Add a line of text after the nth line in a txt file using c file handling

Question

I want to add a line of text after the fifth line in a text file.This is my code:

#include <stdio.h> 

int main(){
  FILE *f;
  f = fopen("file1.txt", "r");

  int nCount = 0;
  char c = fgetc(f);
  while(nCount != 5){
    if(c == '\n'){
      nCount++;
    }
    // c = fgetc(f);
    fseek(f, 1, SEEK_SET);
  }

  c = fgetc(f);

  fprintf(f, "I have appended a line");
  
 
  return 0;
}

Line is not being appended.What is wrong?

Answer 1

If your text file is of a reasonable size (to fit into memory) the following works with one open file... I leave it as an exercise to deal with the Windows/DOS convention of CRLF where LF works just as well. (This is just a rough cut, but it may lead you to the results you want.)

It could do with more testing of return values, especially the fwrite() calls.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int my_main() {
    char *fname = "log.txt";

    FILE *fp = fopen( fname, "rb+" );
    if( fp == NULL ) {
        fprintf( stderr, "Cannot open %s r/w\n", fname );
        return 1;
    }

    // measure space needed
    fseek( fp, 0, SEEK_END );
    size_t size = ftell( fp );
    fseek( fp, 0, SEEK_SET );

    // get space
    char *iobuf = (char*)malloc( size );
    if( iobuf == NULL ) {
        fprintf( stderr, "Cannot allocate %ld byte buffer\n", size );
        fclose( fp );
        return 1;
    }

    // load entire file
    size_t nread = fread( iobuf, sizeof(char), size, fp );
    if( nread != size ) {
        fprintf( stderr, "Expecting %ld - Read %ld\n", size, nread );
        return 1;
    }

    int lnCnt = 5; // skip past 5 lines
    for( char *cp = iobuf; cp < iobuf + size; cp++ )
        if( *cp == '\n' && --lnCnt == 0 )
            break;
    cp += 1;
    // Up to you to handle situation where #lines < 5

    // position after 5th '\n'
    fseek( fp, cp - iobuf, SEEK_SET );

    // write the new string
    char *newText = "I wish I was a fish\n"; // Written after whatever line
    fwrite( newText, sizeof newText[0], strlen( newText ), fp );

    // then write back the rest
    fwrite( cp, sizeof *cp, size - (cp - iobuf), fp );

    fclose( fp );

    return 0;
}

Output:

This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
I wish I was a fish
This is line 6
This is line 7