Added a call back interface to rrd_dump to be able to call the function from c. See...

[rrdtool.git] / doc / librrd.pod

diff --git a/doc/librrd.pod b/doc/librrd.pod
index 52cf18e..04a295d 100644 (file)
--- a/doc/librrd.pod
+++ b/doc/librrd.pod
@@ -58,4 +58,41 @@ source pointer will be NULL and the count will be zero.
     rrd_free_ptrs(&arr, &arr_size);
     /* here, arr == NULL && arr_size == 0 */
 
+=item B<rrd_dump_cr_r(char *filename, int opt_header, rrd_output_callback_t cb, void *user)>
+
+In some situations it is necessary to get the output of C<rrd_dump> without
+writing it to a file or the standard output. In such cases an application
+can ask B<rrd_dump_cb_r> to call an user-defined function each time there
+is output to be stored somewhere. This can be used, to e.g. directly feed
+an XML parser with the dumped output or transfer the resulting string
+in memory.
+
+The arguments for B<rrd_dump_cb_r> are the same as for B<rrd_dump_opt_r>
+except that the output filename parameter is replaced by the user-defined
+callback function and an additional parameter for the callback function
+that is passed untouched, i.e. to store information about the callback state
+needed for the user-defined callback to function properly.
+
+Recent versions of B<rrd_dump_opt_r> internally use this callback mechanism
+to write their output to the file provided by the user.
+
+    size_t rrd_dump_opt_cb_fileout(
+        const void *data,
+        size_t len,
+        void *user)
+    {
+        return fwrite(data, 1, len, (FILE *)user);
+    }
+
+The associated call for B<rrd_dump_cb_r> looks like
+
+    res = rrd_dump_cb_r(filename, opt_header,
+        rrd_dump_opt_cb_fileout, (void *)out_file);
+
+where the last parameter specifies the file handle B<rrd_dump_opt_cb_fileout>
+should write to. There's no specific condition for the callback to detect
+when it is called for the first time, nor for the last time. If you require
+this for initialization and cleanup you should do those tasks before and
+after calling B<rrd_dump_cr_r> respectively.
+
 =back