open file using int21-stack problem

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 written, open_file. Here is what i mean:
At code part: char* fp;
char* open_file(filename*);
fp = load_file(filename);

Assembly part:
global 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


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. So i think that i have use incorrectly the stack or the handler and not something typical like how i use linking or something about variables' or functions' statements, because 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


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