Creating Shared Memory Segments - c

Question

I have this struct in C:
struct first {
struct list *buf;
struct pointers *ptr;
};
A function to create a shared memory segment:
void * create_shared_memory(char *name, int size){
int *ptr;
int ret;
int fd = shm_open (name, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
if (fd == -1) {
perror ("shm_open error!");
exit (1);
}
ret = ftruncate (fd, sizeof (size));
if (ret == -1) {
perror ("ftruncate error!");
exit (2);
}
ptr = mmap(0, sizeof (size), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (ptr == MAP_FAILED) {
perror ("shm-mmap error!");
exit (3);
}
}
and a function to create a shared memory segment for that structure:
void shared_memory_structure(){
create_shared_memory("struct", sizeof(struct first));
}
However I get an error. I found the problem was that the pointers inside the structure are not being pointed to the shared memory segment I just created. How can I do this ?

Answer 1

try out this code first. it's a demo code taken from http://blog.csdn.net/liuzhanchen1987/article/details/7455208 , which is in Chinese:
#include <sys/mman.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
typedef struct{
char name[4];
int age;
}people;
int
main(int argc, char** argv)
{
int i;
people *p_map;
char temp;
p_map=(people*)mmap(NULL,sizeof(people)*10,PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS,-1,0);
if(fork() == 0)
{
sleep(2);
for(i = 0;i<5;i++)
printf("child read: the %d people's age is %d\n",i+1,(*(p_map+i)).age);
(*p_map).age = 100;
munmap(p_map,sizeof(people)*10);
exit();
}
temp = 'a';
for(i = 0;i<5;i++)
{
temp += 1;
memcpy((*(p_map+i)).name, &temp,2);
(*(p_map+i)).age=20+i;
}
sleep(5);
printf( "parent read: the first people,s age is %d\n",(*p_map).age );
printf("umap\n");
munmap( p_map,sizeof(people)*10 );
printf( "umap ok\n" );
return 0;
}
expected result:
child read: the 1 people's age is 20
child read: the 2 people's age is 21
child read: the 3 people's age is 22
child read: the 4 people's age is 23
child read: the 5 people's age is 24
parent read: the first people,s age is 100
umap
umap ok
in which
mmap(NULL,size,PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS,-1,0);
is all what you need to do before forking(and its return value checking code), and its return value will be the allocated pages starting address(if it's valid).
if all processes are forked. it's very handy to just use mmap to allocate anonymous pages, and additionally, there is no side effect after the processes get collected.
if you use shm_open, then in somewhere there will be a share memory object gets created, and it will be the side effect remains in your system, even after all your processes get collected. however, it's necessary when you plan to have two irrelevant processes talk to each other.