memset memcmp memcpy memmove 自己实现
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2022-07-07 20:57:38
memset memcmp memcpy memmove 自己实现 memset c include include include include void my_memcpy(void dest, const void src, size_t cnt){ assert(NULL != dest ......
memset memcmp memcpy memmove 自己实现
memset
#include <stdio.h> #include <memory.h> #include <assert.h> void* my_memset(void* dest, int c, size_t cnt){ assert(NULL != dest); char *a = (char*)dest; while(cnt-- > 0){ *a++ = c; } return dest; } int main(){ int a[10]; for(int i = 0; i < sizeof(a) / sizeof(int); ++i){ printf("%d ", *(a+i)); } printf("\n"); my_memset(a,0,sizeof(a)); for(int i = 0; i < sizeof(a) / sizeof(int); ++i){ printf("%d ", *(a+i)); } printf("\n"); }
memcmp
#include <stdio.h> #include <memory.h> #include <assert.h> int my_memcmp(const void* s1, const void* s2, size_t cnt){ assert(NULL != s1 && NULL != s2); const char *t1 = s1; const char *t2 = s2; int res = 0; while(cnt-- > 0){ if(*t1 > *t2){ res =1; break; } else if(*t1 < *t2){ res = -1; break; } else{ t1++; t2++; } } return res; } int main(){ char* s1 = "abcdaaa"; char *s2 = "abcdaa"; int res = my_memcmp(s1,s2,18); if(0 == res) printf("s1 == s2\n"); else if(res > 0) printf("s1 > s2\n"); else printf("s1 < s2\n"); int i1[] = {255,2,3,4,5}; int i2[] = {511,22,23,5}; int res1 = my_memcmp(i1,i2,1); if(0 == res1) printf("i1 == i2\n"); else if(res1 > 0) printf("i1 > i2\n"); else printf("i1 < i2\n"); }
memcpy
如果copy元的首地址 + copy的字节数的结果 >= copy先的首地址的话,就会出现覆盖,得不到正确的结果。
下面的实现结果太笨了,倒过来复制是最简单的,请看后面的memmove的实现方式。
include <stdio.h> #include <memory.h> #include <assert.h> #include <malloc.h> void* my_memcpy(void* dest, const void* src, size_t cnt){ assert(NULL != dest && NULL != src); char *tmp = dest; const char *st = src; //判断元的首地址+要copy的字节数是否大于copy先的首地址 size_t bit = st + cnt - tmp; char *tt = NULL; char *tm = NULL; //为了free使用 char *ta; //如果copy的首地址大于元的首地址,并且元的首地址+要copy的字节数是否大于copy先的首地址,覆盖就会发生,所以要把将被覆盖的一段内存保存下来,先开辟空间(空间大小是bit),对tt的赋值,在后面的while里。 if(tmp > st && bit > 0){ tt = (char*)malloc(bit); } //由于tt被用于赋值(初始化),所以tt已经不是首地址了;当copy到被覆盖的字节的时候,要从首地址拿值,所以从tm中拿值,不从st中拿值;ta一直指向开辟空间的首地址,最后用于释放这个开辟的空间。 ta = tm = tt; while(cnt-- > 0){ //tt不为NULL,就是说明了,将要发生覆盖,所以把要被覆盖的字节存放到tt中,但要注意不发生覆盖的字节不需要存放进去,所以加了bit-- > 0的条件 if(NULL != tt && bit-- > 0){ *tt++ = *tmp; } //NULL != tt说明了,是覆盖patten,并且到了要被覆盖的字节,所以不从st中取值,从tm中取值。 if(st >= (char*)dest && NULL != tt){ *tmp++ = *tm++; st++; } //说明不是覆盖的patten,无脑复制就可以了。 else{ *tmp++ = *st++; } } free(ta); return dest; } int main(){ char s1[20] = {'a','b','c','d'}; char *s2 = "xyzdef"; //char *s3 = memcpy(s1+1,s1,3); char *s3 = my_memcpy(s1+1,s1,3); printf("s1 = [%s]\n", s1); printf("s3 = [%s]\n", s3); char s11[20] = {'a','b','c','d'}; char *s22 = "xyzdef"; char *s33 = memcpy(s11+1,s11,3); printf("s11 = [%s]\n", s11); printf("s33 = [%s]\n", s33); int i1[10] = {1,2}; int i2[10] = {11,22,33,3}; int i3[10] = {1}; int *pi3 = i3; pi3 = (int*)my_memcpy(i2+2,i2,sizeof(int) * 3); for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){ printf("i1[%d] = %d ",i, i2[i]); } printf("\n"); for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){ printf("i3[%d] = %d ",i, pi3[i]); } printf("\n"); }
memmove
#include <stdio.h> #include <memory.h> #include <assert.h> void* my_memmove(void* dest, const void* src, size_t cnt){ assert(NULL != dest && NULL != src); char* tmp = dest; const char* st = src; //判断出是覆盖的patten,所以从后往前覆盖 if(tmp > st && st + cnt > tmp){ while(cnt-- > 0){ *(tmp + cnt) = *(st + cnt); } } //判断出不是覆盖的patten,所以无脑从前往后覆盖 else { while(cnt-- > 0){ *tmp++ = *st++; } } return dest; } int main(){ char s1[20] = {'a','b','c','d'}; char *s2 = "xyzdef"; char *s3 = my_memmove(s1,s1+1,3); printf("s1 = [%s]\n", s1); printf("s3 = [%s]\n", s3); char s11[20] = {'a','b','c','d'}; char *s22 = "xyzdef"; char *s33 = memmove(s11,s11+1,3); printf("s11 = [%s]\n", s11); printf("s33 = [%s]\n", s33); int i1[10] = {1,2}; int i2[10] = {11,22,33,3}; int i3[10] = {1}; int *pi3 = i3; pi3 = (int*)my_memmove(i2+2,i2,sizeof(int) * 3); for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){ printf("i1[%d] = %d ",i, i2[i]); } printf("\n"); for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){ printf("i3[%d] = %d ",i, pi3[i]); } printf("\n"); }
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