$ cat main.c                                                                                                                                                
#define foo(x)  #x
foo(abc)

$ cpp main.c          
# 1 "main.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 340 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "main.c" 2

#abc

$ cpp -v </dev/null
OpenBSD clang version 10.0.1 
Target: amd64-unknown-openbsd6.9
Thread model: posix
InstalledDir: /usr/libexec
 (in-process)
 "/usr/libexec/cpp" -cc1 -triple amd64-unknown-openbsd6.9 -E -disable-free -disable-llvm-verifier -discard-value-names -main-file-name - -mrelocation-model pic -pic-level 1 -pic-is-pie -mthread-model posix -mframe-pointer=all -relaxed-aliasing -fno-rounding-math -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -target-feature +retpoline-indirect-calls -target-feature +retpoline-indirect-branches -dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v -nostdsysteminc -nobuiltininc -resource-dir /usr/lib/clang/10.0.1 -I /usr/include -fdebug-compilation-dir /home/mario -ferror-limit 19 -fmessage-length 171 -fwrapv -D_RET_PROTECTOR -ret-protector -fgnuc-version=4.2.1 -fobjc-runtime=gnustep -fdiagnostics-show-option -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-valloc -fno-builtin-free -fno-builtin-strdup -fno-builtin-strndup -traditional-cpp -faddrsig -o - -x c -
clang -cc1 version 10.0.1 based upon LLVM 10.0.1 default target amd64-unknown-openbsd6.9
#include "..." search starts here:
#include <...> search starts here:
 /usr/include
End of search list.
# 1 "<stdin>"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 340 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "<stdin>" 2