@ianlancetaylor @bcmills Can you please help me on this ?

@bradfitz can I get help on this ?

clang --version
Apple LLVM version 10.0.1 (clang-1001.0.46.3)
Target: x86_64-apple-darwin18.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

@nitishsaboo, you gave this only 8 hours before you started pinging everybody. Please be patient. Especially because your bug report looks more like a question (perhaps better suited to golang-nuts@ or another https://golang.org/wiki/Questions forum) than a bug report.

I can't reproduce the failure (on Linux with a much older clang).

example.com$ cat >./main.go <<EOF
package main

// #cgo CFLAGS: -g -Wall
// #include <stdlib.h>
// #include "greeter.h"
import "C"
import (
 "unsafe"
)

func main() {
 name := C.CString("Gopher")
 defer C.free(unsafe.Pointer(name))

 year := C.int(2018)

 C.greet(name, year)

}
EOF

example.com$ cat >./greeter.h <<EOF
#ifndef _GREETER_H
#define _GREETER_H

void greet(const char *name, int year);
#endif
EOF

example.com$ cat >./greeter.c <<EOF
#include "greeter.h"
#include <stdio.h>

void greet(const char *name, int year) {
    printf("Greetings, %s from %d! We come in peace :)\n", name, year);
}
EOF

example.com$ cat >./go.mod <<EOF
module example.com

go 1.12
EOF

example.com$ $(go env CC) --version
clang version 4.0.1-10 (tags/RELEASE_401/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

example.com$ go1.12.3 build -o greeter

example.com$ ./greeter
Greetings, Gopher from 2018! We come in peace :)

example.com$

Often with issues on MacOS we find that the problem is some issue with either a bad XCode installation or some additional (interfering) C toolchain installed somewhere in $PATH. It seems likely to be a problem with your C compiler, although a problem in cgo's use of the compiler isn't impossible.

As a diagnostic step, could you try setting CC explicitly in your environment?

You could also try running go clean -cache; go build -x, then running each of the steps individually and checking for errors in the resulting artifacts.

If that doesn't help show us the complete output of go build -x.

I did not make any changes to CC environment variable.
I ran 'go clean -cache' and then 'go build -x'.It generated 'greetings'.
./greetings
Greetings, Gopher from 2018! We come in peace :)90-F0QJJG5M-3R0:greetings nitish.saboo

How did it work this time ?

Output of go build -x

WORK=/var/folders/3n/gnx8m6wx6zndptdxj3z3n61mfnyqn2/T/go-build101211381
mkdir -p $WORK/b001/
cat >$WORK/b001/importcfg.link << 'EOF' # internal
packagefile greetings=/Users/nitish.saboo/Library/Caches/go-build/97/97b03eb447b772cd798d9b3856252b4d5768a05ee3820bef291d484eb5d9f582-d
packagefile runtime/cgo=/usr/local/go/pkg/darwin_amd64/runtime/cgo.a
packagefile syscall=/usr/local/go/pkg/darwin_amd64/syscall.a
packagefile runtime=/usr/local/go/pkg/darwin_amd64/runtime.a
packagefile errors=/usr/local/go/pkg/darwin_amd64/errors.a
packagefile internal/race=/usr/local/go/pkg/darwin_amd64/internal/race.a
packagefile sync=/usr/local/go/pkg/darwin_amd64/sync.a
packagefile internal/bytealg=/usr/local/go/pkg/darwin_amd64/internal/bytealg.a
packagefile internal/cpu=/usr/local/go/pkg/darwin_amd64/internal/cpu.a
packagefile runtime/internal/atomic=/usr/local/go/pkg/darwin_amd64/runtime/internal/atomic.a
packagefile runtime/internal/math=/usr/local/go/pkg/darwin_amd64/runtime/internal/math.a
packagefile runtime/internal/sys=/usr/local/go/pkg/darwin_amd64/runtime/internal/sys.a
packagefile sync/atomic=/usr/local/go/pkg/darwin_amd64/sync/atomic.a
EOF
mkdir -p $WORK/b001/exe/
cd .
/usr/local/go/pkg/tool/darwin_amd64/link -o $WORK/b001/exe/a.out -importcfg $WORK/b001/importcfg.link -buildmode=exe -buildid=nCIAoLwR6blAM0b35qOA/0L87jRCIU8h8q-J6CJt0/NrO_ik_2-gUxsOVCuIwi/nCIAoLwR6blAM0b35qOA -extld=clang /Users/nitish.saboo/Library/Caches/go-build/97/97b03eb447b772cd798d9b3856252b4d5768a05ee3820bef291d484eb5d9f582-d
/usr/local/go/pkg/tool/darwin_amd64/buildid -w $WORK/b001/exe/a.out # internal
mv $WORK/b001/exe/a.out greetings
rm -r $WORK/b001/

But,
Output of go build maing.go

command-line-arguments

Undefined symbols for architecture x86_64:
"_greet", referenced from:
__cgo_d6ff402930ca_Cfunc_greet in _x002.o
(maybe you meant: __cgo_d6ff402930ca_Cfunc_greet)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

go build succeeds whereas go build main.go fails. Am i missing something here ?

go build succeeds whereas go build main.go fails. Am i missing something here ?

go build without further arguments builds the entire package in the current directory, including all Go and C source files.

go build main.go builds only the file main.go, as if that file is the entire Go package. (Search for “If the arguments are a list of .go files” in https://golang.org/cmd/go/#hdr-Compile_packages_and_dependencies.)

In the context of a cgo build, go build main.go would assume that all of the necessary C libraries are already compiled and installed (and presumably specified using a // #cgo LDFLAGS: -lfoo comment or similar). So if that's the difference you observed, then it sounds like everything is working as documented.

@bcmills Thanks for the clarification.So if my C libraries are not compiled and I use 'go build' to build the program and it generates a file(like greetings in above case), does it mean that I have no issues on MacOS with respect to environment and C compiler ?
The reason I am asking this is because I have a different project where I am facing the same issue 'Undefined symbols for architecture x86_64:...'

does it mean that I have no issues on MacOS with respect to environment and C compiler ?

Probably yes. Your problems more likely have something to do with how the program is being built.

i my case i put declartion of function in greeter.c file into greeter.h and it worked!

#ifndef _GREETER_H
#define _GREETER_H
int greet(const char *name, int year, char *out) {
    int n;
    n = sprintf(out, "Greetings, %s from %d! We come in peace :)", name, year);

    return n;
}
#endif

something like that.