In C, file operations are managed using a pointer variable that references a file, known as a file pointer. This is analogous to file objects in other languages like Java.

File Pointer Declaration

FILE *filePointer;

Opening Files with fopen The fopen function is used to open a file. Its syntax is:

filePointer = fopen("filename", "mode");

• filePointer: A variable declared as FILE *.
• filename: A string specifying the file name.
• mode: A string indicating the file access type.

Closing Files with fclose After file operations, close the file with fclose to prevent data loss:

fclose(filePointer);

• Returns 0 on success; non-zero indicates an error.

Example Workflow

1. Include the standard I/O header:

#include <stdio.h>

2. Declare a file pointer:

FILE *fp = NULL;

3. Open a file:

fp = fopen("example.txt", "r");
if (fp == NULL) {
    printf("Error opening file.\n");
    exit(1);
}

4. Perform file operations.
5. Close the file:

fclose(fp);

File Access Modes

• r: Read from an existing file.
• w: Write to a new file; overwrites if exists.
• a: Append to an existing file; creates if not exists.
• r+: Read and write to an existing file.
• w+: Read and write to a new file; overwrites if exists.
• a+: Append and read from a file; creates if not exists.
• t: Text file (default).
• b: Binary file.

Key Points

• Modes combine characters: r, w, a, t, b, +.
• Use r only for existing files.
• w creates or overwrites files.
• a appends data; creates files if needed.
• Check fopen return for NULL to handle errors.
• Text files involve ASCII-binary conversion; binary files do not.
• Standard streams (e.g., stdin, stdout) are pre-opened.

Character I/O Functions

• fgetc: Reads a single character; returns EOF at end.
• fputc: Writes a single character.

String I/O Functions

• fgets: Reads a string up to a specified length or newline/EOF.

fgets(buffer, size, fp);

• fputs: Writes a string; returns number of characters written.

Block I/O Functions

• fread: Reads blocks of data.

fread(dataBuffer, elementSize, count, fp);

• fwrite: Writes blocks of data.

fwrite(dataBuffer, elementSize, count, fp);

Formatted I/O Functions

• fprintf: Writes formatted data to a file.

fprintf(fp, "%d %s", num, str);

• fscanf: Reads formatted data from a file.

fscanf(fp, "%d %s", &num, str);

Random File Access Use rewind or fseek to move the file position pointer.

• rewind(fp): Resets pointer to file start.
• fseek(fp, offset, origin): Moves pointer relative to origin (0=start, 1=current, 2=end).

fseek(fp, 50L, SEEK_SET);

• Best used with binary files due to text file conversion issues.

File Status Functiosn

• feof(fp): Returns non-zero if end-of-file is reached.
• ferror(fp): Checks for errors during file operations.
• clearerr(fp): Clears error and end-of-file flags.