are malloc, free and exit. Any use of functions like printf, puts, calloc, realloc etc… is forbidden
Allowed syscalls: read, write, open, close
You are allowed to use _putchar
You don’t have to push _putchar.c, we will use our file. If you do it won’t be taken into account
In the following examples, the main.c files are shown as examples. You can use them to test your functions, but you don’t have to push them to your repo (if you do we won’t take them into account). We will use our own main.c files at compilation. Our main.c files might be different from the one shown in the examples
The prototypes of all your functions and the prototype of the function _putchar should be included in your header file called main.h
Don’t forget to push your header file
All your header files should be include guarded
Tip: always prefer using symbolic constants (POSIX) vs numbers when it makes sense. For instance read(STDIN_FILENO, ... vs read(0, ...

Quiz questions

Question #0

Which of these answers are the equivalent of O_RDWR on Ubuntu 14.04 LTS? (select all correct answers):

3 | 2
1
0
O_RDONLY
(O_RDONLY | O_WRONLY)
3 & 2
2
(O_RDONLY << 1)
(O_WRONLY << 1)
1 << 1
O_WRONLY
(O_RDONLY & O_WRONLY)
(O_RDONLY && O_WRONLY)
3
(O_RDONLY + O_WRONLY)

Tips: Use printf or read the headers to see the definitions/values of these macros.

Question #1

why? #AlwaysAskWhy

I don’t care I never ask why, just let me access the tasks!
Because most of the time, I will already have stdin (value 0), stdout (value 1) and stderr (value 2) opened when my program starts executing.
Because this will be the first opened file descriptor and in CS we start counting starting from 0
Because this will be the first opened file descriptor and we start counting starting from 1
Because this will be the third opened file descriptor for my process
Because this will be the second opened file descriptor for my process

Question #2

What system call would you use to write to a file descriptor? (select all correct answers)

printf
fprintf
write

Question #3

Without context, on Ubuntu 14.04 LTS, write is a … (please select all correct answers):

kernel routine
executable
system call
game
library call

Question #4

What happens if you try to write “Best” to the standard input on Ubuntu 14.04 LTS?

The text will be printed on the terminal on the standard output
Nothing
Segmentation fault
The text will be printed on the terminal but I can’t pipe it

Tips: Just try it! :)

Question #5

What is the unistd symbolic constant for the standard input?

STDIN_FILENO
STDOUT_FILENO
STDERR_FILENO

Question #6

What is the oflag used to open a file with the mode read only?

O_WRONLY
O_RDONLY
O_RDWR

Question #7

What is the unistd symbolic constant for the standard output?

STDIN_FILENO
STDOUT_FILENO
STDERR_FILENO

Question #8

What is the return value of the system call open if it fails?

0
-1
98

Question #9

What is the unistd symbolic constant for the Standard error?

STDIN_FILENO
STDOUT_FILENO
STDERR_FILENO

Question #10

What is the oflag used to open a file in mode read + write?

O_WRONLY
O_RDONLY
O_RDWR

Question #11

is open a function or a system call? (select all valid answers)

it’s a kernel routine
it’s a function
it’s a system call
it’s a function provided by the kernel
it’s a library call

Question #12

What is the correct combination of oflags used to open a file with the mode write only, create it if it doesn’t exist and append new content at the end if it already exists?

O_RDWR | O_CREAT | O_APPEND
O_WRONLY
O_WRONLY | O_CREAT | O_EXCL
O_WRONLY | O_CREAT | O_APPEND

Question #13

When I am using O_WRONLY | O_CREAT | O_APPEND -> the | are bitwise operators.

False
True

Question #14

Most of the time, on a classic, modern Linux system, what will be the value of the first file descriptor you will get after opening a new file with open (if open succeeds of course):

6
0
5
1
4
3
2

Tasks

0. Tread lightly, she is near

Write 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

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$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x15-file_io
File: 0-read_textfile.c

1. Under the snow

Create 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 failure (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

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$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x15-file_io
File: 1-create_file.c

2. Speak gently, she can hear

Write 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 failure
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 exist or if you do not have the required permissions to write the file

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$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x15-file_io
File: 2-append_text_to_file.c

3. cp

Write 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 first 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 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

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$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x15-file_io
File: 3-cp.c

4. elf

Write a program that displays the information contained in the ELF header at the start of an ELF file.

Usage: elf_header elf_filename
Displayed 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 have as many functions as you want in your source file
You are allowed to use printf

man elf, readelf

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$

Repo:

GitHub repository: alx-low_level_programming
Directory: 0x15-file_io
File: 100-elf_header.c

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Tasks

0. Tread lightly, she is near

1. Under the snow

2. Speak gently, she can hear

3. cp

4. elf

Files

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Tasks

0. Tread lightly, she is near

1. Under the snow

2. Speak gently, she can hear

3. cp

4. elf