INFO: Consecutive Calls to ctime() Overwrite Previous Values

• The C Run-Time (CRT), included with:
  • Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
  • Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
  • 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

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:

Keywords : kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Version : winnt:1.0,2.0,4.0,5.0,6.0
Platform : winnt
Issue type : kbinfo