Browsing su sources of login/shadow package

apt source login # got shadow-4.13

Find src/su.c file with password prompt call

struct passwd *pw = xgetpwnam (name);

But i cant find a xgetpwnam() sources. Just only libs/prototypes.h with next signature

/* xgetpwnam.c */ extern /*@null@*/ /*@only@*/struct passwd *xgetpwnam (const char *);

File xgetpwnam.c not exists in this package. Google by this name find only getpwnam.c in libc repo. But it is not what i want.

I search it on hdd, on ubuntu packages site – nothing.

Where i can find xgetpwnam.c file with xgetpwnam() source code?

Thanks.