Is it safe to use memcpy to shift elements in an array?

Question

Let's say I have an integer array that contains junk and data. I wish to shift this array such that the data is at the start of the array.

Current:
[?, ?, ?, ?, 1, 1, 1, 1, 1]
 ----------  -------------
 JUNK        DATA

Desired:
[1, 1, 1, 1, 1, ?, ?, ?, ?]
 -------------  ----------
 DATA           JUNK

Is it safe to use memcpy to perform this shift?

memcpy(my_array, my_array + 4, 5)

I'm curious about the copy order of memcpy possibly corrupting the data shift.

Note: Before anyone asks, I'm working with legacy code. I think the true solution to my problem requires a circular buffer such that I don't have to shift data within an array.

See https://stackoverflow.com/questions/4415910/memcpy-vs-memmove and https://riptutorial.com/c/example/2150/copying-overlapping-memory — jarmod, Mar 07 '21 at 16:57

score 3 · Accepted Answer · answered Mar 07 '21 at 16:58

3

You don't have to read far in the description of memcpy to find that you can't overlap source and destination.

Here's the standard:

7.21.2.1 The memcpy function

Description

2 The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined

answered Mar 07 '21 at 16:58

Paul Hankin

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2

You can use `memmove` when the source and destination overlap. – icebp Mar 07 '21 at 17:04

Is it safe to use memcpy to shift elements in an array?

1 Answers1