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

Peng Yu via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 26 17:03:23 PST 2019

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

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