C - Why is my buffer not printing properly?

问题

I have two objects, Header, and DF. lets say

header = CCCCCC7E

and

DF = 01020304,

shouldnt the value of the buffer be CCCCCC7E01020304?

for some reason when i printed it i got:

7EFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCCFFFFFFCC00FFFF FFCC00000000000004030201FFFFFF8967341200000000

this is how i printed it:

for (int i = 0; i < sizeof(buffer); i++)
{ printf("%02X", buffer[i]); }

this is the code:

    struct Header header;
    struct Data_Format DF;
    unsigned char buffer[TOTAL_SIZE];

    header.Start = 0x7E;
    header.Options = 0x00;
    header.PacketLength = 0x00;
    header.VCP = 0x00;
    header.Reserved = 0x00;
    header.Return = 0x00;

    DF.Address = 0x01020304; //real value: NULL
    DF.Result = 0x1234; //real value: NULL
    DF.Size = 0x6789; //real value: NULL

    memcpy(buffer,&header, sizeof(Header));
    memcpy(buffer+sizeof(Header), &DF, sizeof(Data_Format));

回答1:

From the partial code given it doesn't give much idea but Some point need to do.

Do a memset always before coping to the buffer.

memset(buffer, 0, sizeof(buffer))

This will prevent you to get the junk.

回答2:

The objects are not neccessarily packed together. Acctually, most compilers allign objects at 8 bytes border so that the data will be accessed at higher speed. The bytes between the objects will just be leaved uninitialized (sometimes the debug-version of runtime environment will fill the gap with invalid data).

回答3:

This is not a proper way [because of compiler optimizations and alignment] to achieve what you want.

However, just for a logical suggestion, in your code, change

for (int i = 0; i < sizeof(buffer); i++)

for (int i = 0; i < (sizeof(Header) + sizeof(Data_Format)); i++).

This will limit the loop till valid entries. Remember, even after this change you're not guaranteed [actually,not supposed to] get the proper result.

EDIT

you can achieve your target using snprintf(). A common usage looks like

snprintf(buf, sizeof(buf), "%x%x%x", header.start, header.options, DF.size);  //incomplete list
printf("%s", buf);

来源：https://stackoverflow.com/questions/25760847/c-why-is-my-buffer-not-printing-properly

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