0x15-file_io

C - File I/O

In this project, I learned about the three standard file descriptors and their POSIX names as well as the difference between function and system calls. I practiced using the I/O system calls open, close, read, and write and the flags O_RDONLY, WR_ONLY, and O_RDWR to create, open, close, read, write, set permissions of files in C.

Header File 📁

• main.h: Header file containing prototypes for all functions written in the project.

File	Prototype
0-read_textfile.c	ssize_t read_textfile(const char *filename, size_t letters);
1-create_file.c	int create_file(const char *filename, char *text_content);
2-append_text_to_file.c	int append_text_to_file(const char *filename, char *text_content);

Tasks 📃

• 0. Tread lightly, she is near

  • 0-read_textfile.c: C function that reads a text file and prints it to the POSIX standard output.
  • The parameter letters is the number of letters the function should read and print.
  • If the file is NULL or cannot be opened or read - returns 0.
  • If the write call fails or does not write the expected number of bytes - returns 0.
  • Otherwise - returns the actual number of bytes the function can read and print.

• 1. Under the snow

  • 1-create_file.c: C function that creates a file.
  • The paramter filename is the name of the file to create.
  • The parameter text_content is a null-terminated string to write to the file.
  • If text_content is NULL, the function creates an empty file.
  • The created file has the permissions rw-------.
  • If the file already exists, the existing permissions are not changed.
  • Existing files are truncated.
  • If filename is NULL or the funciton fails - returns -1.
  • Otherwise - returns 1 on success.

• 2. Speak gently, she can hear

  • 2-append_text_to_file.c: C function that appends text at the end of a file.
  • The parameter filename is the name of the file.
  • The parameter text_content is a null-terminated string to append to the file.
  • The function does not create the file if it does not exist.
  • If text_content is NULL, nothing is added to the file.
  • If the function fails or filename is NULL - returns -1.
  • If the file does not exist or the user lacks write permissions on the file - returns -1.
  • Otherwise - returns 1.

• 3. cp

  • 3-cp.c: C program that copies the contents of a file to another file.
  • Usage: cp file_from file_to
  • If file_to already exists, it is truncated.
  • The created file has the permissions rw-rw-r--.
  • If the file already exists, the existing permissions are not changed.
  • If the number of arguments is incorrect, the function prints Usage: cp file_from file_to, followed by a new line on the POSIX standard error and exits with code 97.
  • If file_from does not exist or the user lacks read permissions on it, the function prints Error: Can't read from file NAME_OF_THE_FILE, followed by a new line and exits with code 98.
    • Where NAME_OF_THE_FILE is the first argument passed to the program.
  • If files cannot be created or if write to file_to fails, the function prints Error: Can't write to NAME_OF_THE_FILE, followed by a new line and exits with code 99.
    • Where NAME_OF_THE_FILE is the second argument passed to the program.
  • If the user cannot close a file descriptor, the function prints Error: Can't close fd FD_VALUE, followed by a new line on the POSIX standard error and exits with code 100.
    • Where FD_VALUE is the value of the file descriptor.

• 4. elf

  • 100-elf_header.c: C program that displays the information contained in the ELF header at the start of an ELF file.
  • Usage elf_header elf_filename
  • Displayed information:
    • Magic
    • Class
    • Data
    • Version
    • OS/ABI
    • ABI Version
    • Type
    • Entry point address
  • Format is identical to readelf -h (version 2.26.1).
  • If an error occurs or the file is not an ELF file, the function displays a corresponding error message to stderr and exits with a status value of 98.