0x15-file_io

0x15. C - File I/O

Resource

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

Tasks

• All *-main.c files are in the main directory.

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0. 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
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.
2. 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 ""
3. 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
4. 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:

         [main !?] root@0331426d386b : [ ~/alx-low_level_programming/0x15-file_io ] 
         $ 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:               0x1120
         Start of program headers:          52 (bytes into file)
         Start of section headers:          178660 (bytes into file)
         Flags:                             0x0
         Size of this header:               52 (bytes)
         Size of program headers:           32 (bytes)
         Number of program headers:         10
         Size of section headers:           40 (bytes)
         Number of section headers:         26
         Section header string table index: 25

     • Then confirm with your compiled program:

        [main !?] root@0331426d386b : [ ~/alx-low_level_programming/0x15-file_io ] 
        $ ./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:               0x1120

     • Other resources to use:
       • ELF: Behind the process
       • Executable and Linkable Format
       • Elf from scratch
       • Exploring ELF
       • The 101 of ELF files on Linux: Understanding and Analysis
       • Elf file structure
       • Linux Foundation Specs: ELF Header
       • Linux man-pages v5 elf