ctime(3) gives incorrect results

[Pietro Cerutti]

Hi group,

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

int main(void)
{
time_t t1, t2;
char *st1, *st2;

t1 = time(NULL);

if(sleep(1)) {
printf("Signal received, exiting\n");
return (1);
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long)t1);
printf("t2 is %lu\n", (unsigned long)t2);

st1 = ctime(&t1);
st2 = ctime(&t2);

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return(0);
}

The code above gives this output:
t1 is 1183506197
t2 is 1183506198
st1 is Wed Jul 4 01:43:18 2007
st2 is Wed Jul 4 01:43:18 2007

How can it happen that two outputs of ctime(3), given two different
inputs (t1 != t2) are the same (st1 == st2) ?

Moreover, on a shell console:

$ date -r 1183506197
Wed Jul 4 01:43:17 CEST 2007
$ date -r 1183506198
Wed Jul 4 01:43:18 CEST 2007

Here date gives a correct result (i.e. the first date is one second
before the second date).

I doubt it has something to do with the compiler grouping the two calls
to time(NULL). If it's the case, how can I avoid it?

Thank you.

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Lew Pitcher]

Pietro Cerutti wrote:

Hi group,

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

int main(void)
{
time_t t1, t2;
char *st1, *st2;

t1 = time(NULL);

if(sleep(1)) {
printf("Signal received, exiting\n");
return (1);
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long)t1);
printf("t2 is %lu\n", (unsigned long)t2);

st1 = ctime(&t1);
st2 = ctime(&t2);

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return(0);
}

The code above gives this output:
t1 is 1183506197
t2 is 1183506198
st1 is Wed Jul 4 01:43:18 2007
st2 is Wed Jul 4 01:43:18 2007

How can it happen that two outputs of ctime(3), given two different
inputs (t1 != t2) are the same (st1 == st2) ?

Well, you get these results because both st1 and st2 point to the same string.

ctime() returns a pointer to a statically allocated character array in which
the date/time string is stored (by asctime(), under the covers). This array is
reused with each call to ctime(), so your second call to ctime() wiped out the
string generated by the first call to ctime().

What you /should/ have done is something like....

printf("t1 is %s\n",ctime(&t1 ));
printf("t2 is %s\n",ctime(&t2 ));

If you needed to defer the printing until after both times were collected, you
should strcpy() the results of the first ctime() call to somewhere safe before
calling ctime() the second time.

HTH
--
Lew Pitcher

Master Codewright & JOAT-in-training | Registered Linux User #112576
http://pitcher.digitalfreehold.ca/ | GPG public key available by request
---------- Slackware - Because I know what I'm doing. ------

[Pietro Cerutti]

Lew Pitcher wrote:

Pietro Cerutti wrote:

>Hi group,

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

int main(void)
{
time_t t1, t2;
char *st1, *st2;

t1 = time(NULL);

if(sleep(1)) {
printf("Signal received, exiting\n");
return (1);
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long)t1);
printf("t2 is %lu\n", (unsigned long)t2);

st1 = ctime(&t1);
st2 = ctime(&t2);

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return(0);
}

The code above gives this output:
t1 is 1183506197
t2 is 1183506198
st1 is Wed Jul 4 01:43:18 2007
st2 is Wed Jul 4 01:43:18 2007

How can it happen that two outputs of ctime(3), given two different
inputs (t1 != t2) are the same (st1 == st2) ?

Well, you get these results because both st1 and st2 point to the same string.

ctime() returns a pointer to a statically allocated character array in which
the date/time string is stored (by asctime(), under the covers). This array is
reused with each call to ctime(), so your second call to ctime() wiped out the
string generated by the first call to ctime().

What you /should/ have done is something like....

printf("t1 is %s\n",ctime(&t1 ));
printf("t2 is %s\n",ctime(&t2 ));

If you needed to defer the printing until after both times were collected, you
should strcpy() the results of the first ctime() call to somewhere safe before
calling ctime() the second time.

HTH

Uh, thank you!

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Martin Ambuhl]

Pietro Cerutti wrote:

Hi group,

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

int main(void)
{
time_t t1, t2;
char *st1, *st2;

t1 = time(NULL);

if(sleep(1)) {
printf("Signal received, exiting\n");
return (1);
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long)t1);
printf("t2 is %lu\n", (unsigned long)t2);

st1 = ctime(&t1);
st2 = ctime(&t2);

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return(0);
}

The code above gives this output:
t1 is 1183506197
t2 is 1183506198
st1 is Wed Jul 4 01:43:18 2007
st2 is Wed Jul 4 01:43:18 2007

How can it happen that two outputs of ctime(3), given two different
inputs (t1 != t2) are the same (st1 == st2) ?

/* mha: off-topic non-standard functionality retained here, although
flagged. The problem does not lie with those non-standard
features, and I was too lazy to create a new example. */

#include <stdio.h>
#include <unistd.h /* mha: not a standard C header */
#include <time.h>
#include <stdlib.h /* mha: added so the error return
returns a portably defined value */
#include <string.h /* mha: added for strcpy */

int main(void)
{
time_t t1, t2;
char st1[26], st2[26]; /* mha: replaced pointers with arrays */

t1 = time(NULL);

if (sleep(1) /* mha: not a standard C function */ ) {
printf("Signal received, exiting\n");
return EXIT_FAILURE; /* mha: replaced the (1) return value */
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long) t1);
printf("t2 is %lu\n", (unsigned long) t2);

/* mha: below were two assignments to two pointers of a pointers to
the same static array. I have fixed that. These two lines are not
actually needed, since the calls to ctime() could be done in
the arguments to printf. */
strcpy(st1, ctime(&t1));
strcpy(st2, ctime(&t2));

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return 0;
}

[Army1987]

"Pietro Cerutti" <ga**@gahr.ch ha scritto nel messaggio news:87******** *************** ****@news.hispe ed.ch...

Hi group,

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

int main(void)
{
time_t t1, t2;
char *st1, *st2;

t1 = time(NULL);

if(sleep(1)) {
printf("Signal received, exiting\n");
return (1);
}

t2 = time(NULL);

printf("t1 is %lu\n", (unsigned long)t1);
printf("t2 is %lu\n", (unsigned long)t2);

st1 = ctime(&t1);
st2 = ctime(&t2);

7.23.3 Time conversion functions
1 Except for the strftime function, these functions each return a pointer to one of two
types of static objects: a broken-down time structure or an array of char. Execution of
any of the functions that return a pointer to one of these object types may overwrite the
information in any object of the same type pointed to by the value returned from any
previous call to any of them.

Here the second call of ctime overwrites the result of the first.
Try declaring st1 and st2 as arrays of chars, and to use strcpy().
char st1[26];
char st2[26];
/* ... */
strcpy(st1, ctime(&t1));
strcpy(st2, ctime(&t2));

printf("st1 is %s", st1);
printf("st2 is %s", st2);

return(0);
}

The code above gives this output:
t1 is 1183506197
t2 is 1183506198
st1 is Wed Jul 4 01:43:18 2007
st2 is Wed Jul 4 01:43:18 2007

How can it happen that two outputs of ctime(3), given two different
inputs (t1 != t2) are the same (st1 == st2) ?

Moreover, on a shell console:

$ date -r 1183506197
Wed Jul 4 01:43:17 CEST 2007
$ date -r 1183506198
Wed Jul 4 01:43:18 CEST 2007

Here date gives a correct result (i.e. the first date is one second
before the second date).

I doubt it has something to do with the compiler grouping the two calls
to time(NULL). If it's the case, how can I avoid it?

If this were the case, you could declare t1 and t2 as volatile. But
it isn't, as t1 < t2.