Header parsing problem

Hi.

I need to parse a header read from a socket, which has the following structure.

Bit 1. indicating if this is the final fragment of a message
Bit 2,3,4. is 0
Bit 5,6,7,8. is an opcode, indicating the type of the message (text/binary/control etc)
Bit 9. indicating that the message is masked or not
Bit 10-16. If its value is < 126, denotes the actual length of the message, if = 126, than the following 16 bits will indicate the length of the message, if = 127, the following 64 bits will be the actual length.

The header is actually the websocket protocol header, and it contains some other informations, but they are irrelevant for my question.

I write the parser in C, and I don`t know how to access a specific value in the header. I created a structure and mapped the values from the header to it, and it worked, but because the length of the header itself can change in some conditions, this is no longer an option.

I saw an example where the author extracted the length of the message (starting with the second byte in the string) with bitwise operations, like this:

int message length = tmp_buff[1] & 127

It gives the value correctly, but can somebody explain to me, how this actually works and maybe give me an example, how can I extract, let`s say the opcode, or the value from the two bytes, following the length?

Thank you.

bitwise operator tutorial - Google Search
You need to make sure you're up to speed on the whole & | ^ << >> deal when you start messing around with bits.

It appears that the bit numbering proceeds from left to right in your example.

Code:

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1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16
f  0  0  0  p  p  p  p  m  l  l  l  l  l  l  l

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I also assume the 16 bits are accessed as two byes from char tmp_buf[0] and tmp_buf[1].

To extract 'masked' bit use (tmp_buf[1] >> 7) and check if that's 0 or non zero as required
To extract 'opcode' field use (tmp_buf[0] & 15).
To extract "final' bit use (tmp_buf[0] & 128) and check if that's 0 or non zero as required

Quote:

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int message length = tmp_buff[1] & 127

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I'll explain it with a 4 bit unsigned variable
When you have the number, say 7, it is represented as 0111 in binaray -> I'll write binary numbers like this 0bxxxx to avoid confusion.

Code:

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7 & 4 = 4

is the same as
0b0111 & 0b0100
= 0b0100
 
Or more generally
0bx1xx & 0b0100
= 0b0100

0bx0xx & 0b0100
= 0b0000

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Notice that if bit '2' was set (with lsb = bit 0), it was going to be 4 regardless of what the other bits are. If bit '2' was not set, it would be 0. So by using the bitwise 'and', the code can isolate different parts of a value.

With the example you gave, 127 is 0b01111111 -> So you can see that the code is isolating the first 7 bits of a variable tmp_buff[1]

Quote:

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Originally Posted by raczzoli

Hi.

I need to parse a header read from a socket, which has the following structure.

Bit 1. indicating if this is the final fragment of a message
Bit 2,3,4. is 0
Bit 5,6,7,8. is an opcode, indicating the type of the message (text/binary/control etc)
Bit 9. indicating that the message is masked or not
Bit 10-16. If its value is < 126, denotes the actual length of the message, if = 126, than the following 16 bits will indicate the length of the message, if = 127, the following 64 bits will be the actual length.

The header is actually the websocket protocol header, and it contains some other informations, but they are irrelevant for my question.

I write the parser in C, and I don`t know how to access a specific value in the header. I created a structure and mapped the values from the header to it, and it worked, but because the length of the header itself can change in some conditions, this is no longer an option.

I saw an example where the author extracted the length of the message (starting with the second byte in the string) with bitwise operations, like this:

int message length = tmp_buff[1] & 127

It gives the value correctly, but can somebody explain to me, how this actually works and maybe give me an example, how can I extract, let`s say the opcode, or the value from the two bytes, following the length?

Thank you.

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I recommend making your life a little easier and use "bit fields" in a structure.. Below is a link explaining bit fields in more depth.

Bit Fields in C

Quote:

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Originally Posted by twiki

I recommend making your life a little easier and use "bit fields" in a structure.. Below is a link explaining bit fields in more depth.

Bit Fields in C

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Careful. The order of bit fields in a struct is implementation-defined.