Date: Tue, 17 Nov 1998 10:33:44 -0500 From: sbabkin@dcn.att.com To: nate@mt.sri.com, hackers@FreeBSD.ORG Subject: RE: Wrapping a function Message-ID: <C50B6FBA632FD111AF0F0000C0AD71EE018C1D9D@dcn71.dcn.att.com>
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> From: Nate Williams [SMTP:nate@mt.sri.com]
>
> Does anyone have an easy way of 'wrapping' an already existing library
> function so that any programs linked against your .o will call your
> function, but so your function can call the 'real' library function?
>
> Example:
>
> my_malloc.c:
>
> void *malloc(size_t size)
> {
> void *ret;
>
> printf("Calling malloc\n");
> ret = REALMALLOC(size);
> printf("Leaving malloc\n");
> return ret;
> }
>
> Ignoring all of the functions where there is loss of errno and such, are
> they any good ideas? Note, the use of the dl* functions is explicitly
> not allowed since those happen to be the functions I want to wrap in
> this case.
>
I see no way to do it in C, but I have done such things
multiple times with the object files. The key is to
rename the entries in the symbol tables of object files.
Here are two ways:
1. Rename the calls from another object file. This is useful
is you want to substitute your function for only one object
file in program, and leave others as is (a good example
would be a binary driver linked to the kernel). Take
this object file and using a binary editor (xvi, for example)
change all the occurrences of "malloc" in the symbol table
to something like "xxxloc". Then name your wrapper
function "xxxloc" and that's it.
2. Rename the calls from all user-level object file. Copy
the library with "malloc", extract the object file with
"malloc" from it, change "malloc" to "xxxloc", insert back
into the library and link this changed library explicitly instead
of the standard one. Your wrapper function should then be named
"malloc" and call "xxxloc".
-Sergey
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