28.3.2 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 *errnop, 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:

NSS_STATUS_TRYAGAIN

numeric value -2

NSS_STATUS_UNAVAIL

numeric value -1

NSS_STATUS_NOTFOUND

numeric value 0

NSS_STATUS_SUCCESS

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.

In case the interface function has to return an error it is important that the correct error code is stored in *errnop. Some return status value have only one associated error code, others have more.

NSS_STATUS_TRYAGAIN	EAGAIN	One of the functions used ran temporarily out of resources or a service is currently not available.
	ERANGE	The provided buffer is not large enough. The function should be called again with a larger buffer.
NSS_STATUS_UNAVAIL	ENOENT	A necessary input file cannot be found.
NSS_STATUS_NOTFOUND	ENOENT	The requested entry is not available.

These are proposed values. There can be other error codes and the described error codes can have different meaning. With one exception: when returning NSS_STATUS_TRYAGAIN the error code ERANGE must mean that the user provided buffer is too small. Everything is non-critical.

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:

• the return value is int;
• the name is as explained in see NSS Module Names;
• the first arguments are identical to the arguments of the non-reentrant function;
• the next three arguments are:

  STRUCT_TYPE *result_buf

  pointer to buffer where the result is stored. STRUCT_TYPE is normally a struct which corresponds to the database.
  

  char *buffer

  pointer to a buffer where the function can store additional data for the result etc.
  

  size_t buflen

  length of the buffer pointed to by buffer.

• possibly a last argument h_errnop, for the host name and network name lookup functions.

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