Unknown Byte in C write struct

Question

I'm sending a c struct with write to a server. The struct and write look like this.

typedef struct MyStruct {
    uint8_t         flag; 
    char            str[50];
    int32_t         number;
    time_t          time; 

} MyStruct ;

...

// Create mystruct

memset(&mystruct->flag,   '\1', sizeof(uint8_t));
memset(&mystruct->str, '\0', sizeof(char) * 50);
memset(&mystruct->number, '\2', sizeof(int32_t)); 
memset(&mystruct->time,   '\3', sizeof(time_t)); 

write(sockfd, mystruct, sizeof(MyStruct));

The server, in Java, receives the information in a nio ByteBuffer and then gets a byte[] with ByteBuffer.array. When I examine the byte[] its contents are:

[ 0]         = 1
[ 1] to [50] = 0
[51]         = 70
[52] to [55] = 2
[56] to [63] = 3

If you add the lengths up 1 + 50 + 4 + 8 you get 63, but the length with the odd 70 byte is 64.

Why is the 70 byte here? Does it have something to do with networking or c structs? Also how, if I can, get rid of it?

Please just don't do this. You're asking for a world of pain. If you're going to use a channel that sends streams of bytes, define precisely what the stream of bytes you're going to exchange is. Don't say "I'll send whatever the C struct happens to be and hope it happens to make sense to the other side". — David Schwartz, Jun 30 '16 at 18:25
Aside, do you realise that `memset(&mystruct->number, '\2', sizeof(int32_t));` sets the variable to `0x02020202`, or decimal `33686018`? — Weather Vane, Jun 30 '16 at 18:26

score 2 · Accepted Answer · edited May 23 '17 at 11:58

2

Why is the 70 byte here? Does it have something to do with networking or c structs? Also how, if I can, get rid of it?

It probably has to do with structure padding. You can try and serialize each member of the structure separately if that's the case.

Also here is a post which well explains all the pitfalls with serialization in C.

Serialization in C is more tricky than say in C# or I think also in Java, for example in C# you know that integer is always 4 bytes in size, which is not the case in C, where the size of integer can vary on platforms.

edited May 23 '17 at 11:58

Community

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answered Jun 30 '16 at 18:23

Giorgi Moniava

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So by serialize, in C, I should make a method that creates a byte array set the way I want to? – ezPaint Jun 30 '16 at 18:29
@ezPaint Yes check the link in my answer – Giorgi Moniava Jun 30 '16 at 18:30
@ezPaint There is also smth like pragma pack - but often it is advised against – Giorgi Moniava Jun 30 '16 at 18:30

score 2 · Answer 2 · answered Jun 30 '16 at 18:29

It's almost certainly due to structure padding; most likely, your compiler/processor requires 32-bit values to be 32-bit aligned, so since your uint8_t and your char[50] add up to 51 bytes, there's a one-byte pad between str and number.

As was mentioned in the comments - you really want to handle your serialization explicitly. There are a number of libraries that can handle defining Java and C/C++ types, along with methods to serialize/deserialize in each language.

Unknown Byte in C write struct

2 Answers2