Author Topic: C Structure to NASM  (Read 11433 times)

Offline Gunner

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C Structure to NASM
« on: August 04, 2012, 10:45:19 PM »
Been looking over the NASM docs, and searching the net and the answer eludes me.  I am beginning the process of porting one of my MASM windows app over to Linux making it a terminal app in the process for the time being.

Don't want to write a command line parser ATM, so I came across getopt and it does what I want.  Except I want to expand it by using getopt_long.  Problem is, this function uses a structure that I have no idea how to use.
Code: [Select]
struct option {
    const char *name;
    int         has_arg;
    int        *flag;
    int         val;
};

The C code would initialize it like this:
Code: [Select]
static struct option long_options[] = {
   {"add",     required_argument, 0,  0 },
   {"append",  no_argument,       0,  0 },
   {"create",  required_argument, 0, 'c'},
   {0,         0,                 0,  0 }

In MASM, without using a structure I could do this in the data section:
Code: [Select]
long_options    dd  offset szAdd, 1, 0, 0, offset szAppend, 0, 0, 0, offset szCreate, 1, 0, offset szC, 0, 0, 0, 0
How do I use this structure?

Offline Gunner

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Re: C Structure to NASM
« Reply #1 on: August 04, 2012, 11:10:23 PM »
Let me answer my own question:
Code: [Select]
long_options    dd  szAdd, 1, 0, 0,  szAppend, 0, 0, 0,  szCreate, 1, 0, 0, 0, 0, 0, 0
Works just fine.  But I still would like to know how to use that structure.

Offline TightCoderEx

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Re: C Structure to NASM
« Reply #2 on: August 04, 2012, 11:57:54 PM »
I'm sure Frank or Keith will provide a better answer about structures than I, but what I would like to suggest is parsing command line options and subsequent environment variables without getopt.

I've written a command line parser in MASM for M$ and functionally it did nothing more that what linux is already doing.  If you emit strings in GDB using x/80s 0x7fffffffe50d, you'll not only find all the arguments passed to your application, but a whole host of other information about your system and application. Keep in mind, the address depicted in example is always different, but always the contents of stack + 4.

To use getop IMHO has merit when using "C", but we are afforded a lot simpler way of doing things in NASM or more specifically assembly because we are not confined with the idiosyncrasies of higher level languages.


Offline Bryant Keller

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Re: C Structure to NASM
« Reply #3 on: August 05, 2012, 01:48:50 AM »
Let me answer my own question:
Code: [Select]
long_options    dd  szAdd, 1, 0, 0,  szAppend, 0, 0, 0,  szCreate, 1, 0, 0, 0, 0, 0, 0
Works just fine.  But I still would like to know how to use that structure.

Code: (C Structure Declaration) [Select]
struct option {
    const char *name;
    int         has_arg;
    int        *flag;
    int         val;
};

Code: (NASM Structure Declaration) [Select]
struc option
    .name:      resd 1
    .has_arg:   resd 1
    .flag:      resd 1
    .val:       resd 1
endstruc

Code: (C Structure Initialization) [Select]
static struct option long_options[] = {
   {"add",     required_argument, 0,  0 },
   {"append",  no_argument,       0,  0 },
   {"create",  required_argument, 0, 'c'},
   {0,         0,                 0,  0 };

Code: (NASM Structure Initialization) [Select]
%define required_argument 1
%define no_argument 0

long_options:
long_options_0:
   istruc option
   at option.name, dd szAdd
   at option.has_name, dd required_argument
   at option.flag, dd 0
   at option.val, dd 0
   iend
long_options_1:
   istruc option
   at option.name, dd szAppend
   at option.has_name, dd no_argument
   at option.flag, dd 0
   at option.val, dd 0
   iend
long_options_2:
   istruc option
   at option.name, dd szCreate
   at option.has_name, dd required_argument
   at option.flag, dd 0
   at option.val, dd 0x00000063
   iend
long_options_3:
   istruc option
   at option.name, dd 0
   at option.has_name, dd 0
   at option.flag, dd 0
   at option.val, dd 0
   iend

In the assembly initialization you'll notice I added extra labels with a numeric index suffix to make accessing the structures easier later on. This isn't specifically required. STRUC and ISTRUC can seem "wordy" but (IMO) it adds a bit of readability.

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Offline Gunner

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Re: C Structure to NASM
« Reply #4 on: August 05, 2012, 02:16:53 AM »
Outstanding, thank you!