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The Interface of the Function in NSS Modules

Now we know about the functions contained in the modules. It is now time to describe the types. When we mentioned the reentrant versions of the functions above, this means there are some additional arguments (compared with the standard, non-reentrant version). The prototypes for the non-reentrant and reentrant versions of our function above are:

struct hostent *gethostbyname (const char *name)

int gethostbyname_r (const char *name, struct hostent *result_buf,
                     char *buf, size_t buflen, struct hostent **result,
                     int *h_errnop)

The actual prototype of the function in the NSS modules in this case is

enum nss_status _nss_files_gethostbyname_r (const char *name,
                                            struct hostent *result_buf,
                                            char *buf, size_t buflen,
                                            int *h_errnop)

I.e., the interface function is in fact the reentrant function with the change of the return value and the omission of the result parameter. While the user-level function returns a pointer to the result the reentrant function return an enum nss_status value:

numeric value -2
numeric value -1
numeric value 0
numeric value 1

Now you see where the action items of the `/etc/nsswitch.conf' file are used.

If you study the source code you will find there is a fifth value: NSS_STATUS_RETURN. This is an internal use only value, used by a few functions in places where none of the above value can be used. If necessary the source code should be examined to learn about the details.

The above function has something special which is missing for almost all the other module functions. There is an argument h_errnop. This points to a variable which will be filled with the error code in case the execution of the function fails for some reason. The reentrant function cannot use the global variable h_errno; gethostbyname calls gethostbyname_r with the last argument set to &h_errno.

The getXXXbyYYY functions are the most important functions in the NSS modules. But there are others which implement the other ways to access system databases (say for the password database, there are setpwent, getpwent, and endpwent). These will be described in more detail later. Here we give a general way to determine the signature of the module function:

This table is correct for all functions but the set...ent and end...ent functions.

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