Explain the Random accessing files in C language

Random access in C allows you to move the file pointer to any position within a file, enabling non-sequential reading and writing operations. This is accomplished using three key functions that control file pointer positioning.

Syntax

long ftell(FILE *stream);
void rewind(FILE *stream);
int fseek(FILE *stream, long offset, int whence);

ftell() Function

The ftell() function returns the current position of the file pointer as a long integer representing the byte offset from the beginning of the file.

#include <stdio.h>

int main() {
    FILE *fp;
    long position;
    
    fp = fopen("test.txt", "w");
    if (fp == NULL) {
        printf("Error opening file
");
        return 1;
    }
    
    fprintf(fp, "Hello World");
    position = ftell(fp);
    printf("Current position: %ld
", position);
    
    fclose(fp);
    return 0;
}

Current position: 11

rewind() Function

The rewind() function moves the file pointer to the beginning of the file, equivalent to calling fseek(fp, 0, SEEK_SET).

#include <stdio.h>

int main() {
    FILE *fp;
    long position;
    
    fp = fopen("test.txt", "w+");
    if (fp == NULL) {
        printf("Error opening file
");
        return 1;
    }
    
    fprintf(fp, "Hello World");
    printf("Position after writing: %ld
", ftell(fp));
    
    rewind(fp);
    position = ftell(fp);
    printf("Position after rewind: %ld
", position);
    
    fclose(fp);
    return 0;
}

Position after writing: 11
Position after rewind: 0

fseek() Function

The fseek() function moves the file pointer to a specific location in the file based on an offset and a reference position.

Parameters

• offset − Number of bytes to move (can be positive or negative)
• whence − Reference position:
  • SEEK_SET (0) − Beginning of file
  • SEEK_CUR (1) − Current position
  • SEEK_END (2) − End of file

#include <stdio.h>

int main() {
    FILE *fp;
    char ch;
    
    fp = fopen("test.txt", "w+");
    if (fp == NULL) {
        printf("Error opening file
");
        return 1;
    }
    
    fprintf(fp, "ABCDEFGHIJ");
    
    /* Move to 5th position from beginning */
    fseek(fp, 5, SEEK_SET);
    ch = fgetc(fp);
    printf("Character at position 5: %c
", ch);
    
    /* Move 2 positions backward from current */
    fseek(fp, -2, SEEK_CUR);
    ch = fgetc(fp);
    printf("Character after moving back 2: %c
", ch);
    
    /* Move to 3rd position from end */
    fseek(fp, -3, SEEK_END);
    ch = fgetc(fp);
    printf("3rd character from end: %c
", ch);
    
    fclose(fp);
    return 0;
}

Character at position 5: F
Character after moving back 2: E
3rd character from end: H

Common Usage Examples

• fseek(fp, 0, SEEK_END) − Move to end of file
• fseek(fp, 0, SEEK_SET) − Move to beginning of file
• fseek(fp, 10, SEEK_SET) − Move 10 bytes from beginning
• fseek(fp, -5, SEEK_END) − Move 5 bytes back from end

Key Points

• fseek() returns 0 on success, non-zero on failure
• Random access works best with binary files
• Text files may have platform-specific line ending issues
• Always check return values for error handling

Conclusion

Random file access in C provides powerful control over file operations through ftell(), rewind(), and fseek() functions. These functions enable efficient data manipulation by allowing direct positioning within files without sequential reading.