Unexpectedly confused by printf and ctime under MinGW using GCC

At first glance the code seems quite simple and straightforward:

test.c:

Code:

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

int main(void)
{
	time_t t_0 = 1238011200;
	time_t t_1 = 1255348800;

	printf("%s", ctime(&t_0));
	printf("%s", ctime(&t_1));

	printf("Again\n");

	printf("%s%s", ctime(&t_0), ctime(&t_1));

	return 0;
}

And I expect the output to be:

Code:

Thu Mar 26 04:00:00 2009
Mon Oct 12 20:00:00 2009
Again
Thu Mar 26 04:00:00 2009
Mon Oct 12 20:00:00 2009

However, after compiling under MinGW using GCC, the result is:

Code:

Thu Mar 26 04:00:00 2009
Mon Oct 12 20:00:00 2009
Again
Thu Mar 26 04:00:00 2009
Thu Mar 26 04:00:00 2009

The last two record equals. Why?

Because ctime has a static buffer that it uses over and over again, so subsequent calls change the value of the pointer. So that second print statement is using the same pointer value, so will print the same string.

Because you cannot do that. ctime() apparently returns a pointer to a static string (within the ctime() function). Whatever evaluates last is what gets assigned to printf().

Originally Posted by tabstop

Because ctime has a static buffer that it uses over and over again, so subsequent calls change the value of the pointer. So that second print statement is using the same pointer value, so will print the same string.

Originally Posted by Kennedy

Because you cannot do that. ctime() apparently returns a pointer to a static string (within the ctime() function). Whatever evaluates last is what gets assigned to printf().

Thanks. My understanding is, it is:

1) the order that printf evaluates the arguments
2) ctime holds a statically allocated memory

that matters, right?

Originally Posted by babyifan

Thanks. My understanding is, it is:

1) the order that printf evaluates the arguments
2) ctime holds a statically allocated memory

that matters, right?

Because of (2), your one-shot print statement will always print the same thing twice. Which one it prints is a result of (1).