I'm writing some code which stores some data structures in a special named binary section. These are all instances of the same struct which are scattered across many C files and are not within scope of each other. By placing them all in the named section I can iterate over all of them.
This works perfectly with GCC and GNU ld. Fails on Mac OS X due to missing __start___mysection
and __stop___mysection
symbols. I guess llvm ld is not smart enough to provide them automatically.
In GCC and GNU ld, I use __attribute__((section(...))
plus some specially named extern pointers which are magically filled in by the linker. Here's a trivial example:
#include <stdio.h>
extern int __start___mysection[];
extern int __stop___mysection[];
static int x __attribute__((section("__mysection"))) = 4;
static int y __attribute__((section("__mysection"))) = 10;
static int z __attribute__((section("__mysection"))) = 22;
#define SECTION_SIZE(sect) \
((size_t)((__stop_##sect - __start_##sect)))
int main(void)
{
size_t sz = SECTION_SIZE(__mysection);
int i;
printf("Section size is %u\n", sz);
for (i=0; i < sz; i++) {
printf("%d\n", __start___mysection[i]);
}
return 0;
}
What is the general way to get a pointer to the beginning/end of a section with FreeBSD linker. Anyone have any ideas?
For reference linker is:
@(#)PROGRAM:ld PROJECT:ld64-127.2
llvm version 3.0svn, from Apple Clang 3.0 (build 211.12)
Similar question was asked about MSVC here: How to get a pointer to a binary section in MSVC?
You can get the Darwin linker to do this for you.
#include <stdio.h>
extern int start_mysection __asm("section$start$__DATA$__mysection");
extern int stop_mysection __asm("section$end$__DATA$__mysection");
// If you don't reference x, y and z explicitly, they'll be dead-stripped.
// Prevent that with the "used" attribute.
static int x __attribute__((used,section("__DATA,__mysection"))) = 4;
static int y __attribute__((used,section("__DATA,__mysection"))) = 10;
static int z __attribute__((used,section("__DATA,__mysection"))) = 22;
int main(void)
{
long sz = &stop_mysection - &start_mysection;
long i;
printf("Section size is %ld\n", sz);
for (i=0; i < sz; ++i) {
printf("%d\n", (&start_mysection)[i]);
}
return 0;
}
Using Mach-O information:
#include <mach-o/getsect.h>
char *secstart;
unsigned long secsize;
secstart = getsectdata("__SEGMENT", "__section", &secsize);
The above gives information about section declared as:
int x __attribute__((section("__SEGMENT,__section"))) = 123;
More information: https://developer.apple.com/library/mac/documentation/developertools/conceptual/machoruntime/Reference/reference.html
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