Assigning values from an array to an array of structs

Question

I'm trying to assign values from an array containing all strings to an array of structs, where some of the struct members are integers. The way I've attempted it causes some undefined behavior, though. Like the code below generates the following:

0 surname ▒▒
1 forename &
2 id 0

when it should say

0 surname Boatswain
1 forename Michael Jr
2 id 109993267

I'm not exactly sure what's wrong with the way I've assigned these values though.

Code:

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

//There are 7 values for each student
#define VALUES 7

struct Students{
  int term;
  int id; // NEED TO GENERATE ERROR IF WRONG NUM OF CHARACTERS
  char surname[15];
  char forename[15];
  char subject[3];
  int catnum;
  char section[3];
};

int main(int argc, char *argv[]){

//Get contents of input file
  unsigned char filename;
  printf("Enter the input file name: \n");
  scanf("%s", &filename);
  FILE *file = fopen(&filename, "r");
  int filesize=0;
  fseek(file, 0L, SEEK_END);
  filesize = ftell(file);
  fseek(file, 0L, SEEK_SET);
  char *contents = malloc(filesize+1);
  size_t size=fread(contents,1,filesize,file);
  contents[size]=0;  // what does all this do exactly?

//Get number of lines in input file
  int total_line = 0;
  const char *str;
  for(str = contents; *str; ++str)
  total_line += *str == '\n';

//Tokenize string
  int n=0,nn;
  char *b[VALUES*total_line];
  char *ds=strdup(contents);
  b[n]=strtok(ds, ",=\"\r\n\"");
  while(b[n] && n<((VALUES*total_line)-1)) b[++n]=strtok(NULL, ",=\"\r\n\"");
  //for(nn=0; nn<=n; ++nn) printf("%s %d   ", b[nn], nn);
  //putchar('\n');
  free(ds);

  struct Students record[total_line];

  int i, j;
  for(i=0;i++;i<total_line){
    for(j=0;j++;j<VALUES){
      int n=(7*i)+j;
      record[i].term=atoi(b[n]);
      record[i].id=atoi(b[n]);
      strcpy(record[i].surname, b[n]);
      strcpy(record[i].forename, b[n]);
      strcpy(record[i].subject, b[n]);
      record[i].catnum=atoi(b[n]);
      strcpy(record[i].section, b[n]);
    }
  }

// try printing some values here
  printf("0 surname %s\n",record[0].surname); 
  printf("1 forename %s\n",record[1].forename);
  printf("2 id %d\n", record[2].id);

  free(contents);
  return 0;
}

Input file:

1301,107515018,"Boatswain","Michael R.",CSE, 230,="R01"
1301,109993269,"Castille","Michael Jr",CSE, 230,="R03"
1301,109993267,"Castille","Janice",CSE, 230,="R03"

Thank you in advance for the help!

Edit: What is the issue in changing the for loop like so?

for(i=0;i<total_line;i++) { 
  record[i].term=atoi(b[(7*i)]);
  record[i].id=atoi(b[(7*i)+1]);
  strcpy(record[i].surname, b[(7*i)+2]);
  strcpy(record[i].forename, b[(7*i)+3]);
  strcpy(record[i].subject, b[(7*i)+4]);
  record[i].catnum=atoi(b[(7*i)+5]);
  strcpy(record[i].section, b[(7*i)+6]);
}

Edit: I got it to work. Just needed to change the for-loop to a while-loop. Edit2: ohh I had the i++ and i total_line switched. derp. fixed now. Well I guess that solves this mystery.

Answer 1

In your for(j=0;j++;j<VALUES) loop you are converting the same b[n] multiple times. You most likely don't need this loop but to convert b[n++] instead.

Answer 2

As a preliminary, this line (which apparently you already find suspicious) ...

  contents[size]=0;  // what does all this do exactly?

... adds a string terminator after the bytes read from the file, so the whole thing can safely be interpreted as one big C string.

@DavetheSax makes a valid observation about the indexes of your tokens. That problem does not cause undefined behavior, but the resulting behavior is surely not what you wanted.

You indeed do have undefined behavior in your program, however. You fill array b by duping contents into ds, and then tokenizing ds with strtok(). That in itself is ok, but needful only if you need to preserve contents as it was originally read, which you probably don't. After the tokenization, however, you free ds. That, again, is not a problem in itself, but it invalidates all the token pointers you just computed, because they point into the memory previously allocated for ds (from which they were tokenized). You need to preserve that until you've done analyzing the tokens. Dereferencing those token pointers after freeing the space into which they point produces UB.

As a matter of principle, one cannot predict UB (else it would not be UB). Still, the particular results you report seem an unlikely manifestation, so perhaps there is yet more going on here.