example for diff void** void*

voidpointer.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <memory.h>
#define  IPC_PERMS           0664
#define  KEY_SHM             0x2000

// use follow command create example binary file
// python -c 'print("\x55\xAA\x55\xAA")' > struct.bin

typedef struct {

    short   a;               
    short   b;               

    char tail;
} TEST_STRUCT;

TEST_STRUCT *g_test_struct;

int infd = 0;

static int get_shm(const int shmsize, const int mode )
{
    int    shmid = 0;

    if( ( shmid = shmget(KEY_SHM , shmsize, mode )) < 0 )
        return( -1 );

    if( ( g_test_struct = (TEST_STRUCT *) shmat( shmid, (char *) 0, 0 ) )
           == (TEST_STRUCT *) -1 )
    {
        shmctl( shmid, IPC_RMID, (struct shmid_ds *) 0 );
        return( -1 );
    }
    return( shmid );
}

static int open_shm_file(int *shmsize )
{
    if( ( infd = open( "struct.bin" , O_RDONLY) ) == -1 )
    {
        close( infd );
        return( -1 );
    }

    struct stat   sbuf;

    memset( &sbuf, 0, sizeof( struct stat ) );

    if( fstat( infd, &sbuf ) < 0 )
    {
        close( infd );
        return( -1 );
    }

    *shmsize = sbuf.st_size;

    return( 0 );
}

static int load_shm_from_bin_voidpp(const int fd
                 , void**  table
                 , const int shmsize)
{
    int len = 0;
    if( ( len = read( fd, *table, shmsize ) ) != shmsize )
    {
        printf( " read(%d) error %d\n", len, errno );
        close( fd );
        return( -1 );
    }

    close( fd );

    return( 0 );
}

static int load_shm_from_bin_voidp(const int fd
                 , void*  table
                 , const int shmsize)
{
    int len = 0;
    if( ( len = read( fd, table, shmsize ) ) != shmsize )
    {
        printf( " read(%d) error %d\n", len, errno );
        close( fd );
        return( -1 );
    }

    close( fd );

    return( 0 );
}


static int load_table_from_shm_voidpp(void** table)
{
    int shmsize = 0;

    if( open_shm_file( &shmsize ) < 0 )
        return( -1 );

    if( get_shm( shmsize, IPC_PERMS | IPC_CREAT ) < 0 )
        return( -1 );

    if( load_shm_from_bin_voidpp( infd, table, shmsize ) < 0 )
        return( -1 );

    return( 0 );
}

static int load_table_from_shm_voidp(void* table)
{
    int shmsize = 0;

    if( open_shm_file(&shmsize ) < 0 )
        return( -1 );

    if( get_shm( shmsize, IPC_PERMS | IPC_CREAT ) < 0 )
        return( -1 );

    if( load_shm_from_bin_voidp( infd, table, shmsize ) < 0 )
        return( -1 );

    return( 0 );
}
int main(){
    //case for void**
   // load_table_from_shm_voidpp((void**)&g_test_struct);
    //case for void*
    load_table_from_shm_voidp((void*)g_test_struct);
    return 0;
}