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Defined in header <string.h> | ||
void* memmove(void* dest,constvoid* src,size_t count); | (1) | |
errno_t memmove_s(void* dest, rsize_t destsz,constvoid* src, rsize_t count); | (2) | (since C11) |
memmove_s is only guaranteed to be available if__STDC_LIB_EXT1__ is defined by the implementation and if the user defines__STDC_WANT_LIB_EXT1__ to the integer constant1 before including<string.h>.Contents |
| dest | - | pointer to the object to copy to |
| destsz | - | max number of bytes to modify in the destination (typically the size of the destination object) |
| src | - | pointer to the object to copy from |
| count | - | number of bytes to copy |
memmove may be used to set theeffective type of an object obtained by an allocation function.
Despite being specified "as if" a temporary buffer is used, actual implementations of this function do not incur the overhead or double copying or extra memory. A common approach (glibc and bsd libc) is to copy bytes forwards from the beginning of the buffer if the destination starts before the source, and backwards from the end otherwise, with a fall back to the more efficientmemcpy when there is no overlap at all.
Wherestrict aliasing prohibits examining the same memory as values of two different types,memmove may be used to convert the values.
#define __STDC_WANT_LIB_EXT1__ 1#include <inttypes.h>#include <stdint.h>#include <stdio.h>#include <stdlib.h>#include <string.h> int main(void){char str[]="1234567890";puts(str); memmove(str+4, str+3,3);// copy from [4,5,6] to [5,6,7]puts(str); // setting effective type of allocated memory to be intint* p=malloc(3*sizeof(int));// allocated memory has no effective typeint arr[3]={1,2,3}; memmove(p, arr,3*sizeof(int));// allocated memory now has an effective type // reinterpreting datadouble d=0.1;// int64_t n = *(int64_t*)(&d); // strict aliasing violationint64_t n; memmove(&n,&d,sizeof d);// OKprintf("%a is %"PRIx64" as an int64_t\n", d, n); #ifdef __STDC_LIB_EXT1__ set_constraint_handler_s(ignore_handler_s);char src[]="aaaaaaaaaa";char dst[]="xyxyxyxyxy";int r= memmove_s(dst,sizeof dst, src,5);printf("dst =\"%s\", r = %d\n", dst, r); r= memmove_s(dst,5, src,10);// count is greater than destszprintf("dst =\"");for(size_t ndx=0; ndx<sizeof dst;++ndx){char c= dst[ndx]; c?printf("%c", c):printf("\\0");}printf("\", r = %d\n", r);#endif}
Possible output:
123456789012344568900x1.999999999999ap-4 is 3fb999999999999a as an int64_tdst = "aaaaayxyxy", r = 0dst = "\0\0\0\0\0yxyxy", r = 22
(C11) | copies one buffer to another (function)[edit] |
(C95)(C11) | copies a certain amount of wide characters between two, possibly overlapping, arrays (function)[edit] |
C++ documentation formemmove | |
