[llvm-dev] How does LLVM know where to resolve declared only functions?

[Jeremy Lakeman via llvm-dev]

Note, I'm using a linux system.

On Sun, 27 Jan 2019 at 11:33, Peng Yu <pengyu.ut at gmail.com> wrote:

> > It's the linkers job to hook together functions and definitions that end
> up in the same binary. Your OS will then hook in functions from other
> binaries when your executable is loaded into memory.
>
> How does it know whether it is a system function or a user-defined
> function?
>
> It seems that user functions have higher priorities over system
> functions as demonstrated by the following example.
>
> Is there any way to always use system functions for certain function
> calls (but not always default to system functions) when there are
> local functions with the same name?
>
> ==> main.c <==
> /* vim: set noexpandtab tabstop=2: */
> #include <stdio.h>
>
> int main() {
>     puts("Hello World!");
>     return 0;
> }
>
> ==> myputs.c <==
> /* vim: set noexpandtab tabstop=2: */
> #include <stdio.h>
>
> int myputs(char *s) {
>     return printf("myputs:%s\n", s);
> }
>
> $ ./main1.sh
> clang -Wall -pedantic -S -emit-llvm -c -o main.ll main.c
> clang -Wall -pedantic -S -emit-llvm -c -o myputs.ll myputs.c
> clang -Wall -pedantic -c -o main.o main.ll
>

objdump -t main.o
...
0000000000000000         *UND*    0000000000000000 puts
...
At this point the .o doesn't specify where puts should be found.

clang -Wall -pedantic -c -o myputs.o myputs.ll
>
clang main.o myputs.o -o ./main.exe
>

objdump -t main.exe
...
0000000000000000       F *UND*    0000000000000000              puts@
@GLIBC_2.2.5
...
The linker found puts in glibc. However using LD_PRELOAD you could still
link against a different implementation at runtime.

./main.exe
> Hello World!
> sed -i 's/@myputs\>/@puts/g' myputs.ll
> clang -Wall -pedantic -c -o myputs.o myputs.ll
> clang main.o myputs.o -o ./main.exe
>

objdump -t main.exe
...
0000000000000000       F *UND*    0000000000000000              printf@
@GLIBC_2.2.5
...
No external reference to puts, because it was found during the link step.


> ./main.exe
> myputs:Hello World!
>
> $ cat ./main1.sh
> #!/usr/bin/env bash
> # vim: set noexpandtab tabstop=2:
>
> set -v
> clang -Wall -pedantic -S -emit-llvm -c -o main.ll main.c
> clang -Wall -pedantic -S -emit-llvm -c -o myputs.ll myputs.c
> clang -Wall -pedantic -c -o main.o main.ll
> clang -Wall -pedantic -c -o myputs.o myputs.ll
> clang main.o myputs.o -o ./main.exe
> ./main.exe
> sed -i 's/@myputs\>/@puts/g' myputs.ll
> clang -Wall -pedantic -c -o myputs.o myputs.ll
> clang main.o myputs.o -o ./main.exe
> ./main.exe
>
>
> --
> Regards,
> Peng
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190128/2ca90ea7/attachment.html>