How to override _SC_NPROCESSORS_CONF when you know what you're doing.

numcpu_override.c

// SPDX-License-Identifier: Apache-2.0
//
// Override sysconf CPU count detection. Why? Because cgroups exist. A process
// running on a 256-core machine, but with 2 cores of CPU granted, should not start
// 256 worker threads. I'm looking at you, gRPC.
//
// If you wanted, you could parse the cgroup knobs automatically.
//
// compile with
// gcc -shared -fPIC -Wall -Wextra -Werror numcpu_override.c -o numcpu_override.so -ldl
//
// run with
// LD_PRELOAD=$PWD/numcpu_override.so NUMCPUS=2 ./sysconf


#define _GNU_SOURCE
#include <dlfcn.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

typedef long (*orig_sysconf_f_type)(int name);

long numcpus = 0;

long sysconf(int name) {
  orig_sysconf_f_type orig;
  orig = (orig_sysconf_f_type)dlsym(RTLD_NEXT, "sysconf");

  if (name != _SC_NPROCESSORS_CONF && name != _SC_NPROCESSORS_ONLN) {
    return orig(name);
  }

  if (numcpus > 0) {
    return numcpus;
  }

  char *env = getenv("NUMCPUS");
  if (env == NULL || strlen(env) == 0) {
    return orig(name);
  }

  numcpus = strtol(env, NULL, 10);
  if (errno != 0 || numcpus < 1) {
    return orig(name);
  }
  return numcpus;
}