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Code Issues Releases Wiki Activity

util: New function bitwise_zero().

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Signed-off-by: Ben Pfaff <blp@nicira.com>
This commit is contained in:
Ben Pfaff
2012-01-17 16:53:29 -08:00
parent ddc4f8e27f
commit 6cc7ea5ea0
3 changed files with 94 additions and 1 deletions
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48
lib/util.c
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@@ -803,6 +803,54 @@ bitwise_copy(const void *src_, unsigned int src_len, unsigned int src_ofs,
}
}
/* Zeros the 'n_bits' bits starting from bit 'dst_ofs' in 'dst'. 'dst' is
* 'dst_len' bytes long.
*
* If you consider all of 'dst' to be a single unsigned integer in network byte
* order, then bit N is the bit with value 2**N. That is, bit 0 is the bit
* with value 1 in dst[dst_len - 1], bit 1 is the bit with value 2, bit 2 is
* the bit with value 4, ..., bit 8 is the bit with value 1 in dst[dst_len -
* 2], and so on.
*
* Required invariant:
* dst_ofs + n_bits <= dst_len * 8
*/
void
bitwise_zero(void *dst_, unsigned int dst_len, unsigned dst_ofs,
unsigned int n_bits)
{
uint8_t *dst = dst_;
if (!n_bits) {
return;
}
dst += dst_len - (dst_ofs / 8 + 1);
dst_ofs %= 8;
if (dst_ofs) {
unsigned int chunk = MIN(n_bits, 8 - dst_ofs);
*dst &= ~(((1 << chunk) - 1) << dst_ofs);
n_bits -= chunk;
if (!n_bits) {
return;
}
dst--;
}
while (n_bits >= 8) {
*dst-- = 0;
n_bits -= 8;
}
if (n_bits) {
*dst &= ~((1 << n_bits) - 1);
}
}
/* Copies the 'n_bits' low-order bits of 'value' into the 'n_bits' bits
* starting at bit 'dst_ofs' in 'dst', which is 'dst_len' bytes long.
*