How do I delete a specific line from text file in C?

Question

Example:

George 50 40
Lime 30 20
Karen 10 60

do {
    printf("\nInput player name:[1..10] ");
    fgets(name,10,stdin);
}

Input name: Lime

Then the text file will be:

George 50 40
Karen 10 60

Answer 1

Consider a file like an array. If we want to delete an element at index i of an array, we shift all elements from i+1 one step towards left and then logically reduce the size of array.

I am applying same princliple on files. In the end ftruncate() is used to reduce the size of file and hence removing the end parts of the file.This code works fine for large files too, as at any time only one byte is being stored in memory.

#include<errno.h>
#include<unistd.h>
#include<stdio.h>
/*
 * Description: fdelete() deletes 'bytes' bytes of data from the stream pointed to by fp. 
 *              bytes will be deleted from the CURRENT FILE POSITION.
 *              File must be opened in read + write mode while passing file pointer 
 *              to this function.
 *              File position before calling the function and after the 
 *              function returns will be the same.
 * Return Values: returns 0 on success and errno on failure. Kindly use perror("") 
 *                to print error if non-0 return value returned.
 */
int fdelete(FILE* fp, int bytes) {
    
    // to store each byte/char from file
    char byte;
    long readPos = ftell(fp) + bytes, writePos = ftell(fp), startingPos = writePos;
    // start reading from the position which comes after the bytes to be deleted
    fseek(fp, readPos, SEEK_SET);
    while (fread(&byte, sizeof(byte), 1, fp)) {
        // modify readPos as we have read right now
        readPos = ftell(fp);
        // set file position to writePos as we are going to write now
        fseek(fp, writePos, SEEK_SET);
        
        // if file doesn't have write permission
        if (fwrite(&byte, sizeof(byte), 1, fp) == 0) 
            return errno;
        // modify writePos as we have written right now
        writePos = ftell(fp);
        // set file position for reading
        fseek(fp, readPos, SEEK_SET);
    }

    // truncate file size to remove the unnecessary ending bytes
    ftruncate(fileno(fp), writePos);
    // reset file position to the same position that we got when function was called.
    fseek(fp, startingPos, SEEK_SET); 
    return 0;
}

Answer 2

Try this:

 /* C Program Delete a specific Line from a Text File
 */
#include <stdio.h>

int main()
{
    FILE *fileptr1, *fileptr2;
    char filename[40];
    char ch;
    int delete_line, temp = 1;

    printf("Enter file name: ");
    scanf("%s", filename);
    //open file in read mode
    fileptr1 = fopen(filename, "r");
    ch = getc(fileptr1);
   while (ch != EOF)
    {
        printf("%c", ch);
        ch = getc(fileptr1);
    }
    //rewind
    rewind(fileptr1);
    printf(" \n Enter line number of the line to be deleted:");
    scanf("%d", &delete_line);
    //open new file in write mode
    fileptr2 = fopen("replica.c", "w");
    ch = 'A';
    while (ch != EOF)
    {
        ch = getc(fileptr1);
        //except the line to be deleted
        if (temp != delete_line)
        {
            //copy all lines in file replica.c
            putc(ch, fileptr2);
        }
        if (ch == '\n')
        {
            temp++;
        }
    }
    fclose(fileptr1);
    fclose(fileptr2);
    remove(filename);
    //rename the file replica.c to original name
    rename("replica.c", filename);
    printf("\n The contents of file after being modified are as follows:\n");
    fileptr1 = fopen(filename, "r");
    ch = getc(fileptr1);
    while (ch != EOF)
    {
        printf("%c", ch);
        ch = getc(fileptr1);
    }
    fclose(fileptr1);
    return 0;
}

Reference - http://www.sanfoundry.com/c-program-delete-line-text-file/