Casting pointer to reuse a buffer

Question

there: I have declaration like this:

typedef struct
{
    quint8  mark1;
    quint16 content1;
    quint16 content2;
    quint8  mark2;
}FrameHead;

and in function,I defined a buffer,and valued it:

quint8 buf[40];
FrameHead *frame  = (FrameHead *)buf;
frame->mark1 = 0x68;
frame->content1 = 0x3A;
frame->content1 = 0x3A;
frame->mark2 = 0x68;

So as I thought, the first 6 bytes of buf should be "68 3A 00 3A 00 68".But the fact is, the first 8 bytes of buf were "68 90 3A 00 3A 00 68 80".And I used:

qDebug() << (quint8 *)(++frame) - buf// output "8" but should be "6"

seemingly the numbers "0x68" were not stored as quint8 type but quint16 type to keep consistency with other quint16 types.Anyone has encounted the same problems and has any suggestions.

score 1 · Accepted Answer · edited May 23 '17 at 11:43

1

This is due to alignment. The 16-bit types are aligned on 16-bit adresses. Since your first member only uses 8 bit, 1 byte padding is inserted.

If you reorder your struct to

typedef struct
{
    quint8  mark1;
    quint8  mark2;
    quint16 content1;
    quint16 content2;
}FrameHead;

it will only use 6 bytes, as no padding is required to align content1.

See Structure padding and packing for a more detailed explanation.

Edit after comment:

So either you use #pragma pack(1) (see: http://msdn.microsoft.com/en-us/library/2e70t5y1%28v=VS.100%29.aspx) (not sure what you mean by

to set alignment in Qt like "#Pragma" in VC"

) or you simply use an array:

typedef quint[6] FrameHead;

....

FrameHead fh1 = {0x68, 0x00, 0x00, 0x00, 0x00, 0x68};
// or 
FrameHead fh2;
memset( fh2, 0, sizeof(FrameHead));
fh2[0] = 0x68;
fh2[5] = 0x68;

edited May 23 '17 at 11:43

Community

1
1

answered May 01 '13 at 15:01

Johannes S.

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thanks johannes S.but I need to format the "buf". The begining byte and end byte of the packet should be "0x68", thats why I insert the "contents1","contents2" into the middle of a struct.Now you taught me the reason causing this problem should be alignment, Is there any way to set alignment in Qt like "#Pragma" in VC. – wolfguolei May 01 '13 at 15:11
@wolfguolei, #pragma pack(1) – richselian May 01 '13 at 15:18
@wolfguolei, also use #pragma pack(push) and #pragma pack(pop) to save and restore the default alignment settings. – richselian May 01 '13 at 15:23

Casting pointer to reuse a buffer

1 Answers1