Q33795: INFO: Consecutive Calls to ctime() Overwrite Previous Values

Article: Q33795
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 26-MAR-2002

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft C for MS-DOS 
   - Microsoft C for OS/2 
   - Microsoft C/C++ for MS-DOS 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 5.0, 6.0 
   - Microsoft Visual C++.NET (2002) 
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NOTE: Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.

SUMMARY
=======

Subsequent calls to the ctime() function overwrite the results of a previous
call. The code example below demonstrates this behavior by printing the same
time for the start time and finish time in its first printf() call. However, the
start and finish times should be different. To correct this situation, do not
call ctime twice in one call to printf. Then the finish time is later than the
start time, as expected.

This is expected behavior. The ctime() function uses one static buffer to store
its results. Therefore, when the second ctime() call is made in the printf
function, ctime() overwrites the value returned by the first ctime() call.

MORE INFORMATION
================

Sample Code
-----------

  /*
   * Compile options needed: None
   */ 

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

  time_t start, finish=0;

  void main(void)
  {

    //loop until 3 seconds pass
    for (time(&start); finish-start < 3; time(&finish))
    ;

    /* If the following statement is used, the start and finish
       display times do not differ, both parameters get a copy of
       the same buffer which is overwritten twice before entering
       printf: */ 

    printf("Start time was %s and ending time was %s", ctime(&start),
       ctime(&finish));

    /* If the following statements are used instead, the start and
       finish times are different, as expected: */ 

       printf("Start time was %s", ctime(&start));
       printf("and ending time was %s\n", ctime(&finish));
  }

Additional query words:

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Keywords          : kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbinfo

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