llvm dev - Jan 2019 - How does LLVM know where to resolve declared only functions?

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

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
>
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> At this point the .o doesn't specify where puts should be found.
If the glibc's puts() call a function f() in glibc, and I define
another f() in main.ll, will the f() in main.ll be called by glibc's
puts() or the f() in glibc be called? Thanks.

-- 
Regards,
Peng