在cout上打印时间时奇怪的C ++行为

Question

Running following code:

  1 #include <iostream>
  2 int main(int argc, char** argv)
  3 {
  4     time_t t1 = time(0);
  5     time_t t(0);
  6     std::cout<<"BUG LINE"<<std::endl<< ctime(&t) << ctime(&t1) ;
  7     std::cout<<"PRINTS FINE HERE"<<std::endl;
  8     std::cout<< ctime(&t);
  9     std::cout<< ctime(&t1);
 10     return 0;
 11 }

Building: g++ test1.cpp

Output:
./a.out
BUG LINE
Thu Jan  1 01:00:00 1970
Thu Jan  1 01:00:00 1970
PRINTS FINE HERE
Thu Jan  1 01:00:00 1970
Wed Jul 10 16:31:48 2013

Why is the stream going weird in the #6 of the code??

Answer 1

http://www.cplusplus.com/reference/ctime/ctime/

The returned value points to an internal array whose validity or value may be altered by any subsequent call to asctime or ctime .

http://en.cppreference.com/w/cpp/chrono/c/ctime

The string may be shared between std::asctime and std::ctime , and may be overwritten on each invocation of any of those functions .

In your example, on line 6, ctime(&t1) gets evaluated first, then ctime(&t) which overwrites the string (whichever is evaluated first is unspecified, and compilers often evaluate in reverse order ( often , not always)).

TL;DR: Reading the docs may help.

Answer 2

The function ctime returns a static string buffer - so the string returned is exactly the same string each time), which means that calling it multiple times on the same line will give undefined behaviour (because the C and C++ standards do not tell in what order the calls are made).

The solution is to call in two distinct statements (with ; between), or use reentrant-safe variant, where you pass in the buffer to store the result in), eg ctime_r

Answer 3

The problem here is that ctime() uses an internal allocated char array which is modified with each call to ctime() .

The other problem here is that your ctime() s can be evaluated in any order in that expression. So what is happening here is that both of them on line 6 are evaluated, but clearly the second is evaluated second. Then the operators are applied from left to right, but both calls to ctime have already been evaluated, the left most one was done afterwards and now both strings refer to the same thing.

If you placed them on different lines, you would get your expected behaviour.

http://www.cplusplus.com/reference/ctime/ctime/

A C-string containing the date and time information in a human-readable format.

The returned value points to an internal array whose validity or value may be altered by any subsequent call to asctime or ctime.

Answer 4

The returned value points to an internal array whose validity or value may be altered by any subsequent call to asctime or ctime.

This is exactly what is happening here. One of the two calls to ctime in that line gets executed before the other, but both get executed before cout can print them. Since they (probably) return the same temporary char buffer, you get the same result by the time cout gets to the printing.