How do realloc and memcpy work?

c memory-management realloc
Ahmed Mounir picture Ahmed Mounir · Dec 12, 2008 · Viewed 20.6k times · Source

I have two questions.

  1. Do realloc() and memcpy() copy the entries in an array to another in a way faster than just iterating on each element O(N) ? If the answer is yes then what do you think is its complexity ?

  2. If the size allocated is smaller than the original size, does realloc() copy the entries to somewhere else or just leave them as they are decreasing the size of the array ?

Answer

Sean picture Sean · Dec 12, 2008

1 - No. They copy a block at a time. See http://www.embedded.com/design/configurable-systems/4024961/Optimizing-Memcpy-improves-speed for a pretty good analysis.

2 - This is implementation dependent. See http://www.gnu.org/software/libtool/manual/libc/Changing-Block-Size.html for glibc details. "In several allocation implementations, making a block smaller sometimes necessitates copying it"

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