Article ID: 51434
Article Last Modified on 6/29/2004
APPLIES TO
- The C Run-Time (CRT), when used with:
- Microsoft C/C++ Professional Development System 7.0
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
This article was previously published under Q51434
SUMMARY
The following code sample allows you to determine how many CRT file handles are available at run time for a particular process. The function makes use of the predefined pointers _iob and _lastiob, which are set to point to the first and the last input/output information blocks, respectively. The I/O blocks are examined to determine whether they are in use, and a total number of available file handles is produced. There is no equivalent C library function currently available.
NOTE: This is not necessarily an indication of the maximum number of physical files that can be opened. The system-wide limit is set in MS-DOS by FILES= in the CONFIG.SYS file. In OS/2, this limit is arbitrarily governed by system resources and is modified for each process by DosSetMaxFH().
MORE INFORMATION
Sample Code
/*
Compile options needed: /DNO_EXT_KEYS under Windows NT, none otherwise
NOTE: This code will not compile unless it is compiled as a .C file
and not a .CPP.
*/
#define FILE struct _iobuf
#ifndef NO_EXT_KEYS /* Extensions enabled */
#define _CDECL cdecl
#define _NEAR near
#else /* Extensions not enabled */
#define _CDECL
#define _NEAR
#endif /* NO_EXT_KEYS */
#define _IOREAD 0x01 /* Open for read bit */
#define _IOWRT 0x02 /* Open for write bit */
#define _IORW 0x80 /* Open for read/write bit */
FILE /* File handle table entry */
{
char *_ptr;
int _cnt;
char *_base;
char _flag;
char _file;
}_NEAR _CDECL _iob[], /*Set to first I/O block at run time*/
*_lastiob; /* Set to last I/O block */
/*
All of the above definitions were taken from the STDIO.H header
file except for _lastiob[], which is not defined. This information
was extracted to make the example more clear.
*/
/*
- The following macro will check the availability of a file handle
by examining the _flag member of the I/O block
*/
#define inuse(s) ((s)->_flag & (_IOREAD|_IOWRT|_IORW))
/*
- The following routine loops through the total number of I/O blocks
and checks the flags to see if it is used or not. The number of
unused handles is returned, which can be 1 to the maximum number of
file handles as set by the operating system or the FILES= command
in the CONFIG.SYS file.
*/
int Number_Of_Handles(void)
{
FILE *stream = _iob;
int count;
count = 0;
do
if (inuse(stream)) count++;
while(stream++ < _lastiob);
return(((_lastiob - _iob)+1)-count);
}
void main(void)
{
int i;
i = Number_Of_Handles();
/* i is now set to the number of available file handles */
}
The table of I/O blocks that is being checked here was allocated at run time according to the maximum number of file handles allowed per process.
Keywords: kbhowto kbcrt KB51434
