memcpy, memcpy_s
From cppreference.com
| 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:
-
destorsrcis a null pointer -
destszorcountis greater than RSIZE_MAX -
countis greater thandestsz(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_sis 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 includingstring.h.
Contents |
[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
dest2) 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
| (C11) |
moves one buffer to another (function) |
| (C95)(C11) |
copies a certain amount of wide characters between two non-overlapping arrays (function) |
| C++ documentation for memcpy
| |