Loop in a file being read in C

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Well, I’m trying to read from a file, and create a loop so I can go through all the characters until I find '<' character. After finding this character, the program must read and save the handle information until finding a ' n', to be processed later, and until then everything happens well.

The problem happens if before these actions occur I insert a while((ch = fgetc(in_file)) != EOF) intended to start reading the text again until it finds the next character '<' to perform the operation again, and stop only at the end of the file.

With while included, I get a Segmentation fault. I’ve tried to create a temporary variable to store the variable position ch, but the Segmentation fault continues to occur.

How can I solve this problem?

#include <stdio.h>

int main(void) {
  int i = 0;
  char s[100];
  char ch;
  FILE *in_file  = fopen("name_of_file", "r"); 
  
  while((ch = fgetc(in_file)) != EOF){  //aqui acontece o erro. sem essa tentativa de loop, os 2 for's que se seguem ocorrem sem erros

      for(ch = fgetc(in_file);ch != '<';ch = fgetc(in_file)){
          }
      i=0;
      for(ch = fgetc(in_file);ch != '\n';ch = fgetc(in_file)){
        s[i] = ch;
        i++;
      }
      s[i] = '\0';
  }
  return 0;
}
c filing-cabinet loop
  • 2

    EOF returns when you reached the end of the file, or some error happened. What happens if while you’re in 1st or 2nd you don’t find any '<' or ' n' it will read endlessly. You need to check if you have reached the end of the file while reading it. while will only test this, when the two Fors finish their execution by their own tests.

  • You should just use a loop, testing on that if you get to the < or to the \n or at the end of the file. This change of logic solves the problem that has in its entirety.

1 answer

3


The problem is that you are doing several readings and in only one of them, in the while, is that it checks if it has reached the end of the file. It can solve the problem by swapping the various loops for one. Assuming that you need to build strings with everything that comes between < and \n then you can change the logic of your program to the following:

#include <stdio.h>

int main(void) {
  int i = 0, save_str = 0, pos = 0; //mais duas variáveis de controle
  char s[100];
  char ch;
  FILE *in_file  = fopen("name_of_file", "r");

  while((ch = fgetc(in_file)) != EOF){
    if (ch == '\n'){ //chegou ao \n fecha a string construida
        s[pos] = '\0'; //coloca o terminador que já tinha
        //utilizar a string
        save_str = 0;
    }
    if (save_str){
        s[pos++] = ch;
    }
    if (ch == '<'){ //achou < começa a construir a string daqui para a frente
        save_str = 1;
        pos = 0;
    }
  }
  return 0;
}

Note that:

  • The test of \n is done before adding the character to the string so that the \n is not included in the string
  • The save_str is actually a boolean in the style of C to indicate when or not to build the string.

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