Digital Mars DOS _fmalloc not giving pointer to usable memory?

Question

I'm using the Digital Mars C compiler to write a 16-bit DOS program, in the small memory model. This program needs to save a file to the hard disk, and it also needs to allocate 64,000 bytes in memory to temporarily store data in preparation to save it, along with some other data, to the hard disk. The problem is that it was saving gibberish to the file, when it shouldn't be. I made a separate small program so that I could separate my possibly bugged code from code that shouldn't have any problems. So far, I've been able to track the problem down to the _fmalloc() function in Digital Mars. When I write data that has been allocated with _fmalloc() the data gets corrupted with what seems like program data. When I allocate memory off of the stack, there is no problem and the program works as intended. .
.

File Test Program

This is the small program I wrote to eliminate possible problems in my code and figure out where the problem is.

#include <io.h>
#include <sys\stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

int main(int argc, char *argv[])
{

    //unsigned char data[256]; <-- using this way works just fine

    unsigned char __far *data; <-- this way does not work
    data = _fmalloc(256);

    if(data==NULL)
    {
        printf("Not enough memory.\n");
        exit(1);
    }

    //initialize data to 1
    _fmemset(data, 1, 256);


    int file;   //file handle
    unsigned int err = 0;

    // Create File
    file = _creat("file.bin", _S_IREAD | _S_IWRITE);

    if(file==-1){
        printf("Error creating file.\n");
        exit(1);
    }else{
        printf("File created successfully.\n");
        err = 0;
    }

    // Write Data to File
    int bytes_written = 0;
    bytes_written = _write(file, &data, 256);

    if(bytes_written==-1){
        printf("Error writing to file.\n");
        exit(1);
    }else{
        if(bytes_written == 256){
            printf("File written successfully.\n");
            err = 0;
        }else{
            printf("Error wrong amount of data written.\n");
            exit(1);
        }
    }

    // Close File
    err = _close(file);

    if(err==-1){
        printf("Error closing file.\n");
        exit(1);
    }else{
        printf("File closed.\n");
    }

    _ffree(data);

    return 0;
}  

.
.

File Output

.
.
Using data[256];
This is what the output should look like. 256 bytes total.

01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01

Using data = _fmalloc(256);
NO BUENO! Note that this also produces an extra byte, for a grand total of 257 bytes.

00 00 38 14 00 01 05 00 00 00 00 00 00 00 E5 02 A2 01 01 00 1F 01 7E 2B 43 3A 5C 46 49 4C 45 2E 45 58 45 00 3C 00 50 41 54 48 3D 5A 3A 5C 00 43 4F 4D 53 50 45 43 3D 5A 3A 5C 43 4F 4D 4D 41 4E 44 2E 43 4F 4D 00 42 4C 41 53 54 45 52 3D 41 32 32 30 20 49 35 20 44 31 20 48 35 20 54 36 00 00 0D 0A 00 42 2B 4B 2B 62 2B 00 00 06 09 6A D4 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

text view

..8...........å.¢.....~+C:\FILE.EXE.<.PATH=Z:.COMSPEC=Z:\COMMAND.COM.BLASTER=A220 I5 D1 H5

T6.....B+K+b+....jÔ.................................................................................................................................................

Answer 1

_write(file, &data, 256) needs to be _write(file, data, 256) for the malloc case. It works for the auto allocated case because data and &data are the same address for arrays (though different types).

Answer 2

So, I ended up needing a _far version of the _write() and _read() functions. These were not provided with the compiler, so I made my own. Forgive me if these seem a little crude, but I've never done anything like this before, so this was a good learning experience for me. Here is the code I came up with.

int _fwrite(int fd, void __far *buffer, unsigned int length)
{
    void *buffer_offset             = 0; // used to pass the offset of buffer to _write()
    unsigned int buffer_segment     = 0; // used to change the value of the data segment register (DS)
    unsigned long int buffer_copy   = 0; // used for temporary, corruptible storage of buffer
    int result                      = 0; // used for returning number of bytes written or an error from _write()

    // store the full __far pointer buffer in buffer_copy as a 32 bit int so that we can use bitwise operators
    buffer_copy   = (unsigned long int)buffer;

    // left shift the address stored in buffer_copy two bytes to get the offset address
    buffer_offset = (void *)(buffer_copy << 16);

    // right shift the address stored in buffer_copy two bytes to get the segment address
    buffer_segment = buffer_copy >> 16;

    // change the data segment register to the one that buffer points to so that _write() will work on far data
    asm
    {
        push ds
        mov ds,buffer_segment
    }

    // do the file write
    result = _write(fd, buffer_offset, length);

    // restore previous data segment register value so that near data can be accessed again
    asm
    {
        pop ds
    }

    return result;
}

int _fread(int fd, void __far *buffer, unsigned int length)
{
    void *buffer_offset             = 0; // used to pass the offset of buffer to _read()
    unsigned int buffer_segment     = 0; // used to change the value of the data segment register (DS)
    unsigned long int buffer_copy   = 0; // used for temporary, corruptible storage of buffer
    int result                      = 0; // used for returning number of bytes written or an error from _read()

    // store the full __far pointer buffer in buffer_copy as a 32 bit int so that we can use bitwise operators
    buffer_copy   = (unsigned long int)buffer;

    // left shift the address stored in buffer_copy two bytes to get the offset address
    buffer_offset = (void *)(buffer_copy << 16);

    // right shift the address stored in buffer_copy two bytes to get the segment address
    buffer_segment = buffer_copy >> 16;

    // change the data segment register to the one that buffer points to so that _read() will work on far data
    asm
    {
        push ds
        mov ds,buffer_segment
    }

    // do the file read
    result = _read(fd, buffer_offset, length);

    // restore previous data segment register value so that near data can be accessed again
    asm
    {
        pop ds
    }

    return result;
}