On Mar 13, 2011, at 11:55 AM, Chris Lattner wrote:
> On Mar 13, 2011, at 11:26 AM, Jack Howarth wrote:
>
>>> Yes, I agree that this is a better solution. This error was put into the
>>> linker to detect some overflow conditions for part of the code that
>>> expected the section number to only be a byte. It is likely that "things
>>> worked" only out of luck before.
>>>
>>> -Chris
>>
>> Chris,
>> Is there any documentation or example code on how to properly use
>> subsections in mach-o?
>> My fear is that we are moving from one poorly documented technique to
>> another which may well
>> have it own slate of hidden bugs.
>
> I'm not sure what you mean here Jack. ld64 is open source on the darwin
> page, and if you have all the developer bits installed, the format is
> "documented" in /usr/include/mach-o/. It's pretty clear all around that you
> can only have 256 sections.
Sorry, I actually mean 255 of course, because of the NO_SECT sentinel. Here
are the relevant bits from nlist.h. I'm not sure how you expect the toolchain
to store more than 256 sections in a uint8_t.
This is not the conspiracy you are looking for.
-Chris
/*
* Format of a symbol table entry of a Mach-O file for 32-bit architectures.
* Modified from the BSD format. The modifications from the original format
* were changing n_other (an unused field) to n_sect and the addition of the
* N_SECT type. These modifications are required to support symbols in a larger
* number of sections not just the three sections (text, data and bss) in a BSD
* file.
*/
struct nlist {
union {
#ifndef __LP64__
char *n_name; /* for use when in-core */
#endif
int32_t n_strx; /* index into the string table */
} n_un;
uint8_t n_type; /* type flag, see below */
uint8_t n_sect; /* section number or NO_SECT */
int16_t n_desc; /* see <mach-o/stab.h> */
uint32_t n_value; /* value of this symbol (or stab offset) */
};
/*
* This is the symbol table entry structure for 64-bit architectures.
*/
struct nlist_64 {
union {
uint32_t n_strx; /* index into the string table */
} n_un;
uint8_t n_type; /* type flag, see below */
uint8_t n_sect; /* section number or NO_SECT */
uint16_t n_desc; /* see <mach-o/stab.h> */
uint64_t n_value; /* value of this symbol (or stab offset) */
};
/*
* If the type is N_SECT then the n_sect field contains an ordinal of the
* section the symbol is defined in. The sections are numbered from 1 and
* refer to sections in order they appear in the load commands for the file
* they are in. This means the same ordinal may very well refer to different
* sections in different files.
*
* The n_value field for all symbol table entries (including N_STAB's) gets
* updated by the link editor based on the value of it's n_sect field and where
* the section n_sect references gets relocated. If the value of the n_sect
* field is NO_SECT then it's n_value field is not changed by the link editor.
*/
#define NO_SECT 0 /* symbol is not in any section */
#define MAX_SECT 255 /* 1 thru 255 inclusive */
