File I/O

File: Represents a sequence of bytes on the disk where a group of related data is stored.

Created for permanent storage of data.
In C, we use file pointers to modify files.
- stdio.h provides the FILE struct for us.

Stream: File is treated as a sequence of bytes,

Access is sequential (i.e., in byte order).
Cannot replace data in the middle of a file.

Raw: Files are sequences of blocks.

Any block can be read and/or written independently.

C File Functions

Function	Description
`fopen()`	create a new file or open a existing file
`fclose()`	closes a file
`fgetc()`	reads a character from a file
`fputc()`	writes a character to a file
`fscanf()`	reads a set of data from a file
`fprintf()`	writes a set of data to a file
`getw()`	reads a integer from a file
`putw()`	writes a integer to a file
`fseek()`	set the position to desire point
`ftell()`	gives current position in the file
`rewind()`	set the position to the beginning point

Note: fclose() returns zero on success and EOF if there’s an error.

EOF is a constant defined in stdio.h

Failing to close can lead to corruption of data.

Opening and Closing a File

Filename and Mode

Filename: Name of the file.

Mode: Purpose of opening the file.

Used when opening/creating a file.

Mode	Description
r	opens a text file in reading mode
w	opens or creates a text file in writing mode
a	opens a text file in append mode
r+	opens a text file in both reading and writing mode
w+	opens a text file in both reading and writing mode
a+	opens a text file in both reading and writing mode
rb	opens a binary file in reading mode
wb	opens or creates a binary file in writing mode
ab	opens a binary file in append mode
rb+	opens a binary file in both reading and writing mode
w+	opens a binary file in both reading and writing mode
a+	opens a binary file in both reading and writing mode

Notes:

w will overwrite an existing file.

r+, w+, a+ can be dangerous.

b is for dealing with raw data.

Examples

Example: Writing to a file

#include <stdio.h>
int main() {
  // Open "one.txt" in write mode
  FILE *fp = fopen("one.txt", "w");
  char ch;
  // Write to the file with user inputted characters.
  printf("Enter data...\n");
  while( (ch = getchar()) != EOF) {
      putc(ch, fp);
  }
  // Close file and return
  fclose(fp);
  return 0;
}

Example: Reading from a file

#include <stdio.h>
int main() {
  // Open "one.txt" in read mode
  FILE *fp = fopen("one.txt", "r");
  char ch;
  // Print contents character by character
  while( (ch = getc(fp)) != EOF) {
      printf("%c",ch);
  }
  // Close file and return
  fclose(fp);
  return 0;
}

`fseek`

fseek(): Function used to move file pointer position to a given location.

Format: int fseek(FILE *fp, long int offset, int whence)

fp: file pointer
offset: Number of bytes/characters to be offset/moved from whence/the current file pointer position
whence: This is the current file pointer position from where offset is added.
- Below 3 constants are used to specify this.

File Operation	Declaration and Description
`SEEK_SET`	Moves file pointer position to the beginning of the file.
`SEEK_CUR`	Moves file pointer position to given location.
`SEEK_END`	Moves file pointer position to the end of file.