可以,但是你必须告诉swap
代码元素有多大:
void generic_swap(void *v1, void *v2, size_t size)
{
char temp[size];
memmove(temp, v1, size);
memmove(v1, v2, size);
memmove(v2, temp, size);
}
它使用 VLA(可变长度数组 - C99 的一项功能和 C11 的可选功能)作为临时空间。本地数组的大小,temp
,在运行时由函数参数控制size
。如果您不相信用户不会请求交换多兆字节的数据,则可以改用动态内存分配,或者仅在大小大于(例如 1 KB)时才使用动态内存分配。
Either:
void generic_swap(void *v1, void *v2, size_t size)
{
size_t chunk = (size > 1024) ? 1024 : size;
size_t offset = 0;
char *s1 = v1;
char *s2 = v2;
char temp[chunk];
while (size > 0)
{
size_t length = (size > chunk) ? chunk : size;
memmove(temp, s1 + offset, length);
memmove(s1 + offset, s2 + offset, length);
memmove(s2 + offset, temp, length);
size -= length;
offset += length;
}
}
Or:
void generic_swap(void *v1, void *v2, size_t size)
{
void *v3 = malloc(size);
if (v3 != 0)
{
memmove(v3, v1, size);
memmove(v1, v2, size);
memmove(v2, v3, size);
free(v3);
}
}
循环版本避免了动态内存分配的开销,并且不会比在三个操作中全部复制慢多少。有多种方法可用于调整循环代码 - 另请参阅comments by rici如果您发现交换代码是瓶颈,则可以了解其他可以优化的方法。
您可以自由选择小于 1024 字节的大小; 64 或 128 也可能是可行的,并且函数中不一定需要 VLA。
交换两个整数:
int i = 37;
int j = 99;
swap_generic(&i, &j, sizeof(i));
交换两个数组char
:
char data[80] = "A tabloid writer's nightmare on steroids";
char info[80] = "Obsequiousness will get you nowhere fast";
swap_generic(data, info, sizeof(data));
等等。请注意,数组的大小必须相同 - 或者更准确地说,您指定的大小必须是较小数组的大小才能安全。
您可以使用memcpy()
代替memmove()
如果你乐于危险地生活——尽管在这种情况下危险是有限的。 (如果你将一个对象与其自身交换,你会调用未定义的行为。否则,它是安全的。)memmove()
总是有效;使用memcpy()
通常有效。比起“大部分”,我更喜欢“总是”。
三种算法的测试工具
编译,例如:
gcc -O3 -g -std=c11 -Wall -Wextra -Werror -DUSE_GENSWAP_3 swap89.c -o swap89
当运行时Valgrind,代码获得了干净的健康证明。
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#if !defined(USE_GENSWAP_1) && !defined(USE_GENSWAP_2) && !defined(USE_GENSWAP_3)
#define USE_GENSWAP_1
#endif
extern void generic_swap(void *v1, void *v2, size_t size);
#ifdef USE_GENSWAP_1
void generic_swap(void *v1, void *v2, size_t size)
{
char temp[size];
memmove(temp, v1, size);
memmove(v1, v2, size);
memmove(v2, temp, size);
}
#endif
#ifdef USE_GENSWAP_2
void generic_swap(void *v1, void *v2, size_t size)
{
size_t chunk = (size > 1024) ? 1024 : size;
size_t offset = 0;
char *s1 = v1;
char *s2 = v2;
char temp[chunk];
while (size > 0)
{
size_t length = (size > chunk) ? chunk : size;
memmove(temp, s1 + offset, length);
memmove(s1 + offset, s2 + offset, length);
memmove(s2 + offset, temp, length);
size -= length;
offset += length;
}
}
#endif
#ifdef USE_GENSWAP_3
void generic_swap(void *v1, void *v2, size_t size)
{
void *v3 = malloc(size);
if (v3 != 0)
{
memmove(v3, v1, size);
memmove(v1, v2, size);
memmove(v2, v3, size);
free(v3);
}
}
#endif
static size_t min_len(size_t x, size_t y) { return (x < y) ? x : y; }
static void dump_long_buffer(const char *tag, size_t length, char buffer[length])
{
int maxpadlen = strlen(tag) + sizeof(" = ") - 1;
printf("%s = ", tag);
size_t offset = 0;
int padlen = 0;
while (length > 0)
{
int linelen = min_len(length, 80 - maxpadlen - sizeof("[]\n"));
printf("%*s[%.*s]\n", padlen, "", linelen, buffer + offset);
offset += linelen;
length -= linelen;
padlen = maxpadlen;
}
}
int main(void)
{
int i = 37;
int j = 99;
printf("i = %d; j = %d\n", i, j);
generic_swap(&i, &j, sizeof(i));
printf("i = %d; j = %d\n", i, j);
char data[80] = "A tabloid writer's nightmare on steroids";
char info[80] = "Obsequiousness will get you nowhere fast";
printf("data = [%s]\ninfo = [%s]\n", data, info);
generic_swap(data, info, sizeof(data));
printf("data = [%s]\ninfo = [%s]\n", data, info);
char maxibuff1[2560];
char maxibuff2[2560];
for (size_t k = 0; k < sizeof(maxibuff1); k++)
{
maxibuff1[k] = k % 64 + '!';
maxibuff2[k] = 'z' - k % 64;
}
/* The aligned output is mostly the result of serendipity */
dump_long_buffer("maxibuff1", sizeof(maxibuff1), maxibuff1);
dump_long_buffer("maxibuff2", sizeof(maxibuff2), maxibuff2);
generic_swap(maxibuff1, maxibuff2, sizeof(maxibuff1));
dump_long_buffer("maxibuff1", sizeof(maxibuff1), maxibuff1);
dump_long_buffer("maxibuff2", sizeof(maxibuff2), maxibuff2);
return 0;
}
示例输出(每种算法的结果相同):
i = 37; j = 99
i = 99; j = 37
data = [A tabloid writer's nightmare on steroids]
info = [Obsequiousness will get you nowhere fast]
data = [Obsequiousness will get you nowhere fast]
info = [A tabloid writer's nightmare on steroids]
maxibuff1 = [!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]
[!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]
…
[!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]
maxibuff2 = [zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
[zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
…
[zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
maxibuff1 = [zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
[zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
…
[zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;]
maxibuff2 = [!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]
[!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]
…
[!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`]