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

Hi

In the generated .ll file, it may have something like this. How does
LLVM know where to look for the definition of printf? Is it documented
somewhere? Thanks.

declare i32 @printf(i8*, ...) #1

-- 
Regards,
Peng

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.

On Sat, 26 Jan 2019 at 12:43, Peng Yu via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Hi
>
> In the generated .ll file, it may have something like this. How does
> LLVM know where to look for the definition of printf? Is it documented
> somewhere? Thanks.
>
> declare i32 @printf(i8*, ...) #1
>
> --
> Regards,
> Peng
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> 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