Q44725: INFO: printf() and 512-Byte I/O Buffer
Article: Q44725
Product(s): Microsoft C Compiler
Version(s): 1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Operating System(s):
Keyword(s): kbCRT kbVC kbVC152 kbVC200 kbVC210 kbVC220 kbVC410 kbVC420 kbVC500 kbVC600
Last Modified: 22-JUL-2001
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The information in this article applies to:
- The C Run-Time (CRT), used with:
- Microsoft Visual C++ for Windows, 16-bit edition, version 1.52
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 5.0, 6.0
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SUMMARY
=======
The function printf() takes up 512 bytes from the default heap in 16-bit
applications and 4096 bytes in 32-bit applications.
MORE INFORMATION
================
The standard I/O buffer gets allocated only when needed. This gives the program
an extra 512 bytes of near-heap space in small and medium memory models or an
extra 512 bytes of far-heap space in compact and large models. Therefore, when
the printf() function is used for the first time, a 512-byte buffer is reserved
for I/O, and the subsequent calls to printf() use that buffer.
The sample program below demonstrates this behavior. When compiled and executed
with current 16-bit versions, the number printed after printf() executes is 512
bytes more than the number printed before the printf() executes.
When this sample program is compiled for release under 32-bit versions and
executed in Windows NT, the output displays 4096 bytes. For debug compile, the
number of bytes shown is 4144, the extra 48 bytes are used to debug related
information. Note that this program displays 0 bytes for 32-bit versions on
Windows 95, Windows 98, or Windows Millennium Edition (Me) because _heapwalk is
not supported in Windows 95, Windows 98, and Windows Me.
Sample Code
-----------
/* Compile options needed: none
*/
#include <stdio.h>
#include <malloc.h>
unsigned long heapdump (void)
{
struct _heapinfo hinfo;
int heapstatus;
unsigned long memUsed = 0;
hinfo._pentry = NULL;
while ((heapstatus = _heapwalk(&hinfo)) == _HEAPOK)
{
if (hinfo._useflag == _USEDENTRY)
memUsed += hinfo._size;
}
switch (heapstatus)
{
case _HEAPEMPTY:
printf("OK - empty heap\n");
break;
case _HEAPEND:
printf("OK - end of heap\n");
break;
case _HEAPBADPTR:
printf("ERROR - bad pointer\n");
break;
case _HEAPBADBEGIN:
printf("ERROR - bad start\n");
break;
case _HEAPBADNODE:
printf("ERROR - bad node\n");
break;
}
return memUsed;
}
void main (void)
{
unsigned long memUsed, memTemp;
memTemp = memUsed = heapdump();
printf("Pre-printf memory used was %lu bytes.\n", memUsed);
memUsed = heapdump();
printf("Post-printf memory used is %lu bytes.\n", memUsed);
printf(
"The difference in memory used between printf's is %lu bytes.\n",
memUsed - memTemp);
}
Additional query words: heapwalk printf
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Keywords : kbCRT kbVC kbVC152 kbVC200 kbVC210 kbVC220 kbVC410 kbVC420 kbVC500 kbVC600
Technology : kbVCsearch kbAudDeveloper kbCRT
Version : :1.52,2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Issue type : kbinfo
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