datum.h
typedef unsigned int uintptr_t;(stdint.h) typedef uintptr_t Datum; (postgres.h) /*------------------------------------------------------------------------- * * datum.h * POSTGRES Datum (abstract data type) manipulation routines. * * These routines are driven by the 'typbyval' and 'typlen' information,* which must previously have been obtained by the caller for the datatype * of the Datum. (We do it this way because in most situations the caller * can look up the info just once and use it for many per-datum operations.) * * Portions Copyright (c) 1996-2012,PostgreSQL Global Development Group * Portions Copyright (c) 1994,Regents of the University of California * * src/include/utils/datum.h * *------------------------------------------------------------------------- */ #ifndef DATUM_H #define DATUM_H /* * datumGetSize - find the "real" length of a datum */ extern Size datumGetSize(Datum value,bool typByVal,int typLen); /* * datumCopy - make a copy of a datum. * * If the datatype is pass-by-reference,memory is obtained with palloc(). */ extern Datum datumCopy(Datum value,int typLen); /* * datumFree - free a datum previously allocated by datumCopy,if any. * * Does nothing if datatype is pass-by-value. */ extern void datumFree(Datum value,int typLen); /* * datumIsEqual * return true if two datums of the same type are equal,false otherwise. * * XXX : See comments in the code for restrictions! */ extern bool datumIsEqual(Datum value1,Datum value2,int typLen); #endif /* DATUM_H */datum.c
/*-------------------------------------------------------------------------
*
* datum.c
* POSTGRES Datum (abstract data type) manipulation routines.
*
* Portions Copyright (c) 1996-2012,Regents of the University of California
*
*
* IDENTIFICATION
* src/backend/utils/adt/datum.c
*
*-------------------------------------------------------------------------
*/
/*
* In the implementation of the next routines we assume the following:
*
* A) if a type is "byVal" then all the information is stored in the
* Datum itself (i.e. no pointers involved!). In this case the
* length of the type is always greater than zero and not more than
* "sizeof(Datum)"
*
* B) if a type is not "byVal" and it has a fixed length (typlen > 0),* then the "Datum" always contains a pointer to a stream of bytes.
* The number of significant bytes are always equal to the typlen.
*
* C) if a type is not "byVal" and has typlen == -1,* then the "Datum" always points to a "struct varlena".
* This varlena structure has information about the actual length of this
* particular instance of the type and about its value.
*
* D) if a type is not "byVal" and has typlen == -2,* then the "Datum" always points to a null-terminated C string.
*
* Note that we do not treat "toasted" datums specially; therefore what
* will be copied or compared is the compressed data or toast reference.
*/
#include "postgres.h"
#include "utils/datum.h"
/*-------------------------------------------------------------------------
* datumGetSize
*
* Find the "real" size of a datum,given the datum value,* whether it is a "by value",and the declared type length.
*
* This is essentially an out-of-line version of the att_addlength_datum()
* macro in access/tupmacs.h. We do a tad more error checking though.
*-------------------------------------------------------------------------
*/
Size
datumGetSize(Datum value,int typLen)
{
Size size;
if (typByVal)
{
/* Pass-by-value types are always fixed-length */
Assert(typLen > 0 && typLen <= sizeof(Datum));
size = (Size) typLen;
}
else
{
if (typLen > 0)
{
/* Fixed-length pass-by-ref type */
size = (Size) typLen;
}
else if (typLen == -1)
{
/* It is a varlena datatype */
struct varlena *s = (struct varlena *) DatumGetPointer(value);
if (!PointerIsValid(s))
ereport(ERROR,(errcode(ERRCODE_DATA_EXCEPTION),errmsg("invalid Datum pointer")));
size = (Size) VARSIZE_ANY(s);
}
else if (typLen == -2)
{
/* It is a cstring datatype */
char *s = (char *) DatumGetPointer(value);
if (!PointerIsValid(s))
ereport(ERROR,errmsg("invalid Datum pointer")));
size = (Size) (strlen(s) + 1);
}
else
{
elog(ERROR,"invalid typLen: %d",typLen);
size = 0; /* keep compiler quiet */
}
}
return size;
}
/*-------------------------------------------------------------------------
* datumCopy
*
* make a copy of a datum
*
* If the datatype is pass-by-reference,memory is obtained with palloc().
*-------------------------------------------------------------------------
*/
Datum
datumCopy(Datum value,int typLen)
{
Datum res;
if (typByVal)
res = value;
else
{
Size realSize;
char *s;
if (DatumGetPointer(value) == NULL)
return PointerGetDatum(NULL);
realSize = datumGetSize(value,typByVal,typLen);
s = (char *) palloc(realSize);
memcpy(s,DatumGetPointer(value),realSize);
res = PointerGetDatum(s);
}
return res;
}
/*-------------------------------------------------------------------------
* datumFree
*
* Free the space occupied by a datum CREATED BY "datumCopy"
*
* NOTE: DO NOT USE THIS ROUTINE with datums returned by heap_getattr() etc.
* ONLY datums created by "datumCopy" can be freed!
*-------------------------------------------------------------------------
*/
#ifdef NOT_USED
void
datumFree(Datum value,int typLen)
{
if (!typByVal)
{
Pointer s = DatumGetPointer(value);
pfree(s);
}
}
#endif
/*-------------------------------------------------------------------------
* datumIsEqual
*
* Return true if two datums are equal,false otherwise
*
* NOTE: XXX!
* We just compare the bytes of the two values,one by one.
* This routine will return false if there are 2 different
* representations of the same value (something along the lines
* of say the representation of zero in one's complement arithmetic).
* Also,it will probably not give the answer you want if either
* datum has been "toasted".
*-------------------------------------------------------------------------
*/
bool
datumIsEqual(Datum value1,int typLen)
{
bool res;
if (typByVal)
{
/*
* just compare the two datums. NOTE: just comparing "len" bytes will
* not do the work,because we do not know how these bytes are aligned
* inside the "Datum".We assume instead that any given datatype is
* consistent about how it fills extraneous bits in the Datum.
*/
res = (value1 == value2);
}
else
{
Size size1,size2;
char *s1,*s2;
/*
* Compare the bytes pointed by the pointers stored in the datums.
*/
size1 = datumGetSize(value1,typLen);
size2 = datumGetSize(value2,typLen);
if (size1 != size2)
return false;
s1 = (char *) DatumGetPointer(value1);
s2 = (char *) DatumGetPointer(value2);
res = (memcmp(s1,s2,size1) == 0);
}
return res;
}
relevant knowledge：
上面提到的varlena是一个结构体，其定义在c.h文件中，结构非常简单，如下所示：
/* ----------------
* Variable-length datatypes all share the 'struct varlena' header.
* NOTE: for TOASTable types,this is an oversimplification,since the value
* may be compressed or moved out-of-line. However datatype-specific routines
* are mostly content to deal with de-TOASTed values only,and of course
* client-side routines should never see a TOASTed value. But even in a
* de-TOASTed value,beware of touching vl_len_ directly,as its representation
* is no longer convenient. It's recommended that code always use the VARDATA,* VARSIZE,and SET_VARSIZE macros instead of relying on direct mentions of
* the struct fields. See postgres.h for details of the TOASTed form.
* ----------------
*/
struct varlena{
char vl_len_[4];
char vl_dat[1];
};
值传递和引用传递（Pass-by-value&pass-by-reference）:详见下一篇博客:C/C++中函数参数传递详解版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 dio@foxmail.com 举报，一经查实，本站将立刻删除。