Question

I’m having a hard time using sscanf to scan hour and minutes from a list. Below is a small snip of the list.

1704 86 2:30p 5:50p Daily
1711 17 10:40a 2:15p 5
1712 86 3:10p 6:30p 1
1731 48 6:25a 9:30a 156
1732 100 10:15a 1:30p Daily
1733 6 2:15p 3:39p Daily

I’ve tried this, but it keeps getting me segmentation Fault.(I’m putting this information into structures).

 for(i=0;i<check_enter;i++){
    sscanf(all_flights[i],
    "%d %d %d:%d%c %d:%d%c %s",
    &all_flights_divid[1].flight_number,
    &all_flights_divid[i].route_id,
    &all_flights_divid[i].departure_time_hour,
    &all_flights_divid[i].departure_time_minute,
    &all_flights_divid[i].departure_time_format,
    &all_flights_divid[i].arrival_time_minute,
    &all_flights_divid[i].arrival_time_minute,
    &all_flights_divid[i].arrival_time_format,
    &all_flights_divid[i].frequency);

    printf("%d ",all_flights_divid[i].flight_number);
    printf("%d ",all_flights_divid[i].route_id);
    printf("%d ",all_flights_divid[i].departure_time_hour);
    printf("%d ",all_flights_divid[i].departure_time_minute);
    printf("%c ",all_flights_divid[i].departure_time_format);
    printf("%d ",all_flights_divid[i].arrival_time_hour);
    printf("%d ",all_flights_divid[i].arrival_time_minute);
    printf("%c ",all_flights_divid[i].arrival_time_format);
    printf("%s\n",all_flights_divid[i].frequency);
  }

This is how I declared it.

struct all_flights{
  int flight_number;
  int route_id;
  int departure_time_hour;
  int departure_time_minute;
  char departure_time_format;
  int arrival_time_hour;
  int arrival_time_minute;
  char arrival_time_format;
  char frequency[10];
};
struct all_flights all_flights_divid[3000];

These are the results I get

0 86 2 30 p 0 50 p Daily
0 17 10 40 a 0 15 p 5
0 86 3 10 p 0 30 p 1
0 48 6 25 a 0 30 a 156
0 100 10 15 a 0 30 p Daily
0 6 2 15 p 0 39 p Daily

Answer 1

Look carefully at your list of output targets in sscanf. Do you see the difference between &all_flights_divid[i].departure_time_minute and all_flights_divid[i].departure_time_format? Similary for .arrival_time_format and .frequency.

What do you think the & ampersand is for? Hint: what is one way of returning multiple values from a single function call, and what does this have to do with the & ampersand?

A segmentation fault arises when your program tries to write data into memory the operating system has never instructed the CPU to make available to the program. Segmentation faults do not always occur when data is misplaced, because sometimes data is misplaced within available memory. By way of analogy, if you inadvertently put a book in the wrong place on the bookshelf, you’ll not easily find the book later, but the book is still on a bookshelf and does not seem to anyone to be out of place. On the other hand, if you inadvertently put the same book in the refrigerator, well, when mother goes to get the milk she’s going to issue you a segmentation fault! That’s the analogy, anyway.

In general, it is hard to guess whether misplacing data will cause a segmentation fault (as misplaced into the refrigerator) or not (as misplaced on the bookshelf) until you run the program. The segmentation fault (refrigerator) is preferable because it makes the mistake obvious, so the operating system tries to give you as many segmentation faults as it can by affording the program as little memory as possible.

I am avoiding giving a 100 percent direct answer because of your “homework” tag. See if you cannot figure out the & ampersand matter, then come back here if it still does not make sense.