"setrlimit()" is not affecting the running process - c++

Question

I am trying the simulate the Error Scenario of a Process in Linux that Heap is not enough to allocate the memory in a C++ Linux Application.
But Eventhough I use the "setrlimit" to reduce the Heap Memory available to the Process, still the heap memory is getting allocated successfully.
struct rlimit the_limit = { 1, 1 };
if (-1 == setrlimit(RLIMIT_DATA, &the_limit)) {
perror("setrlimit failed");
}
try
{
char *n = new char[5600];
if (n==NULL)
{
cout <<"\nAllocation Failure\n";
}
}
catch (std::bad_alloc& ba)
{
std::cerr << "bad_alloc caught: " << ba.what() << '\n';
}

Answer 1

Most C++ standard libs including the one supplied with g++ start off with some heap memory preallocated.
5600 is a small request and as such, on my Linux system it gets satisfied from the preallocated memory as evidenced
from an strace:
Modified example:
#include <stdio.h>
#include <sys/resource.h>
int main()
{
struct rlimit the_limit = { 1, 1 };
if (-1 == setrlimit(RLIMIT_DATA, &the_limit)) { perror("setrlimit failed"); }
puts("ALLOC");
#if __cplusplus
try { char *n = new char[5600]; } catch (...) { perror("alloc failure"); }
#else
{ char *n = malloc(1); if(!n) perror("alloc failure"); }
#endif
}
End of example's strace:
...
write(1, "ALLOC\n", 6ALLOC
) = 6
exit_group(0) = ?
Either increasing the request size, e.g. in my case to at least 1<<16, or switching to plain C, causes the allocation request to be served from the OS, and then the limit does apply:
End of strace with an 1<<16 allocation request:
write(1, "ALLOC\n", 6ALLOC
) = 6
brk(0x561bcc5d4000) = 0x561bcc5b2000
mmap(NULL, 1048576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
dup(2) = 3
fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
fstat(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 14), ...}) = 0
write(3, "alloc failure: Cannot allocate m"..., 38alloc failure: Cannot allocate memory
) = 38
close(3) = 0
exit_group(0) = ?
Note that generic allocator implementations generally use sbrk and/or
mmap to get memory directly from the OS, and as you can glean from the setrlimit manpage, RLIMIT_DATA will only apply to a mmap-backed allocation iff you're on a Linux >= 4.7.