I have a structure that contains a pointer to a byte array.
To set the pointer I’ve tried the following two ways:
1 Use malloc then memcpy byte array data (commented out in code below).
2 Simply copy pointer.
#include "stdlib.h"
#include "string.h"
#include "stdio.h"
typedef struct _element
{
unsigned char *pValue;
int nLength;
} Element;
Element* ElementCreate(void)
{
Element *pElement = (Element*)malloc(sizeof(*pElement));
pElement->pValue = NULL;
pElement->nLength = 0;
return pElement;
}
void ElementDestroy(Element **ppElement)
{
Element *pElement = NULL;
if (!ppElement)
return;
pElement = *ppElement;
//free(pElement->pValue);
//pElement->pValue = NULL;
free(pElement);
*ppElement = NULL;
}
void ElementSetValue(Element *pElement, unsigned char *pValue, int nLength)
{
//pElement->pValue = (unsigned char*)malloc(nLength * sizeof(*(pElement->pValue)));
//if (!(pElement->pValue))
// return;
//memcpy(pElement->pValue, pValue, nLength);
pElement->pValue = pValue;
pElement->nLength = nLength;
}
void ElementWriteValue(const Element *pElement)
{
int nIndex = 0;
for (; nIndex < pElement->nLength; nIndex++)
printf("%02X ", pElement->pValue[nIndex]);
}
int main(void)
{
//unsigned char myValue[] = { 0x01, 0x02, 0x03 };
//int nLength = sizeof(myValue) / sizeof(myValue[0]);
Element *pElement = ElementCreate();
{
unsigned char myValue[] = { 0x01, 0x02, 0x03 };
int nLength = sizeof(myValue) / sizeof(myValue[0]);
ElementSetValue(pElement, myValue, nLength);
}
// How come this writes out correct value?
ElementWriteValue(pElement);
ElementDestroy(&pElement);
return 0;
}
(Error checks are omitted for brevity)
Which way is correct?
I’d expect 2 to fail because myValue will be destroyed after the “}†so
printf("%02X ", pElement->pValue[nIndex]);
would write out rubbish data but it seems to work OK. Why?
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