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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/../c/string/byte/memcpy.html below:

memcpy, memcpy_s - cppreference.com

(1) void* memcpy( void *dest, const void *src, size_t count ); (until C99) void* memcpy( void *restrict dest, const void *restrict src, size_t count ); (since C99)

errno_t memcpy_s( void *restrict dest, rsize_t destsz,
                  const void *restrict src, rsize_t count );

(2) (since C11)

1) Copies count characters from the object pointed to by src to the object pointed to by dest. Both objects are interpreted as arrays of unsigned char.

The behavior is undefined if access occurs beyond the end of the

dest

array. If the objects overlap

(which is a violation of the restrict contract)(since C99)

, the behavior is undefined. The behavior is undefined if either

dest

or

src

is an invalid or null pointer.

2)

Same as

(1)

, except that the following errors are detected at runtime and cause the entire destination range

[dest, dest+destsz)

to be zeroed out (if both

dest

and

destsz

are valid), as well as call the currently installed

constraint handler

function:

The behavior is undefined if the size of the character array pointed to by

dest

<

count

<=

destsz

; in other words, an erroneous value of

destsz

does not expose the impending buffer overflow.

As with all bounds-checked functions, memcpy_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 constant 1 before including <string.h>.
[edit] Parameters 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 [edit] Return value

1) Returns a copy of dest

2) Returns zero on success and non-zero value on error. Also on error, if dest is not a null pointer and destsz is valid, writes destsz zero bytes in to the destination array.

[edit] Notes

memcpy may be used to set the effective type of an object obtained by an allocation function.

memcpy is the fastest library routine for memory-to-memory copy. It is usually more efficient than strcpy, which must scan the data it copies or memmove, which must take precautions to handle overlapping inputs.

Several C compilers transform suitable memory-copying loops to memcpy calls.

Where strict aliasing prohibits examining the same memory as values of two different types, memcpy may be used to convert the values.

[edit] Example 
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#include <string.h>
#include <stdlib.h>
 
int main(void)
{
    // simple usage
    char source[] = "once upon a midnight dreary...", dest[4];
    memcpy(dest, source, sizeof dest);
    for(size_t n = 0; n < sizeof dest; ++n)
        putchar(dest[n]);
 
    // setting effective type of allocated memory to be int
    int *p = malloc(3*sizeof(int));   // allocated memory has no effective type
    int arr[3] = {1,2,3};
    memcpy(p,arr,3*sizeof(int));      // allocated memory now has an effective type
 
    // reinterpreting data
    double d = 0.1;
//    int64_t n = *(int64_t*)(&d); // strict aliasing violation
    int64_t n;
    memcpy(&n, &d, sizeof d); // OK
    printf("\n%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 = memcpy_s(dst,sizeof dst,src,5);
    printf("dst = \"%s\", r = %d\n", dst,r);
    r = memcpy_s(dst,5,src,10);            //  count is greater than destsz  
    printf("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:

once
0x1.999999999999ap-4 is 3fb999999999999a as an int64_t
dst = "aaaaayxyxy", r = 0
dst = "\0\0\0\0\0yxyxy", r = 22
[edit] References
[edit] See also

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