C: Parsing a file with fgetc() - EOF infinite loop

Question

Hello everbody @Stackoverflow,

currently I am trying to write a simple parser for the .obj File format but somehow my parser is stuck in an infinite loop after reading the last comment in my file.txt. But it also happens in-between.

The parser should do following:

If the current char is a '#' it should print "COMMENT" and skip this line, if it is a 'v' it should print "VERTEX" and skip the line.

Even though I stepped through my code with a debugger I still can't find the problem.

file.txt:

# Comment1
# Comment2
# Comment3
v Vertex1
# Comment4

Code:

int main()
{
    FILE *file = fopen("file.txt", "r");
    if(file==NULL)
    {
        return -1;
    }

    int currentChar=0;

    while(currentChar!=EOF)
    {
        currentChar=fgetc(file);
        printf("CURR CHAR: %c\n", currentChar);
        switch(currentChar)
        {
            case '#':
                {
                    printf("COMMENT\n");
                    currentChar=fgetc(file); //Read Whitespace 1x
                    while(currentChar!='\n')
                        currentChar=fgetc(file);
                    break;
                }
            case 'v':
                {
                    printf("VERTEX\n");
                    currentChar=fgetc(file); //Read Whitespace 1x
                    while(currentChar!='\n')
                        currentChar=fgetc(file);
                    break;
                }
            }
    }

    return 0;
}

I still can not see where the problem lies.

Sincerely, toxic

Answer 1

It’s looks like it’s getting stuck waiting for a newline character. The loop within your switch case is probably the culprit. Change your loop to be:

    while(currentChar != EOF && currentChar != '\n')

Answer 2

You probably want this:

#include <stdio.h>

int main()
{
  FILE *file = fopen("file.txt", "r");
  if (file == NULL)
  {
    return -1;
  }

  int currentChar = 0;

  while (currentChar != EOF)
  {
    currentChar = fgetc(file);

    if (currentChar == EOF)    // if EOF stop loop immediately,
                               // it's pointless to continue
      break;

    printf("CURR CHAR: %c\n", currentChar);
    switch (currentChar)
    {
      case '#':
      {
        printf("COMMENT\n");
        currentChar = fgetc(file); 
        while (currentChar != EOF && currentChar != '\n')   // skip until \n or EOF
          currentChar = fgetc(file);
        break;
      }
      case 'v':
      {
        printf("VERTEX: <");
        currentChar = fgetc(file);

        while (currentChar != EOF && currentChar != '\n')   // read until \n or EOF
        {
          printf("%c", currentChar);
          currentChar = fgetc(file);
        }

        printf(">\n");
        break;
      }
    }
  }

  return 0;
}

It's heavily based upon your original code. All comments are mine.