Unix Programming - finding sizeof structure using gdb

Author

finding sizeof structure using gdb

junky_fellow@yahoo.co.in

2008-01-28, 4:33 am

Guys,

Is there any way by which I can find out the size of a particular
structure using GDB ?Also, is there a way, I can find out the offset
of a member inside a structure ?

Many thanks for any help..

fnegroni

2008-01-28, 4:33 am

I think the command you are looking for is:
print _expr_
where _expr_ is the expression you want to evaluate at runtime.
You can place a breakpoint anywhere in your program where you want to
evaluate an expression, and type 'print' followed by the expression
such as 'sizeof (struct_type)'
To find out the offset of a member it necessitates some casts:

Say you have a struct defined as:

struct kelp_file {
FILE *f;
char *name;
};

and at some point an object of type struct kelp_file called k comes
into visibility.

Use gdb to print offset of name compared to start of structure:

print (size_t)&(k.name) - (size_t)(&k)

Spoon

2008-01-28, 4:33 am

junky fellow wrote:

> Is there any way by which I can find out the size of a particular
> structure using GDB?

Did you try the obvious 'print sizeof(my_type)'

> Also, is there a way, I can find out the offset of a member inside
> a structure ?

You could define a wrapper in your code and call that.

junky_fellow@yahoo.co.in

2008-01-28, 8:23 am

On Jan 28, 2:54=A0pm, fnegroni <f.e.negr...@googlemail.com> wrote:
> I think the command you are looking for is:
> print _expr_
> where _expr_ is the expression you want to evaluate at runtime.
> You can place a breakpoint anywhere in your program where you want to
> evaluate an expression, and type 'print' followed by the expression
> such as 'sizeof (struct_type)'
> To find out the offset of a member it necessitates some casts:
>
> Say you have a struct defined as:
>
> struct kelp_file {
> FILE *f;
> char *name;
>
> };
>
> and at some point an object of type struct kelp_file called k comes
> into visibility.
>
> Use gdb to print offset of name compared to start of structure:
>
> print (size_t)&(k.name) - (size_t)(&k)

Thanks Guys, that was really helpful.
Can you please also tell me how to typecast a memory address to a
particular structure type and print the values of various membes of
that structure.
For eg. Say I have some "structure x" at address 0xffff1000
Now, I want to typecast this memory address 0xffff1000 with the
"structure x" and print the contents of all the members in "structure
x". Can it be done in GDB Or I have to dump the contents of memory and
manually interpret the value of each member ?

fnegroni

2008-01-28, 8:23 am

Yes, you can typecast an address and gdb will try and reinterpret the
memory location (it is what in C++ is known as reinterpret cast)

For example, recalling the example I gave earlier, say that you
encunter a variable called r which effectively is the address of a
struct kelp_file.

You can issue the command:
print *((struct kelp_file *)ret)

And gdb will print something like

$2 = {
f = 0x5fbff6a000000000,
name = 0x15bc00007fff <Address 0x15bc00007fff out of bounds>
}

Obviously if ret is not the address of a struct kelp type object, you
are printing garbage... but if it is the address of a portion of a
buffer in memory, that could work well... provided the alignment and
padding is the same.

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