ANSI C的位操作库

Question

Does anyone knows a good bit manipulation library for ANSI C? What I basically need, is ability, like in Jovial to set specific bits in a variable, something like

// I assume LSB has index of 0
int a = 0x123;
setBits(&a,2,5, 0xFF);
printf("0x%x"); // should be 0x13F

int a = 0x123;
printf("0x%x",getBits(&a,2,5)); // should be 0x4

char a[] = {0xCC, 0xBB};
char b[] = {0x11, 0x12};
copyBits(a,/*to=*/4,b,/*from=*/,4,/*lengthToCopy=*/8);
// Now a == {0x1C, 0xB2}

There's a similar library called bitfile , but it doesn't seem to support direct memory manipulation. It only supports feeding bits to file streams.

It's not hard to write, but if there's something tested - I won't reinvent the wheel.

Maybe this library exists as a part of bigger library ( bzip2 , gzip are the usual suspects)?

Answer 1

I think is considered "too simple" for a library; most functions would only be a statement or two, which would make the overhead of calling a library function a bit more than typical C programmers tolerate. :)

That said, the always-excellent glib has two of the more complicated bit-oriented functions: g_bit_nth_lsf() and g_bit_nth_msf() . These are used to find the index of the first bit set, searching from the lowest or the highest bit, respectively.

Answer 2

You will come a long way with the following macros:

#define SETBITS(mem, bits)      (mem) |= (bits)
#define CLEARBITS(mem, bits)    (mem) &= ~(bits)
#define BIN(b7,b6,b5,b4, b3,b2,b1,b0)                      \
(unsigned char)(                                           \
    ((b7)<<7) + ((b6)<<6) + ((b5)<<5) + ((b4)<<4) +        \
    ((b3)<<3) + ((b2)<<2) + ((b1)<<1) + ((b0)<<0)          \
)

Then you can write

int a = 0x123;
SETBITS(a, BIN(0,0,0,1, 1,1,1,0));
printf("0x%x", a); // should be 0x13F

Answer 3

也许算法从“FXT”一书（在页面的底部链接）将是有益的。

Answer 4

This seems to be the problem I was tackling in my question

Algorithm for copying N bits at arbitrary position from one int to another

There are several different alternatives provided, with the fastest being the assembly solution by fnieto.