Q23869: malloc() and free() Do Not Return Memory to DOS

Article: Q23869
Product(s): See article
Version(s): 3.00 4.00 5.00 5.10 6.00 6.00a
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 15-JAN-1991

Problem:

My large model program calls system() to do a directory, and then I
malloc() a large amount of space. I can free() it all but I get a "Not
enough memory" error when I try to call system() again. Apparently
free() is not returning the memory to DOS.

Response:

When you malloc() a large amount of space (more than is available
within the current heap), the malloc() function will execute a DOS
call to get more memory in an attempt to expand the heap. If this is
successful, DOS will allocate the desired memory to your program and
malloc() will return successful. If you then free() this memory, it
will be marked as free within your program's heap and will be
available for use again by your program.

However, DOS still sees this memory as being used by the program
because it now resides within the program's heap. If you attempt a
subsequent system() call, DOS may find it does not have enough free
memory to load the code required to perform the system() function.

An alternative is to use halloc() and hfree(), which will return the
memory to DOS, rather than retaining it for use by the currently
executing program.

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