SHM and HugePages

/*
Author Dawid Mocek
SHM exmaple code from https://beej.us/guide/bgipc/output/html/multipage/shm.html

Testing Hugepages:

[oracle@we-pr-002-2 ~]$ ulimit -l
1048576
[oracle@we-pr-002-2 ~]$ cat /proc/meminfo
MemTotal:        8176208 kB
MemFree:         5985184 kB
Buffers:           13556 kB
Cached:           952644 kB
SwapCached:            0 kB
Active:           806600 kB
Inactive:         212156 kB
Active(anon):      52588 kB
Inactive(anon):      324 kB
Active(file):     754012 kB
Inactive(file):   211832 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       2097148 kB
SwapFree:        2097148 kB
Dirty:               244 kB
Writeback:             0 kB
AnonPages:         52584 kB
Mapped:             9144 kB
Shmem:               336 kB
Slab:              62416 kB
SReclaimable:      17324 kB
SUnreclaim:        45092 kB
KernelStack:         840 kB
PageTables:         3728 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     5660964 kB
Committed_AS:     112860 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       15700 kB
VmallocChunk:   34359711467 kB
HardwareCorrupted:     0 kB
HugePages_Total:     512
HugePages_Free:      507
HugePages_Rsvd:      251
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        8192 kB
DirectMap2M:     8380416 kB
[oracle@we-pr-002-2 ~]$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x52030b0f 131073     oracle     644        536870912  0

------ Semaphore Arrays --------
key        semid      owner      perms      nsems

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages





*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define SHM_SIZE 536870912   /* make it a 512M shared memory segment */



int main(int argc, char *argv[])
{
    key_t key;
    int shmid;
    char *data;
    int mode;
    char c;
    FILE *fh = NULL;
    long size;
    if (argc > 2) {
        fprintf(stderr, "usage: shmdemo [big_text_file]\n");
        exit(1);
    }

    /* make the key: */
    if ((key = ftok("shm.c", 'R')) == -1) {
        perror("ftok");
        exit(1);
    }

    printf("Geting shared memory for key....\n");
    /* connect to (and possibly create) the segment: */
    if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT | SHM_HUGETLB)) == -1) {
        perror("shmget");
        exit(1);
    }
    printf("... got. Waiting to attach to shared memory\n");
    c = getc(stdin);
    /* attach to the segment to get a pointer to it: */
    data = shmat(shmid, (void *)0, 0);
    if (data == (char *)(-1)) {
        perror("shmat");
        exit(1);
    }
    printf("Attached to shared memory\n");


    /* read or modify the segment, based on the command line: */
    if (argc == 2) {
        printf("Modifing shared memory...\n");
        printf("Writing to segment file: \"%s\"\n", argv[1]);
        fh = fopen(argv[1], "r+");
        fseek(fh, 0L, SEEK_END);
        size = ftell(fh);
        fseek(fh, 0L, SEEK_SET);
        fread(data, size, sizeof(char), fh);
        fclose(fh);

//      strncpy(data, argv[1], SHM_SIZE);
        printf("Shared memory modified\n");
    } else {
        printf("Reading shared memory\n");
        printf("Segment contains: \"%s\"\n", data);
        printf("Done reading shared memory\n");
        }
    printf("Detaching shared memory\n");
    /* detach from the segment: */
    if (shmdt(data) == -1) {
        perror("shmdt");
        exit(1);
    }

    return 0;
}