C time.h wrap around - c

Question

I would like to assign max possible time to a time_t variable then convert it to a string and print out the result.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <limits.h>
int main()
{
time_t maxTime;
maxTime= LONG_LONG_MAX;
char *strOfMaxTime;
*strOfMaxTime = ctime(maxTime);
printf("%s",strOfMaxTime);
return 0;
}

Answer 1

OP's code is using char *ctime(const time_t *timer) incorrectly.
time_t maxTime;
char *strOfMaxTime;
// *strOfMaxTime = ctime(maxTime);
strOfMaxTime = ctime(&maxTime);
Yet simply assigning maxTime= LONG_LONG_MAX; is not necessary the correct way to determine the maximum time a system can handle.
Below is a trial and error method - likely with various implementation limitations. localtime() returns NULL when time_t is out of range.
#include <stdio.h>
#include <time.h>
time_t max_time() {
time_t t0, t1;
time_t delta = 1;
time(&t0); // now
while (t0 != -1) {
t1 = t0 + delta;
if (localtime(&t1) == NULL) { // If conversion fail, quit doubling.
break;
}
delta *= 2; // 2x for the next increment.
t0 = t1;
}
while (delta) {
t1 = t0 + delta;
if (localtime(&t1) != NULL) { // if succeeds, update t0
t0 = t1;
}
delta /= 2; // try smaller and smaller deltas.
}
printf("%s %lld\n", ctime(&t0), (long long) t0);
return t0;
}
int main(void) {
max_time();
return 0;
}
Output (Note that 17:59:59 depends on timezone and the year 2,147,483,647 is the max 32-bit signed integer. YMMV.)
Tue Dec 31 17:59:59 2147483647
67767976233532799

Answer 2

From C Standards#7.27.1p4
The range and precision of times representable in clock_t and time_t are implementation-defined.
First, you need to fix the issue in your program. The below statement must be giving error while compiling:
*strOfMaxTime = ctime(maxTime);
Change this to:
strOfMaxTime = ctime(&maxTime);
You can use perror() to get the error message for the given input - LONG_LONG_MAX, like this:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <limits.h>
#include <errno.h>
int main()
{
time_t maxTime;
maxTime= LONG_LONG_MAX;
char *strOfMaxTime;
strOfMaxTime = ctime(&maxTime);
if (errno != 0)
perror ("Error");
else
printf("%d,%s",errno, strOfMaxTime);
return 0;
}
On my setup I am getting this output:
Error: Value too large to be stored in data type
Indeed, LONG_LONG_MAX is invalid input.
As the standard mentions that the range of time_t is implementation-defined, so if I give UINT_MAX I am getting the output:
0,Sun Feb 7 11:58:15 2106

Answer 3

This is wrong:
*strOfMaxTime = ctime(maxTime);
This tries to assign the return value of ctime (a pointer to a char) to *strOfMaxTime a char.
Instead call:
strOfMaxTime = ctime(&maxTime);
And then check the return value of strOfMaxTime as it may be NULL if ctime fails to convert maxTime

Answer 4

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <limits.h>
int main()
{
time_t maxTime;
maxTime = INT_MAX;
char *strOfMaxTime = ctime(&maxTime);
printf("%s",strOfMaxTime);
return 0;
}
The maximum year is 2038, and this is known as Year 2038 problem:
https://en.wikipedia.org/wiki/Year_2038_problem

Answer 5

Numerous errors have been pointed out in other postings (assigning output of ctime() to *strOfMaxTime, LONG_LONG_MAX, etc). On my 64bit Ubuntu 16.04 Linux system, time_t is defined as a long int and a long int is defined as 8 bytes as is a long long int. However assigning LLONG_MAX to maxTime still causes ctime() to fail. So I modified your code to get a range of what the upper limit of valid values ctime() will accept.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>
#include <limits.h>
int main()
{
time_t maxTime;
maxTime= LONG_MAX;
char *strOfMaxTime;
strOfMaxTime = ctime(&maxTime);
while( strOfMaxTime == NULL )
{
perror("ctime error");
printf("%ld div by 2\n", maxTime);
maxTime /= 2;
strOfMaxTime = ctime(&maxTime);
}
printf("%s\n",strOfMaxTime);
return 0;
}
Running it yields the following output:
ctime error: Invalid argument
9223372036854775807 div by 2
ctime error: Invalid argument
4611686018427387903 div by 2
ctime error: Invalid argument
2305843009213693951 div by 2
ctime error: Invalid argument
1152921504606846975 div by 2
ctime error: Invalid argument
576460752303423487 div by 2
ctime error: Invalid argument
288230376151711743 div by 2
ctime error: Invalid argument
144115188075855871 div by 2
ctime error: Invalid argument
72057594037927935 div by 2
Sat Jun 12 22:26:07 1141709097