source: GPL/trunk/lib32/strscpy.c@ 752

/*
 *  linux/lib/string.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

#include <string.h>
#include <types.h>
#include <asm/errno.h>

#define REPEAT_BYTE(x)  ((~0ul / 0xff) * (x))

/*
 * This is largely generic for little-endian machines, but the
 * optimal byte mask counting is probably going to be something
 * that is architecture-specific. If you have a reliably fast
 * bit count instruction, that might be better than the multiply
 * and shift, for example.
 */
struct word_at_a_time {
        const unsigned long one_bits, high_bits;
};

/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
static inline long count_masked_bytes(long mask)
{
        /* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
        long a = (0x0ff0001+mask) >> 23;
        /* Fix the 1 for 00 case */
        return a & mask;
}

#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) }

/* Return nonzero if it has a zero */
static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c)
{
        unsigned long mask = ((a - c->one_bits) & ~a) & c->high_bits;
        *bits = mask;
        return mask;
}

static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits, const struct word_at_a_time *c)
{
        return bits;
}

static inline unsigned long create_zero_mask(unsigned long bits)
{
        bits = (bits - 1) & ~bits;
        return bits >> 7;
}

/* The mask we created is directly usable as a bytemask */
#define zero_bytemask(mask) (mask)

static inline unsigned long find_zero(unsigned long mask)
{
        return count_masked_bytes(mask);
}

/**
 * strscpy - Copy a C-string into a sized buffer
 * @dest: Where to copy the string to
 * @src: Where to copy the string from
 * @count: Size of destination buffer
 *
 * Copy the string, or as much of it as fits, into the dest buffer.
 * The routine returns the number of characters copied (not including
 * the trailing NUL) or -E2BIG if the destination buffer wasn't big enough.
 * The behavior is undefined if the string buffers overlap.
 * The destination buffer is always NUL terminated, unless it's zero-sized.
 *
 * Preferred to strlcpy() since the API doesn't require reading memory
 * from the src string beyond the specified "count" bytes, and since
 * the return value is easier to error-check than strlcpy()'s.
 * In addition, the implementation is robust to the string changing out
 * from underneath it, unlike the current strlcpy() implementation.
 *
 * Preferred to strncpy() since it always returns a valid string, and
 * doesn't unnecessarily force the tail of the destination buffer to be
 * zeroed.  If the zeroing is desired, it's likely cleaner to use strscpy()
 * with an overflow test, then just memset() the tail of the dest buffer.
 */
ssize_t strscpy(char *dest, const char *src, size_t count)
{
        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
        size_t max = count;
        long res = 0;

        if (count == 0)
                return -E2BIG;

        /* If src or dest is unaligned, don't do word-at-a-time. */
        if (((long) dest | (long) src) & (sizeof(long) - 1))
                max = 0;

        while (max >= sizeof(unsigned long)) {
                unsigned long c, data;

                c = *(unsigned long *)(src+res);
                if (has_zero(c, &data, &constants)) {
                        data = prep_zero_mask(c, data, &constants);
                        data = create_zero_mask(data);
                        *(unsigned long *)(dest+res) = c & zero_bytemask(data);
                        return res + find_zero(data);
                }
                *(unsigned long *)(dest+res) = c;
                res += sizeof(unsigned long);
                count -= sizeof(unsigned long);
                max -= sizeof(unsigned long);
        }

        while (count) {
                char c;

                c = src[res];
                dest[res] = c;
                if (!c)
                        return res;
                res++;
                count--;
        }

        /* Hit buffer length without finding a NUL; force NUL-termination. */
        if (res)
                dest[res-1] = '\0';

        return -E2BIG;
}