2. Using ODR
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## 2. Using ODR

### 2.1. ODR Streams

Conceptually, the ODR stream is the source of encoded data in the decoding mode; when encoding, it is the receptacle for the encoded data. Before you can use an ODR stream it must be allocated. This is done with the function

         ODR odr_createmem(int direction);

The `odr_createmem()` function takes as argument one of three manifest constants: `ODR_ENCODE`, `ODR_DECODE`, or `ODR_PRINT`. An ODR stream can be in only one mode - it is not possible to change its mode once it's selected. Typically, your program will allocate at least two ODR streams - one for decoding, and one for encoding.

When you're done with the stream, you can use

         void odr_destroy(ODR o);

to release the resources allocated for the stream.

### 2.2. Memory Management

Two forms of memory management take place in the ODR system. The first one, which has to do with allocating little bits of memory (sometimes quite large bits of memory, actually) when a protocol package is decoded, and turned into a complex of interlinked structures. This section deals with this system, and how you can use it for your own purposes. The next section deals with the memory management which is required when encoding data - to make sure that a large enough buffer is available to hold the fully encoded PDU.

The ODR module has its own memory management system, which is used whenever memory is required. Specifically, it is used to allocate space for data when decoding incoming PDUs. You can use the memory system for your own purposes, by using the function

         void *odr_malloc(ODR o, int size);

You can't use the normal `free(2)` routine to free memory allocated by this function, and ODR doesn't provide a parallel function. Instead, you can call

         void odr_reset(ODR o);

when you are done with the memory: Everything allocated since the last call to `odr_reset()` is released. The `odr_reset()` call is also required to clear up an error condition on a stream.

The function

         int odr_total(ODR o);

returns the number of bytes allocated on the stream since the last call to `odr_reset()`.

The memory subsystem of ODR is fairly efficient at allocating and releasing little bits of memory. Rather than managing the individual, small bits of space, the system maintains a free-list of larger chunks of memory, which are handed out in small bits. This scheme is generally known as a nibble memory system. It is very useful for maintaining short-lived constructions such as protocol PDUs.

If you want to retain a bit of memory beyond the next call to `odr_reset()`, you can use the function

         ODR_MEM odr_extract_mem(ODR o);

This function will give you control of the memory recently allocated on the ODR stream. The memory will live (past calls to `odr_reset()`), until you call the function

         void odr_release_mem(ODR_MEM p);

The opaque `ODR_MEM` handle has no other purpose than referencing the memory block for you until you want to release it.

You can use `odr_extract_mem()` repeatedly between allocating data, to retain individual control of separate chunks of data.

### 2.3. Encoding and Decoding Data

When encoding data, the ODR stream will write the encoded octet string in an internal buffer. To retrieve the data, use the function

         char *odr_getbuf(ODR o, int *len, int *size);

The integer pointed to by len is set to the length of the encoded data, and a pointer to that data is returned. `*size` is set to the size of the buffer (unless `size` is null, signaling that you are not interested in the size). The next call to a primitive function using the same ODR stream will overwrite the data, unless a different buffer has been supplied using the call

         void odr_setbuf(ODR o, char *buf, int len, int can_grow);

which sets the encoding (or decoding) buffer used by `o` to `buf`, using the length `len`. Before a call to an encoding function, you can use `odr_setbuf()` to provide the stream with an encoding buffer of sufficient size (length). The `can_grow` parameter tells the encoding ODR stream whether it is allowed to use `realloc(2)` to increase the size of the buffer when necessary. The default condition of a new encoding stream is equivalent to the results of calling

         odr_setbuf(stream, 0, 0, 1);

In this case, the stream will allocate and reallocate memory as necessary. The stream reallocates memory by repeatedly doubling the size of the buffer - the result is that the buffer will typically reach its maximum, working size with only a small number of reallocation operations. The memory is freed by the stream when the latter is destroyed, unless it was assigned by the user with the `can_grow` parameter set to zero (in this case, you are expected to retain control of the memory yourself).

To assume full control of an encoded buffer, you must first call `odr_getbuf()` to fetch the buffer and its length. Next, you should call `odr_setbuf()` to provide a different buffer (or a null pointer) to the stream. In the simplest case, you will reuse the same buffer over and over again, and you will just need to call `odr_getbuf()` after each encoding operation to get the length and address of the buffer. Note that the stream may reallocate the buffer during an encoding operation, so it is necessary to retrieve the correct address after each encoding operation.

It is important to realize that the ODR stream will not release this memory when you call `odr_reset()`: It will merely update its internal pointers to prepare for the encoding of a new data value. When the stream is released by the `odr_destroy()` function, the memory given to it by `odr_setbuf` will be released only if the `can_grow` parameter to `odr_setbuf()` was nonzero. The `can_grow` parameter, in other words, is a way of signaling who is to own the buffer, you or the ODR stream. If you never call `odr_setbuf()` on your encoding stream, which is typically the case, the buffer allocated by the stream will belong to the stream by default.

When you wish to decode data, you should first call `odr_setbuf()`, to tell the decoding stream where to find the encoded data, and how long the buffer is (the `can_grow` parameter is ignored by a decoding stream). After this, you can call the function corresponding to the data you wish to decode (eg, `odr_integer()` odr `z_APDU()`).

Example 8.1. Encoding and decoding functions

          int odr_integer(ODR o, Odr_int **p, int optional, const char *name);

          int z_APDU(ODR o, Z_APDU **p, int optional, const char *name);

If the data is absent (or doesn't match the tag corresponding to the type), the return value will be either 0 or 1 depending on the `optional` flag. If `optional` is 0 and the data is absent, an error flag will be raised in the stream, and you'll need to call `odr_reset()` before you can use the stream again. If `optional` is nonzero, the pointer pointed to/ by `p` will be set to the null value, and the function will return 1. The `name` argument is used to pretty-print the tag in question. It may be set to `NULL` if pretty-printing is not desired.

If the data value is found where it's expected, the pointer pointed to by the `p` argument will be set to point to the decoded type. The space for the type will be allocated and owned by the ODR stream, and it will live until you call `odr_reset()` on the stream. You cannot use `free(2)` to release the memory. You can decode several data elements (by repeated calls to `odr_setbuf()` and your decoding function), and new memory will be allocated each time. When you do call `odr_reset()`, everything decoded since the last call to `odr_reset()` will be released.

Example 8.2. Encoding and decoding of an integer

The use of the double indirection can be a little confusing at first (its purpose will become clear later on, hopefully), so an example is in order. We'll encode an integer value, and immediately decode it again using a different stream. A useless, but informative operation.

    void do_nothing_useful(Odr_int value)
    {
        ODR encode, decode;
        Odr_int *valp, *resvalp;
        char *bufferp;
        int len;

        /* allocate streams */
        if (!(encode = odr_createmem(ODR_ENCODE)))
            return;
        if (!(decode = odr_createmem(ODR_DECODE)))
            return;

        valp = &value;
        if (odr_integer(encode, &valp, 0, 0) == 0)
        {
            printf("encoding went bad\n");
            return;
        }
        bufferp = odr_getbuf(encode, &len, 0);
        printf("length of encoded data is %d\n", len);

        /* now let's decode the thing again */
        odr_setbuf(decode, bufferp, len, 0);
        if (odr_integer(decode, &resvalp, 0, 0) == 0)
        {
            printf("decoding went bad\n");
            return;
        }
        /* ODR_INT_PRINTF format for printf (such as %d) */
        printf("the value is " ODR_INT_PRINTF "\n", *resvalp);

        /* clean up */
        odr_destroy(encode);
        odr_destroy(decode);
    }

This looks like a lot of work, offhand. In practice, the ODR streams will typically be allocated once, in the beginning of your program (or at the beginning of a new network session), and the encoding and decoding will only take place in a few, isolated places in your program, so the overhead is quite manageable.

### 2.4. Printing

When an ODR stream is created of type `ODR_PRINT` the ODR module will print the contents of a PDU in a readable format. By default output is written to the `stderr` stream. This behavior can be changed, however, by calling the function

          odr_setprint(ODR o, FILE *file);

before encoders or decoders are being invoked. It is also possible to direct the output to a buffer (of indeed another file), by using the more generic mechanism:

          void odr_set_stream(ODR o, void *handle,
                             void (*stream_write)(ODR o, void *handle, int type,
                                                  const char *buf, int len),
                             void (*stream_close)(void *handle));

Here the user provides an opaque handle and two handlers, `stream_write` for writing, and `stream_close` which is supposed to close/free resources associated with handle. The `stream_close` handler is optional and if NULL for the function is provided, it will not be invoked. The `stream_write` takes the ODR handle as parameter, the user defined handle, a type `ODR_OCTETSTRING`, `ODR_VISIBLESTRING` which indicates the type of contents is being written.

Another utility useful for diagnostics (error handling) or as part of the printing facilities is:

          const char **odr_get_element_path(ODR o);

which returns a list of current elements that ODR deals with at the moment. For the returned array, say `ar`, `ar[0]` is the top level element, `ar[n]` is the last. The last element has the property that `ar[n+1] == NULL`.

Example 8.3. Element Path for record

For a database record part of a PresentResponse the array returned by `odr_get_element` is `presentResponse`, `databaseOrSurDiagnostics`, `?`, `record`, `?`, `databaseRecord` . The question mark appears due to unnamed constructions.

### 2.5. Diagnostics

The encoding/decoding functions all return 0 when an error occurs. Until you call `odr_reset()`, you cannot use the stream again, and any function called will immediately return 0.

To provide information to the programmer or administrator, the function

         void odr_perror(ODR o, char *message);

is provided, which prints the `message` argument to `stderr` along with an error message from the stream.

You can also use the function

         int odr_geterror(ODR o);

to get the current error number from the screen. The number will be one of these constants:

Table 8.1. ODR Error codes

code | Description
--- | ---
OMEMORY | Memory allocation failed.
OSYSERR | A system- or library call has failed. The standard diagnostic variable `errno` should be examined to determine the actual error.
OSPACE | No more space for encoding. This will only occur when the user has explicitly provided a buffer for an encoding stream without allowing the system to allocate more space.
OREQUIRED | This is a common protocol error; A required data element was missing during encoding or decoding.
OUNEXPECTED | An unexpected data element was found during decoding.
OOTHER | Other error. This is typically an indication of misuse of the ODR system by the programmer, and also that the diagnostic system isn't as good as it should be, yet.

The character string array

         char *odr_errlist[]

can be indexed by the error code to obtain a human-readable representation of the problem.

### 2.6. Summary and Synopsis

         #include <yaz/odr.h>

         ODR odr_createmem(int direction);

         void odr_destroy(ODR o);

         void odr_reset(ODR o);

         char *odr_getbuf(ODR o, int *len, int *size);

         void odr_setbuf(ODR o, char *buf, int len, int can_grow);

         void *odr_malloc(ODR o, int size);

         NMEM odr_extract_mem(ODR o);

         int odr_geterror(ODR o);

         void odr_perror(ODR o, const char *message);

         extern char *odr_errlist[];

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