I would like to convert an int to a byte in C.
In Java, I'd write:
int num = 167;
byte b = num.toByte(); // -89
In C:
int num = 167;
???
4 Answers
You can simply cast to a byte:
unsigned char b=(unsigned char)num;
Note that if num is more than 255 or less than 0 C won't crash and simply give the wrong result.
1 Comment
Lundin
After edits, the result is well-defined. "Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type." until the value is in the range of the new type.
byte is a java signed integer type with a range of -128 to 127.
The corresponding type in C is int8_t defined in <stdint.h> for architectures with 8-bit bytes. It is an alias for signed char.
You can write:
#include <stdint.h>
void f() {
int num = 167;
int8_t b = num; // or signed char b = num;
...
If your compiler emits a warning about the implicit conversion to a smaller type, you can add an explicit cast:
int8_t b = (int8_t)num; // or signed char b = (signed char)num;
Note however that it is much more common to think of 8-bit bytes as unsigned quantities in the range 0 to 255, for which one would use type uint8_t or unsigned char. The reason java byte is a signed type might be that there is no unsigned type in this language, but it is quite confusing for non-native readers.
byte can also be defined as a typedef:
typedef unsigned char byte; // 0-255;
or
typedef signed char byte; // -128-127;
Do not use type char because it is implementation defined whether this type is signed or unsigned by default. Reserve type char for the characters in C strings, although many functions actually consider these to be unsigned: strcmp(), functions from <ctype.h>...
2 Comments
Lundin
"Note however that C programmers usually prefer to think of 8-bit bytes as unsigned quantities" Not just C programmers. All computer scientists and computer engineers too. Apparently Java
byte was designed by some special snowflakes.
chqrlie
@Lundin: java has no unsigned types, I should probably rephrase this part.
There is no such type as Byte in native C. Although if you don't want to import new libs, you can create one like this :
typedef unsigned char Byte
And then create any variable you'd like with it :
int bar = 15;
Byte foo = (Byte)bar
Comments
In computer science, the term byte is well-defined as an 8 bit chunk of raw data. Apparently Java uses a different definition than computer science...
-89is not the value167"converted to a byte".167already fits in a byte, so no conversion is necessary.-89is the value167converted to signed 2's complement with 8 bits representation.The most correct type to use for signed 2's complement 8 bit integers in C is
int8_tfromstdint.h.Converting from
inttoint8_tis done implicitly in C upon assignment. There is no need for a cast.int num = 167; int8_t b = num;
int8_t b = (int8_t) num;?