`memcpy_s`, `wmemcpy_s`

Article
2022-12-02

Copies bytes between buffers. These functions are versions of memcpy, wmemcpy with security enhancements as described in Security features in the CRT.

Syntax

errno_t memcpy_s(
   void *dest,
   size_t destSize,
   const void *src,
   size_t count
);
errno_t wmemcpy_s(
   wchar_t *dest,
   size_t destSize,
   const wchar_t *src,
   size_t count
);

Parameters

dest
New buffer.

destSize
Size of the destination buffer, in bytes for memcpy_s and wide characters (wchar_t) for wmemcpy_s.

src
Buffer to copy from.

count
Number of characters to copy.

Return value

Zero if successful; an error code on failure.

Error conditions

`dest`	`destSize`	`src`	`count`	Return value	Contents of `dest`
any	any	any	0	0	Not modified
`NULL`	any	any	non-zero	`EINVAL`	Not modified
any	any	`NULL`	non-zero	`EINVAL`	`dest` is zeroed out
any	< `count`	any	non-zero	`ERANGE`	`dest` is zeroed out

Remarks

memcpy_s copies count bytes from src to dest; wmemcpy_s copies count wide characters. If the source and destination regions overlap, the behavior of memcpy_s is undefined. Use memmove_s to handle overlapping regions.

These functions validate their parameters. If count is non-zero and dest or src is a null pointer, or destSize is smaller than count, these functions invoke the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, these functions return EINVAL or ERANGE, and set errno to the return value.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine	Required header
`memcpy_s`	`<memory.h>` or `<string.h>`
`wmemcpy_s`	`<wchar.h>`

For more compatibility information, see Compatibility.

Example

// crt_memcpy_s.c
// Copy memory in a more secure way.

#include <memory.h>
#include <stdio.h>

int main()
{
   int a1[10], a2[100], i;
   errno_t err;

   // Populate a2 with squares of integers
   for (i = 0; i < 100; i++)
   {
      a2[i] = i*i;
   }

   // Tell memcpy_s to copy 10 ints (40 bytes), giving
   // the size of the a1 array (also 40 bytes).
   err = memcpy_s(a1, sizeof(a1), a2, 10 * sizeof (int) );
   if (err)
   {
      printf("Error executing memcpy_s.\n");
   }
   else
   {
     for (i = 0; i < 10; i++)
       printf("%d ", a1[i]);
   }
   printf("\n");
}

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