NAL_DECODE_UINT32

Section: distcache (2)
Updated: 2004.03.23
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NAME

NAL_decode_uint32, NAL_decode_uint16, NAL_decode_char, NAL_decode_bin, NAL_encode_uint32, NAL_encode_uint16, NAL_encode_char, NAL_encode_bin - libnal serialisation functions

SYNOPSIS

 #include <libnal/nal.h>

 int NAL_decode_uint32(const unsigned char **bin, unsigned int *bin_len,
                       unsigned long *val);
 int NAL_decode_uint16(const unsigned char **bin, unsigned int *bin_len,
                       unsigned int *val);
 int NAL_decode_char(const unsigned char **bin, unsigned int *bin_len,
                     unsigned char *val);
 int NAL_decode_bin(const unsigned char **bin, unsigned int *bin_len,
                    unsigned char *val, unsigned int val_len);

 int NAL_encode_uint32(unsigned char **bin, unsigned int *bin_len,
                       const unsigned long val);
 int NAL_encode_uint16(unsigned char **bin, unsigned int *bin_len,
                       const unsigned int val);
 int NAL_encode_char(unsigned char **bin, unsigned int *bin_len,
                     const unsigned char val);
 int NAL_encode_bin(unsigned char **bin, unsigned int *bin_len,
                    const unsigned char *val, const unsigned int val_len);

DESCRIPTION

NAL_decode_uint32(), NAL_decode_uint16(), and NAL_decode_char() attempt to parse different sized integer values from the data pointed to by *bin (both bin and bin_len are passed by reference). If bin_len indicates there is sufficient data to successfully parse a value, then the value will be stored in val, *bin will be incremented to point to the next unparsed byte of data, and *bin_len will be decremented to indicate how much unparsed data remains.

NAL_decode_bin() follows the semantics of the other decode functions except that it decodes a block of binary data of length val_len.

NAL_encode_uint32(), NAL_encode_uint16(), and NAL_encode_char() attempt to encode different sized integer values to the located pointed to by *bin (again, both bin and bin_len are passed by reference). If bin_len indicates there is sufficient room to successfully encode a value, val will be stored at *bin, *bin will be incremented to point to the next unused byte of storage, and *bin_len will be decremented to indicate how much unused storage remains.

NAL_encode_bin() follows the semantics of the other encode functions except that it encodes a block of binary data of length val_len.

RETURN VALUES

All the encode and decode functions return non-zero for success or zero for failure. On failure, bin and bin_len are left unchanged.

NOTES

The reason for passing bin and bin_len by reference to all these functions is to allow (de)serialisation of complex structures to be built up more easily without unnecessary work by the caller. The return value still indicates whether an encoding or decoding was successful, but the caller will not need to increment bin nor decrement bin_len after success before continuing to encode or decode further data.

EXAMPLES

Assume we wish to pass a data structure between applications running on different machines (and potentially on different architectures), and the data structure is defined as follows;

 #define MAX_DATA_SIZE 4096
 typedef struct st_some_data_t {
     unsigned char is_active;      /* boolean */
     unsigned char buffer[MAX_DATA_SIZE];
     unsigned int buffer_used;
 } some_data_t;

We could define two functions for encoding and decoding an object of this type such that they could be serialised and transferred over a connection. The most elegant way to build serialisation of objects is to create functions that use the same form of prototype as the libnal serialisation functions, this way serialisation of complex objects can be performed recursively by serialisation of aggregated types. Although the built-in libnal serialisation functions leave bin and bin_len unchanged on failure, it is generally not worth bothering to preserve this property at higher-levels - these examples do not attempt this.

An encoding function would thus look like;

 int encode_some_data(unsigned char **bin, unsigned int *bin_len,
                      const some_data_t *val)
 {
     if(
             /* Encode the "is_active" boolean */
             !NAL_encode_char(bin, bin_len, val->is_active) ||
             /* Encode the used data */
             !NAL_encode_uint16(bin, bin_len, val->buffer_used) ||
             ((val->buffer_used > 0) &&
             !NAL_encode_bin(bin, bin_len, val->buffer, val->buffer_used)))
         return 0;
     return 1;
 }

Note that other types that include some_data_t objects could implement serialisation using encode_some_data() in the same way that encode_some_data() uses the lower-level libnal functions. A corresponding decode function follows.

 int decode_some_data(const unsigned char **bin, unsigned int *bin_len,
                      some_data_t *val)
 {
     if(
             /* Decode the "is_active" boolean */
             !NAL_decode_char(bin, bin_len, &val->is_active) ||
             /* Decode the used data */
             !NAL_decode_uint16(bin, bin_len, &val->buffer_used) ||
             /* [TODO: check 'val->buffer_used' is acceptable here] */
             ((val->buffer_used > 0) &&
             !NAL_decode_bin(bin, bin_len, val->buffer, val->buffer_used)))
         return 0;
     return 1;
 }

The above examples would be simpler still if a wrapper function were first written to serialise length-prefixed blocks of data. Such functions are not included in libnal because they can vary on what range of sizes are appropriate, what size encoding to use for a length-prefix, whether dynamic allocation should be used on decoding, etc. The above examples use a static buffer and encode the length prefix as 16-bits.

AUTHOR

This toolkit was designed and implemented by Geoff Thorpe for Cryptographic Appliances Incorporated. Since the project was released into open source, it has a home page and a project environment where development, mailing lists, and releases are organised. For problems with the software or this man page please check for new releases at the project web-site below, mail the users mailing list described there, or contact the author at geoff@geoffthorpe.net.

Home Page: http://www.distcache.org