Simple question about pointer in c programming - c

Question

I was writing a simple shared memory test program like below.
And I used a line like below.
e_intf[i % 8].flag = i;
I thought it should be like below.
e_intf[i % 8]->flag = i;
But if I use "->" operator, I have errors.
This is a full source code.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#define SHM_KEY 0x9998
typedef struct _ethernet_interface {
char name[8];
int32_t flag;
} Ethernet_Interface_t;
int32_t memory_update(void)
{
int32_t i = 0;
int32_t shmid;
Ethernet_Interface_t *e_intf;
int32_t ret;
shmid = shmget(SHM_KEY, 8 * sizeof(Ethernet_Interface_t), 0644|IPC_CREAT);
if (shmid == -1)
{
perror("Shared Memory");
return -1;
}
e_intf = shmat(shmid, NULL, 0);
if (e_intf == (void *)-1)
{
perror("Shared Memory Attach");
return -1;
}
while (1)
{
e_intf[i % 8].flag = i;
if (i < 8)
{
sprintf(e_intf[i % 8].name, "%s%d", "eth", (i % 8));
}
printf("memory_update : e_intf[%d].name = %s, e_intf[%d] = %d\n", i % 8, e_intf[i % 8].name, (i % 8), i);
sleep(2);
i++;
}
ret = shmdt(e_intf);
if (ret == -1)
{
perror("shmdt");
return -1;
}
return 0;
}
int32_t memory_read(void)
{
int32_t i;
int32_t shmid;
Ethernet_Interface_t *e_intf;
int32_t ret;
shmid = shmget(SHM_KEY, 8 * sizeof(Ethernet_Interface_t), 0644|IPC_CREAT);
if (shmid == -1)
{
perror("Shared Memory");
return -1;
}
e_intf = shmat(shmid, NULL, 0);
if (e_intf == (void *)-1)
{
perror("Shared Memory Attach");
return -1;
}
i = 0;
while (1)
{
printf("[%d]memory_read : index = %d\n", i, (i % 8));
printf("[%d]memory_read : e_intf[%d].name = %s\n", i, (i % 8), e_intf[i % 8].name);
printf("[%d]memory_read : e_intf[%d].flag = %d\n", i, (i % 8), e_intf[i % 8].flag);
if (e_intf[i % 8].flag == 42)
{
printf("[%d]memory_read : Exiting\n", i);
break;
}
sleep(1);
i++;
}
ret = shmdt(e_intf);
if (ret == -1)
{
perror("shmdt");
return -1;
}
ret = shmctl(shmid, IPC_RMID, 0);
if (ret == -1)
{
perror("shmctl");
return -1;
}
return 0;
}
int32_t main(int32_t argc, char *argv[])
{
pid_t pid;
pid = fork();
if (pid == 0)
{
/* child process */
memory_update();
}
else
{
/* parent process */
memory_read();
}
exit(0);
}
But when I made another simple test program for pointer operation.
I can use "->" operator as I expected.
Here is the source code for the test program for pointer operation.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
typedef struct _ethernet_interface {
char name[8];
int32_t flag;
} Ethernet_Interface_t;
int32_t main(int32_t argc, char *argv[])
{
Ethernet_Interface_t intf;
Ethernet_Interface_t *p_intf;
p_intf = malloc(sizeof(Ethernet_Interface_t));
intf.flag = 1111;
p_intf->flag = 2222;
printf("intf.flag = %d\n", intf.flag);
printf("p_intf->flag = %d\n", p_intf->flag);
return 0;
}
Can someone point out why I am confused with these 2 cases?

Answer 1

The answer is very simple - index dereferences the pointer:
pointer[n] === *(pointer + n)
so in your case:
e_intf[i % 8] === *(e_intf + i % 8)
and
e_intf[i % 8].flag == (*(e_intf + i % 8)).flag
if you want to use the -> operator you need pointer, not referenced object.
(&e_intf[i % 8]) -> flag
or
(e_intf + i % 8) -> flag