Q44178: File Buffers Are Not Allocated until First Accessed

Article: Q44178
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 22-MAY-1989

Problem:

I am using _memavl() to determine the amount of free space on the near
heap. When I use fgets(), the amount of near heap space decreases by
512 bytes.

Response:

File buffers are not allocated when the file is first opened. The
512-byte buffer is allocated on the near heap when the file is first
accessed. The buffer will be used by the file until the file is
closed. At that time, the buffer space will be freed to the system.

File buffers are allocated in the near heap for small and medium
memory models and in the far heap for compact and large memory models.
_memavl() returns the amount of free space on the near heap only.

The following program, compiled in small or medium memory model,
illustrates this behavior:

#include <malloc.h>
#include <stdio.h>

FILE *fp;

void main(void)
{
  char bufs[64];

  printf("Start of program \n");
  Bytes_free();
  fp=fopen("file1.txt","r");

  printf("file1.txt has been opened \n");
  Bytes_free();

  fgets(bufs,5,fp);
  printf(" file has been accessed \n");
  Bytes_free();

  fclose(fp);
  printf(" file has been closed \n");
  Bytes_free();
}

Bytes_free()
{
  printf("\nHeap bytes free: %u \n", _memavl()  );
}

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