A simple utility to determine the largest allocation possible for your operating system, compiler and C library

maxmalloc.c

/*
 * MaxMalloc: a simple test to determine the maximum memory allocation
 *
 * gcc -Os -Wall -Wextra -o maxmalloc maxmalloc.c
 */

#include <getopt.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

void usage(const char *executable) {
	printf("Usage: %s [-m] [-b block_size] [-s small_block_size]\n\t-m: use memset to confirm that allocated memory is actually usable (SLOW!)\n\t-b: set the block size increment used to determine the largest single allocation (0=disable)\n\t-s: set the block size used to determine the largest cumulative allocation (0=disable)\n", executable);
	exit(0);	
}

int main(int argc, char *argv[]) {
	int do_memset                  = 0;
	unsigned long block_size       = 65536L;
	unsigned long small_block_size = 1024L;
	void *p;
	char opt;
	
	while ((opt = getopt(argc, argv, "b:s:m")) != -1) {
		switch (opt) {
			case 'b':
				sscanf(optarg, "%lu", &block_size);
				break;

			case 's':
				sscanf(optarg, "%lu", &small_block_size);
				break;

			case 'm':
				do_memset = 1;
				break;

			default:
				usage(argv[0]);
		}
	}

	if (block_size > 0) {
		#ifdef USE_GOOD_SIZE
				block_size = malloc_good_size(block_size);
		#endif
	
		printf("Determining maximum single allocation: block size = %lu, memset=%d\n", block_size, do_memset);

		unsigned long blocks = 0;
		while ( (p = malloc( blocks * block_size )) != NULL) {
			if (do_memset) memset(p, 0, blocks * block_size);
			free(p); 
			blocks++;
		}

		printf("Maximum single allocation: %0.2fGb (%lu %lu blocks)\n", (float)(blocks * block_size) / 1073741824, blocks, block_size);
	}

	if (small_block_size > 0) {
		#ifdef USE_GOOD_SIZE
				small_block_size = malloc_good_size(small_block_size);
		#endif

		printf("Determining maximum allocation size: small block size = %lu, memset=%d\n", small_block_size, do_memset);

		unsigned long malloc_highwater = 0;
		while ((p = malloc(small_block_size)) != NULL ) {
			malloc_highwater += small_block_size;
			if (do_memset) memset(p, 0, malloc_highwater);
		}

		printf("Maximum total allocation: %0.2fGb (%lu %lu blocks)\n", (float)(malloc_highwater / 1073741824), malloc_highwater / small_block_size, small_block_size);
	}
	
	return 0;	 
}