problem opening a file

I write a program in C but i have not the right to use the standard library. I want to open a file so i use an assembly procedure that i have write, open_file. Here is what i mean:
At C code part:

Code:

                        int fp;
                        int open_file(char* filename);
                ....
                         fp = open_file(filename);

Assembly part:
                 ....
 global open_file

    open_file:

        push bp
        mov bp,sp
        push bx
        push dx

        mov ah, 0x3d

        mov al, 0x00        ; read-only

        mov bx, [bp + 4]    ; file_name segment in DS

        mov ds, bx

        mov dx, [bp + 6]    ; file_name offset in DX

        int 0x21                 ; open file




        jc open_error    ; if carry is set file could not open
        mov  handler, ax
        pop       dx
        pop        bx
        pop        bp
        ret        4

  open_error:

        mov ah,1
        pop dx
        pop bx
        pop bp

        ret        4

The problem is that when i execute my program, when it comes to the part of opening the file it aborts. What do you think is wrong? Other functions that i wrote in such a way, for example function that prints a string or clear the monitor, works well.

I would appreciate your help. Thanks a lot for your time

 

( I had mis-read your post first )
I will look into the ASM code and tell you later.

 

If you're allowed to use the BIOS then it's fairly easy; just write a program in C that does the same, then step through it at the assembler level using the debugger. That'll show you what the standard library does, then you can use that for inspiration.

 

when i try to use the debugger(ddd), i cannot debug and it returns me the message "Program exited with code 0176. You can't do that without a process to debug"

 

ok finally i did that, but when it comes for the function that opens the file, simply refers to it as fopen, so i cannot see what fopen does in assembly language

 

You need to use a debugger that will work at the assembly level and will allow you to step through individual assembly instructions.

See Section 8.6 of the gdb manual; this seems relevant.
http://sourceware.org/gdb/current/onlinedocs/gdb_9.html#SEC64

There's also an apparently relevant section in the ddd documentation:
http://www.gnu.org/manual/ddd/html_mono/ddd.html#Machine-Level Debugging

RTFM!