read lines from file with fgets and compare each line with strncmp in c

Question

i want to read every line from a file which looks something like this:

readEveryLine
{       
  "Bart [m]" -> "Marge [f]";  
  "Lisa [f]" -> "Homer [m]"; 
  ...      
}

i want to use:

1. fgets() to read the file line by line
2. strncmp() to compare every line with a given string or see that it has just the right format

what i have:

while(fgets(*file_string, MAX_INPUT_STDIN, file) != NULL)
{       
  changeLastC(*file_string);  // function to change \n into \0 

    if (strncmp(*file_string, "readEveryLine\0", 14) == 0)
    {
      if (strncmp(*file_string, "{\0", 2) == 0)
      {
        // check the first -> relation
      }
    }
    else
    {
      printf("Error Parsing\n");
    } 
}

so the problem is that it just gives me an Error Parsing and i don`t know what i did wrong here.

Thanks a lot for helping me!

here i made a few things now (parsing the first two lines works now) : maybe anyone has got a good tip for me what i could do better. Thanks a lot.

if ((fp = fopen("df.dot","r")) == NULL)
{
  printf("Error: File Open\n");
  return 1;
}


int row = 0; // check row 1

while (fgets(buffer, MAX_PARSING, fp))
{        
  if ((row == 0) && strncmp(buffer, "readEveryLine\n", 14) == 0)
  {
    printf("%s", buffer);
  }
  else
  {
    printf("Parsing Error 1\n");
  }
}


int row1 = 1; // check row 2

while (fgets(buffer, MAX_PARSING, fp))
{     
  if ((row1 == 1) && strncmp(buffer, "{\n", 2) == 0)
  {
    printf("%s", buffer);
  }
  else
  {
    printf("Parsing Error 2\n");
  }
}


int row2 = 2; // check other rows (dynamic, could be even more or less)

while (fgets(buffer, MAX_PARSING, fp))
{ 
  if ((row2 == 2) && strncmp(buffer, "  ", 2) == 0)
  {
    const char *p1 = strstr(fp, "\"")+1;
    const char *p2 = strstr(p1, " [m]\"");
    const char *p3 = strstr(p1, " [f]\"");

    // extract male persons
    if (p1 && p2)
    {
      size_t len1 = p2 - p1;
      char* res1 = (char*)malloc(sizeof(char)*(len1 + 1));
      strncpy(res1, p1, len1);

      res1[len1] = '\0';

      // give res1 for functionMale() to work on that string
    }

    // extract female persons
    else if (p1 && p3)
    {
      size_t len2 = p3 - p1;
      char* res2 = (char*)malloc(sizeof(char)*(len2 + 1));
      strncpy(res2, p1, len2);

      res2[len2] = '\0';

      // give res2 for functionFemale() to work on that string
    }

    else if (strcmp(buffer, " -> ") == 0)
    {
      // work in progress (quite complicated to do this i think)
      // it has to be a realtion between two people
    }

    else if (strcmp(buffer, ";") == 0)
    {
      // work in progress
      // this sign can either exist like this:
      // "Bart [m]" -> "Marge [f]";

      // or like this:
      // "Marge [f]";
    }

    break;
  }
  else
  {
    printf("Parsing Error 3\n");
  }

  row2++;

}

// and the very last sign has to be }\n

Answer 1

Your algorithm is already broken. You use the very same content of *file_string to compare it against tweo different strings. If you find a match for "readEveryLine" you need to read the next line from your file before you can get the next match for strncmp(). Otherwise the line from the file must match both "readEveryLine" and "{" to pass your second if condition, which is impossible.

Edit: Now as you have done some improvements I still think it will not work with your approach. The loops will not exit when they should and your if-else-cascade also does not seem to be a good idea. In your approach you will get messed up with reading too many lines while only 1 line should be parsed.

Maybe you should read a bit about state machines.

Here is a quick approach how I would address the problem:

enum { STATE_HEADER1, STATE_HEADER2, STATE_BODY, STATE_END} state;
int done = 0;
state = STATE_HEADER1;

while (fgets(buffer, MAX_PARSING, fp) && !done) {        
  if (state == STATE_HEADER1) {
    if (strcmp(buffer, "readEveryLine\n") == 0) {
      printf("%s", buffer);
      state = STATE_HEADER2;
    }
    else {
      printf("Parsing Error 1\n");
      done = 1;
    }        
  }
  else if (state == STATE_HEADER2) {
    if (strcmp(buffer, "{\n") == 0) {
      printf("%s", buffer);
      state = STATE_BODY;
    }
    else {
      printf("Parsing Error 2\n");
      done = 1;
    }
  }
  else if (state == STATE_BODY) {
    if (strcmp(buffer, "  ") == 0) {
      const char *p1 = strstr(buffer, "\"");
      const char *pm = strstr(p1, " [m]\"");
      const char *pf = strstr(p1, " [f]\"");
            char *res;
      const char *ptemp;
      int is_male;

      if (p1 && pf)  {
        p1 ++;
        is_male = 0;
        size_t len1 = pf - p1;
        res = malloc(len1 + 1);
        strcpy(res, p1);
        ptemp = pf+3; // point after closing \"

        // give res for functionFemale() to work on that string
      }
      else if (p1 && pm) {
        p1 ++;
        is_male = 1;
        size_t len1 = pm - p1;
        res = malloc(len1 + 1);
        strcpy(res, p1);
        ptemp = pm+3; // point after closing \"

        // give res for functionMale() to work on that string
      }
      else {
        done = 1;
        printf("Parsing Error 2\n");
      }

      // Now we have res and is_male holding name and gender.

      if (!done)
      {
        if (strncmp(ptemp, " -> ", 4) == 0) {

  // Handle this variant:
  // this sign can either exist like this:
  // "Bart [m]" -> "Marge [f]";

          // Do similar stuff as above for first name

          // Get second name + gender
          // Also check trailing ';' here
        }
        else if (strcmp(temp, ";\n") == 0) {

 // Handle this variant:
 // or like this:
 // "Marge [f]";

        }

      } // found "  "
      else {
        if (strcmp(buffer, "}\n") == 0) {
          state = STATE_END;
          done = 1;
          printf("That's it folks...\n"); 
        }
        else {
          done = 1;
          printf("Parsing Error 3\n");
        }
      }
    }
  } // STATE_BODY
} // while (fgets)

if (state == STATE_END) }
  // Success. :)
} else {
  // Something didn't match.
}

// close file, cleanup, etc.

I did not compile it yet but you should get the idea.