How should I by using these bytes(spaces in between each bytes?) and What type of variable would nf need to be in this context?
if(nf == 0xff 0xd8 0xff 0xe0 || nf == 0xff 0xd8 0xff 0xe1)
-
What exactly you want to do?– sinisterAug 9, 2014 at 1:25
-
@sinister I need to check if the following series of bytes (in that order) are in the certain place in a file– user1475Aug 9, 2014 at 2:05
Add a comment
|
2 Answers
That is not technically correct syntax.
There are least two ways that this could be interpreted into working code, depending on the variable type of nf, which can either be a byte or some other larger value type.
A common scenario is if nf refers to bytes, for example if nf is a char array or pointer, then the if statement needs a comparison for each byte, e.g:
if ((nf[0] == 0xff && nf[1] == 0xd8 && nf[2] == 0xff && nf[3] == 0xe0)
|| (nf[0] == 0xff && nf[1] == 0xd8 && nf[2] == 0xff && nf[3] == 0xe1))
Another possibility is if nf refers to a larger data type. In this case you would be accessing 4 bytes, or 32 bits. You might define nf as a uint32_t, then compare the entire value in one go.
if (nf == 0xffd8ffe0 || nf == 0xffd8ffe1)
Here the hexadecimal values are all concatenated into one long number, without the need for 0x for each byte.
Note that while the second solution is shorter to write, and may work in many cases, it is not semantically equivalent to the first solution and care needs to be taken when porting the code to different platforms.
-
are you sure you mean: if (nf == 0xffd8ffe0 || nf == 0xffd8ffe1) and not if(nf == 0xff0xd80xff0xe0 || nf == 0xff0xd80xff0xe1) (are you saying that that initial 0x is all you need?)– user1475Aug 10, 2014 at 4:46
-
1Yes, the first kind is correct. When you write hex, you're writing a number in a different number base. If you were to just write
ffd8ffe0then the compiler would think you were writing the name of a variable. Prefixing the number with0xtells the compiler that the following digits are hexadecimal. Once the compiler knows that it interprets all the digits as hex until it reaches a space, bracket, newline, semi-colon, or other delimiter. It is not necessary, nor valid, to repeat0xwithin the hex number. Aug 10, 2014 at 8:56
The values 0xff, 0xd8, 0xe0 and 0xe1 are int values that are written in hexadecimal notation.
Since this is for pset5 and, as the pset specification page states, a block is 512 bytes long, we can conclude that none of these values will exceed 0xff(hex) or 255(dec) since each value will be of size 1 byte (8 bits).
As a result, we can use uint8_t (aka BYTE) which is an unsigned int of size 1 byte, declared in stdint.h and defined as BYTE in bmp.h as a data type for the variable nf in your example.
If you don't want to include bmp.h, feel free to define that type by putting this statement atop your program in order to use the type name BYTE instead of uint8_t interchangeably
typedef uint8_t BYTE;
