Question

I have written a program which takes a file as input and whenever it finds a line with length > 80, it adds \ and \n to that file to make it 80 chars in width max.

The problem is that I have used fseek to insert \ and \n whenever the length exceeds 80, so it overrides two characters of that line which exceeds length 80. Is there a way using which I can insert text without overriding the existing text?

Here is my code:-

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

int main(int argc, char *argv[])
{
  FILE *fp1,*fp2;
  int prev=0,now=0;
  char ch;
  int flag=0;
  long cur;
  fp1=fopen(argv[1],"r+");
  if(fp1==NULL){
    printf("Unable to open the file to read. Program will exit.");
    exit(0);
  }
  else{
    while((ch=fgetc(fp1))!=EOF){
      if(ch!=' ' && ch!='\n'){
        now=now+1;
      }
      else{
        if(now>=80){
            fseek(fp1,cur,SEEK_SET);
            fputc('\\',fp1);
            fputc('\n',fp1);
            now=0;
            continue;
        }
        if(ch=='\n'){
          flag=0;
          now=0;
          continue;
          }
        else{
          prev=now;
          cur=ftell(fp1);
        }
        now=now+1;
      }
    }
  }
  fclose(fp1);
  return 0;
}

To run it, you need to do following:-

user@ubuntu$ cc xyz.c
user@ubuntu$ ./a.out file_to_check.txt

Answer 1

No, there’s no way to insert characters into an existing file. You will need to use a second file to do that.