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