The getpwnam function searches the user database for an entry
    with the specified name. The function returns the first user
    entry in the database with the pw_name member of the passwd
    structure that matches the name argument.

    The passwd structure is defined in the <pwd.h> header file as
    follows:

    pw_name    The user's login name.
    pw_uid     The numerical user ID.
    pw_gid     The numerical group ID.
    pw_dir     The home directory of the user.
    pw_shell   The initial program for the user.

                                   NOTE

       All information generated by the getpwnam function is stored
       in a per-thread static area and is overwritten on subsequent
       calls to the function.

    The getpwnam_r function is the reentrant version of getpwnam.
    The getpwnam_r function updates the passwd structure pointed to
    by pwd and stores a pointer to that structure at the location
    pointed to by result. The structure will contain an entry from
    the user database that matches the specified name. Storage
    referenced by the structure is allocated from the memory provided
    with the buffer argument, which is bufsize characters in length.
    The maximum size needed for this buffer can be determined with
    the _SC_GETPW_R_SIZE_MAX parameter of the sysconf function. On
    error or if the requested entry is not found, a NULL pointer is
    returned at the location pointed to by result.

    Applications wishing to check for error situations should set
    errno to 0 before calling getpwnam. If getpwnam returns a NULL
    pointer and errno is nonzero, an error occurred.