Issue
I am searching to find a way to relocate/load a user defined data section of a shared library to specific virtual memory of a C program.
Initially, i was using only static libraries (.a) to link the whole program. some of these libraries (2 of them, let call libdata1.a and libdata2.a) are just a collection of initialized structs. A kind of "C based database files".
for some compatibility reasons, the "data section" of these libraries need to be loaded at a specific memory location in the final program.
in the static case, I just modify the ld script to add a rule to achieve that, and all work fine.
.usrdata1 0x04200000 : { *(.usrdata1 ) } /* data1 is the renamed data section of libdata1 */
.usrdata2 0x04300000 : { *(.usrdata2 ) } /* data2 is the renamed data section of libdata2 */
However now, I need a solution to load these two libs (or the data section that they embed) in a dynamic fashion. So the libs will be dissociated from the final binary. I thought on using shared libraries (.so), however the dynamic linker will just load the two libraries at any virtual memory, and I don't know a mean to force a specific memory location.
Do you have any idea of any applicable solution please ?
If this is not valuable, is it possible to find the memory location where the two "usrdata" sections were loaded in the running binary by the dynamic linker ? If so, this can be sufficient for me, because I can also change some memory references in the inner C code to make it work.
Thanks in advance
Solution
some of these libraries (2 of them, let call libdata1.a and libdata2.a) are just a collection of initialized structs.
However now, I need a solution to load these two libs (or the data section that they embed) in a dynamic fashion.
The easiest solution is to save the raw initialized structs into a file (file1.data, file2.data) and mmap() them at the address you need with MAP_FIXED flag.
Involving the dynamic loader in this endeavor is counter-productive -- you'll just fight it.
is it possible to find the memory location where the two "usrdata" sections were loaded in the running binary by the dynamic linker ?
That is trivial to do using dladdr.
Answered By - Employed Russian Answer Checked By - Candace Johnson (WPSolving Volunteer)