An empty program that does nothing in C++ needs a heap of 204KB but not in C

Consider this empty program:

int main()
{ return 0; }

If I compile it with C++ with g++ main.cpp && strace ./a.out, and analyze the output with strace, I observed that the last lines of the output are (you can add -O3 that the effect is the same):

mprotect(0x7f71af154000, 45056, PROT_READ) = 0
mprotect(0x7f71af38b000, 4096, PROT_READ) = 0
brk(NULL)                               = 0xed2000
brk(0xf05000)                           = 0xf05000
exit_group(0)                           = ?
+++ exited with 0 +++

However, if I do instead: gcc main.cpp && strace ./a.out

mprotect(0x7f4114318000, 16384, PROT_READ) = 0
mprotect(0x7f4114547000, 4096, PROT_READ) = 0
exit_group(0)                           = ?
+++ exited with 0 +++

As you see, in C++ there’s an extra brk that extends the heap by exactly 204KB (0xf05000 – 0xed2000 = 204KB after translating both to decimal). That can be easily verified by replacing that program by (coliru link):

#include <iostream>
#include <unistd.h>

int main()
{
    char buf[1024];
    sprintf(buf, "pmap -XX %u", getpid());
    std::system(buf);
    
    return 0;
}

and you can easily observe that compiling with g++ the [heap] has a size of 204KB, and with gcc the [heap] line even disappeared from the pmap output.

NOTE: By the way, and to my surprise, gcc had no problem at all with include <iostream> and with the presence of std in std::system.

What are these 204KB used for? Not worried or anything about these little 204KB but it caught my attention and I’m curious now.

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