/cc @ianlancetaylor

This is doable in principle--we could apply special handling to an //extern function that returns two values where the second one has type int, but it would be so rarely used that I don't think it's worth the additional complexity.

I think go also needs the ability to set errno when calling into certain functions like getgrent and getpwent. These functions return NULL and set errno on error, but they may also return NULL and not set errno on success (when all the entries have been returned). So, users must explicitly set errno to 0 before calling these functions.

From the macOS man page for getpwent:

RETURN VALUES
The functions getpwent(), getpwnam(), getpwuid(), and getpwuuid() return a
valid pointer to a passwd structure on success or NULL if the entry is not
found or if an error occurs. If an error does occur, errno will be set.
Note that programs must explicitly set errno to zero before calling any of
these functions if they need to distinguish between a non-existent entry
and an error.

It seems the only way to do this is to write a C function to set errno.

As I said earlier, this case is so rarely used that I don't think it's worth the extra complexity in cgo.

For people who need this I recommend writing, in the cgo comment.

/*
#include <errno.h>
#include <sys/types.h>
#include <pwd.h>

static struct passwd *goGetpwent() {
    errno = 0;
    return getpwent();
}
*/
import "C"

Then change your code to call goGetpwent rather than getpwent.

this case is so rarely used that I don't think it's worth the extra complexity in cgo.

I'm the original reporter, and with four years more experience, I fully agree with Ian. Sorry for the noise.