Q46820: Library Support for Near, Far, and Huge Keywords

Article: Q46820
Product(s): See article
Version(s): 5.00 5.10 | 5.00 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | mixed memory model S_QuickC | mspl13_c
Last Modified: 18-AUG-1989

When using the "near", "far", and "huge" keywords to override
addressing conventions within specific memory models, you can usually
use one of the standard libraries. Often, you cannot pass far
pointers, or the addresses of far data items, to a small-model library
routine. Some exceptions are the library routines "halloc", "hfree",
and the "printf" family of functions.

For additional information, refer to the "Microsoft C Optimizing
Compiler User's Guide," Page 145.

Mixed-memory-model programming uses the "near", "far", and "huge"
keywords to locate code or data outside of the segments specified by
the memory model, e.g. declaring a "far" character array in a small
memory model where it would ordinarily be "near".

Mixed-model programming is useful when you have only a bit of data to
put outside of the single data segment and you don't want to switch to
a multiple-data-segment memory model. The standard library routines,
however, were written to support the standard memory models, and so
must be used with care.

The following example demonstrates a function that performs as
expected in large or compact model but returns incorrect results in
the medium or small model because its data, "buffer", was declared
"far":

char far buffer[100];
.
.
.
fwrite ( buffer, size, count, stream);

In single-data-segment models, data addresses are 2-bytes long. In
multiple-data-segment models, data addresses are 4-bytes long. Data
declared "far" also has 4-byte addresses. When fwrite() is called in a
single-data-segment model, it expects 2-byte, not 4-byte, data
addresses.

Removing the "far" keyword or compiling in a multiple-data-segment
model corrects the problem. Another solution is to assign the value of
the far variable to a near variable. In the example below, each
element of the far array is assigned to the near array. A strcpy()
cannot be used in this case, since it is one of the library functions
that operates properly only when the data addresses are consistent
with their memory model:

char far buffer[];
char nearbuffer;   /* near by default in small and medium models */
int i;

for (i = 0; i < 100; i++)
  nearbuffer[i] = buffer[i];
.
.
.
fwrite ( nearbuffer, size, count, stream);

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