Why does ctime returns the same string for different time_t values?

Question

I have a function which prints time_t values.

void Logger::describe()
    {
        cout << m_start_time << " " << m_end_time << "\n";
            if (ctime(&m_start_time) == ctime(&m_end_time))
            {
                cout << "check1\n";
            }
            cout << m_vehicle->getPlate() << " " << ctime(&m_start_time) << " " << ctime(&m_end_time) << "\n";
    }

// m_start_time and m_end_time are private variables of type time_t of the class Logger

For a sample output after waiting a couple of seconds I get

1634907786 1634907791
check1
bike1 Fri Oct 22 18:33:06 2021
 Fri Oct 22 18:33:06 2021

As can be seen m_start_time and m_end_time are different but ctime returns the the same value. Can anyone help explain why ?

I'm using gcc 6.3.0 if it helps.

ctime returns a char*, i.e. a pointer to a string buffer. The function does not have to be thread safe. Therefore the same buffer might be reused. You should print the first result before you call again. Also: You only compare buffers, not string content. At least when it comes to C which you have tagged your question with. — Gerhardh
– Gerhardh, Commented Oct 22, 2021 at 13:18
@Gerhardh I'm using C++, updated tags. Sorry for the confusion. — Devesh Lohumi
– Devesh Lohumi, Commented Oct 22, 2021 at 13:22
You want to use ctime_r(3) with two different preallocated buffers — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 22, 2021 at 14:06

Passerby · Accepted Answer · 2021-10-22 13:21:37Z

4

Read the information on the return value here: ctime

It is a pointer to a static string. You are not comparing the string content (see strcmp), only this pointer.

edited Oct 22, 2021 at 13:21

answered Oct 22, 2021 at 13:18

Passerby

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2 Comments

463035818_is_not_an_ai Over a year ago

that and ... << ctime(&m_start_time) << " " << ctime(&m_end_time) << "\n"; prints the same string

Marek R Over a year ago

demo: godbolt.org/z/Pz1E9Y9TK

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