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# memcpy, memcpy_s

From cppreference.com
< c‎ | string‎ | byte

Defined in header `<string.h>` |  |
| (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 a 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:

  - `dest` or `src` is a null pointer
  - `destsz` or `count` is greater than RSIZE_MAX
  - `count` is greater than `destsz` (buffer overflow would occur)
  - the source and the destination objects overlap

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 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

Run this code

    #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

- C11 standard (ISO/IEC 9899:2011):

  - 7.24.2.1 The memcpy function (p: 362)

  - K.3.7.1.1 The memcpy_s function (p: 614)

- C99 standard (ISO/IEC 9899:1999):

  - 7.21.2.1 The memcpy function (p: 325)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.11.2.1 The memcpy function

### [edit] See also

memmove memmove_s (C11) | moves one buffer to another (function) [edit]
wmemcpy wmemcpy_s (C95)(C11) | copies a certain amount of wide characters between two non-overlapping arrays (function) [edit]
C++ documentation for memcpy

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