mars/meta.h

86 lines
2.7 KiB
C

// (c) 2010 Thomas Schoebel-Theuer / 1&1 Internet AG
#ifndef META_H
#define META_H
/////////////////////////////////////////////////////////////////////////
// metadata descriptions
/* The idea is to describe your C structures in such a way that
* transfers to disk or over a network become self-describing.
*
* In essence, this is a kind of version-independent marshalling.
*
* Advantage:
* When you extend your original C struct (and of course update the
* corresponding meta structure), old data on disk (or network peers
* running an old version of your program) will remain valid.
* Upon read, newly added fields missing in the old version will be simply
* not filled in and therefore remain zeroed (if you don't forget to
* initially clear your structures via memset() / initializers / etc).
* Note that this works only if you never rename or remove existing
* fields; you should only add new ones.
* [TODO: add macros for description of ignored / renamed fields to
* overcome this limitation]
* You may increase the size of integers, for example from 32bit to 64bit
* or even higher; sign extension will be automatically carried out
* when necessary.
* [TODO; NYI]
* Also, you may change the order of fields, because the metadata interpreter
* will check each field individually; field offsets are automatically
* maintained.
*
* Disadvantage: this adds some (small) overhead.
*/
enum field_type {
FIELD_DONE,
FIELD_REF,
FIELD_SUB,
FIELD_STRING,
FIELD_RAW,
FIELD_INT,
FIELD_UINT,
};
struct meta {
//char field_name[MAX_FIELD_LEN];
char *field_name;
int field_type;
int field_size;
int field_offset;
const struct meta *field_ref;
};
#define _META_INI(NAME,STRUCT,TYPE) \
.field_name = #NAME, \
.field_type = TYPE, \
.field_size = sizeof(((STRUCT*)NULL)->NAME), \
.field_offset = offsetof(STRUCT, NAME) \
#define META_INI(NAME,STRUCT,TYPE) { _META_INI(NAME,STRUCT,TYPE) }
#define _META_INI_REF(NAME,STRUCT,REF) \
.field_name = #NAME, \
.field_type = FIELD_REF, \
.field_size = sizeof(*(((STRUCT*)NULL)->NAME)), \
.field_offset = offsetof(STRUCT, NAME), \
.field_ref = REF
#define META_INI_REF(NAME,STRUCT,REF) { _META_INI_REF(NAME,STRUCT,REF) }
#define _META_INI_SUB(NAME,STRUCT,SUB) \
.field_name = #NAME, \
.field_type = FIELD_SUB, \
.field_size = sizeof(((STRUCT*)NULL)->NAME), \
.field_offset = offsetof(STRUCT, NAME), \
.field_ref = SUB
#define META_INI_SUB(NAME,STRUCT,SUB) { _META_INI_SUB(NAME,STRUCT,SUB) }
extern const struct meta *find_meta(const struct meta *meta, const char *field_name);
//extern void free_meta(void *data, const struct meta *meta);
#endif