hugepage-shm.c

 1// SPDX-License-Identifier: GPL-2.0
 2/*
 3 * hugepage-shm:
 4 *
 5 * Example of using huge page memory in a user application using Sys V shared
 6 * memory system calls.  In this example the app is requesting 256MB of
 7 * memory that is backed by huge pages.  The application uses the flag
 8 * SHM_HUGETLB in the shmget system call to inform the kernel that it is
 9 * requesting huge pages.
10 *
11 * Note: The default shared memory limit is quite low on many kernels,
12 * you may need to increase it via:
13 *
14 * echo 268435456 > /proc/sys/kernel/shmmax
15 *
16 * This will increase the maximum size per shared memory segment to 256MB.
17 * The other limit that you will hit eventually is shmall which is the
18 * total amount of shared memory in pages. To set it to 16GB on a system
19 * with a 4kB pagesize do:
20 *
21 * echo 4194304 > /proc/sys/kernel/shmall
22 */
23
24#include <stdlib.h>
25#include <stdio.h>
26#include <sys/types.h>
27#include <sys/ipc.h>
28#include <sys/shm.h>
29#include <sys/mman.h>
30
31#define LENGTH (256UL*1024*1024)
32
33#define dprintf(x)  printf(x)
34
35int main(void)
36{
37	int shmid;
38	unsigned long i;
39	char *shmaddr;
40
41	shmid = shmget(2, LENGTH, SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W);
42	if (shmid < 0) {
43		perror("shmget");
44		exit(1);
45	}
46	printf("shmid: 0x%x\n", shmid);
47
48	shmaddr = shmat(shmid, NULL, 0);
49	if (shmaddr == (char *)-1) {
50		perror("Shared memory attach failure");
51		shmctl(shmid, IPC_RMID, NULL);
52		exit(2);
53	}
54	printf("shmaddr: %p\n", shmaddr);
55
56	dprintf("Starting the writes:\n");
57	for (i = 0; i < LENGTH; i++) {
58		shmaddr[i] = (char)(i);
59		if (!(i % (1024 * 1024)))
60			dprintf(".");
61	}
62	dprintf("\n");
63
64	dprintf("Starting the Check...");
65	for (i = 0; i < LENGTH; i++)
66		if (shmaddr[i] != (char)i) {
67			printf("\nIndex %lu mismatched\n", i);
68			exit(3);
69		}
70	dprintf("Done.\n");
71
72	if (shmdt((const void *)shmaddr) != 0) {
73		perror("Detach failure");
74		shmctl(shmid, IPC_RMID, NULL);
75		exit(4);
76	}
77
78	shmctl(shmid, IPC_RMID, NULL);
79
80	return 0;
81}