Mno, to se da snadno overit, ze...
<![CDATA[
$ cat realloc.c
#include <stdlib.h>
int main(int argc, char *argv[])
{
void *x = malloc (10000000);
malloc (10000000);
x = realloc (x, 20000000);
return 0;
}
$ make realloc
cc realloc.c -o realloc
$ ltrace -S ./realloc
[...]
__libc_start_main(0x400514, 1, 0x7fff34c36568, 0x400560, 0x4005f0 <unfinished ...>
malloc(10000000 <unfinished ...>
SYS_mmap(0, 0x98a000, 3, 34, 0xffffffff) = 0x7f18029f6000
<... malloc resumed> ) = 0x7f18029f6010
malloc(10000000 <unfinished ...>
SYS_mmap(0, 0x98a000, 3, 34, 0xffffffff) = 0x7f180206c000
<... malloc resumed> ) = 0x7f180206c010
realloc(0x7f18029f6010, 20000000 <unfinished ...>
SYS_mremap(0x7f18029f6000, 0x98a000, 0x1313000, 1, 0x1313000) = 0x7f1800d59000
<... realloc resumed> ) = 0x7f1800d59010
SYS_exit_group(0 <no return ...>
+++ exited (status 0) +++
]]>
IMHO by kernel mremap mohl tim premapovanim i resit... Pochopitelne porad plati, ze to musi byt velky blok alokovany pres mapovani.