scanf and time_t segmentation fault

[dead_rabbit]

I was working on a program which reads a input in seconds from the user. This program compiles fine but when I enter a value for the seconds it crashes. I used this program to test the assignment of a value to a time_t variable

#include<stdio.h>
#include<time.h>

int main() {

    time_t t;
    scanf("%lld", &t);
    printf("%lld\n", t);
    return 0;
}

But when I execute this code, after entering a number, I get

Segmentation fault: 11 (core dumped)

any insight?

 

[fonz]

Works just fine on my system. Can you provide a stack trace or something?

Fonz

 

[xibo]

lld expects long long decimal, which is usually a 64 bit integer. What's the length of a time_t on your machine, and especially, is sizeof(time_t) == sizeof(long long int) true?

 

[fonz]

Maybe this will solve your problem?

A couple of remarks:

• For starters, I try to avoid long long int because C90 doesn't allow it.
• time_t is usually a long int, not a long long int.
• Try the following on your machine:

  #include <stdio.h>
  #include <time.h>
  
  int
  main()
  {
    printf("%d %d %d\n",sizeof(long int),sizeof(long long int),sizeof(time_t));
    return 0;
  }

  On an i386 I get the output 4 8 4 (in other words: time_t is a long int). On an amd64 I get the output 8 8 8 (in other words: there's no difference between long int and long long int). This explains why your code segfaults on an i386 but works just fine on an amd64.
• Did you use any warning flags at all? If I compile your code with -Wall I get warnings about type differences.
• The most likely solution is to use ld instead of lld. When I try that your code works fine even on the i386.
• To get rid of any warnings, explicitly cast the time_t to long int as follows:

  #include <stdio.h>
  #include <time.h>
  
  int
  main()
  {
    time_t t;
    scanf("%ld",(long int*)&t);
    printf("%ld\n",(long int)t);
    return 0;
  }

Hope this helps,

Fonz

 

[dead_rabbit]

Thanks a bunch Fonz! I tried explicitly casting time_t to long int as you suggested above and every thing is well now