How to transfer FILE structure using Shared memory(POSIX SKIN) - c

Question

I am using Shared memory (POSIX), to share a FILE pointer(FILE * rt_file), but it's not getting file pointer at client end. Any suggestions please.
/*
* shm_msgserver.c
*
* Illustrates memory mapping and persistency, with POSIX objects.
* This process produces a message leaving it in a shared segment.
* The segment is mapped in a persistent object meant to be subsequently
* open by a shared memory "client".
*
*
* Created by Mij <mij#bitchx.it> on 27/08/05.
* Original source file available at http://mij.oltrelinux.com/devel/unixprg/
*
*/
#include <stdio.h>
/* shm_* stuff, and mmap() */
#include <sys/mman.h>
#include <sys/types.h>
/* exit() etc */
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
/* for random() stuff */
#include <stdlib.h>
#include <time.h>
/* Posix IPC object name [system dependant] - see
http://mij.oltrelinux.com/devel/unixprg/index2.html#ipc__posix_objects */
#define SHMOBJ_PATH "/foo1423"
/* maximum length of the content of the message */
#define MAX_MSG_LENGTH 50
/* how many types of messages we recognize (fantasy) */
#define TYPES 8
/* message structure for messages in the shared segment */
struct msg_s {
int type;
char content[MAX_MSG_LENGTH];
FILE *pt;
};
int main(int argc, char *argv[]) {
int shmfd;
int shared_seg_size = (1 * sizeof(struct msg_s)); /* want shared segment capable of storing 1 message */
struct msg_s *shared_msg; /* the shared segment, and head of the messages list */
FILE *rt_file;
rt_file = fopen ("res","a+");
printf("rt_file : %s\n",rt_file);
/* creating the shared memory object -- shm_open() */
shmfd = shm_open(SHMOBJ_PATH, O_CREAT | O_EXCL | O_RDWR, S_IRWXU | S_IRWXG);
if (shmfd < 0) {
perror("In shm_open()");
exit(1);
}
fprintf(stderr, "Created shared memory object %s\n", SHMOBJ_PATH);
/* adjusting mapped file size (make room for the whole segment to map) -- ftruncate() */
ftruncate(shmfd, shared_seg_size);
/* requesting the shared segment -- mmap() */
shared_msg = (struct msg_s *)mmap(NULL, shared_seg_size, PROT_READ | PROT_WRITE, MAP_SHARED, shmfd, 0);
if (shared_msg == NULL) {
perror("In mmap()");
exit(1);
}
fprintf(stderr, "Shared memory segment allocated correctly (%d bytes).\n", shared_seg_size);
srandom(time(NULL));
/* producing a message on the shared segment */
shared_msg->type = random() % TYPES;
snprintf(shared_msg->content, MAX_MSG_LENGTH, "My message, type %d, num %ld", shared_msg->type, random());
shared_msg->pt = rt_file;
/* [uncomment if you wish] requesting the removal of the shm object -- shm_unlink() */
/*
if (shm_unlink(SHMOBJ_PATH) != 0) {
perror("In shm_unlink()");
exit(1);
}
*/
return 0;
}
Client Code
/*
* shm_msgclient.c
* http://mij.oltrelinux.com/devel/unixprg/#ipc__posix_shm
* Illustrates memory mapping and persistency, with POSIX objects.
* This process reads and displays a message left it in "memory segment
* image", a file been mapped from a memory segment.
*
*
* Created by Mij <mij#bitchx.it> on 27/08/05.
* Original source file available at http://mij.oltrelinux.com/devel/unixprg/
*
*/
#include <stdio.h>
/* exit() etc */
#include <unistd.h>
/* shm_* stuff, and mmap() */
#include <sys/mman.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
/* for random() stuff */
#include <stdlib.h>
#include <time.h>
/* Posix IPC object name [system dependant] - see
http://mij.oltrelinux.com/devel/unixprg/index2.html#ipc__posix_objects */
#define SHMOBJ_PATH "/foo1423"
/* maximum length of the content of the message */
#define MAX_MSG_LENGTH 50
/* how many types of messages we recognize (fantasy) */
#define TYPES 8
/* message structure for messages in the shared segment */
struct msg_s {
int type;
char content[MAX_MSG_LENGTH];
FILE *pt;
};
int main(int argc, char *argv[]) {
int shmfd;
int shared_seg_size = (1 * sizeof(struct msg_s)); /* want shared segment capable of storing 1 message */
struct msg_s *shared_msg; /* the shared segment, and head of the messages list */
FILE *rt_file;
/* creating the shared memory object -- shm_open() */
shmfd = shm_open(SHMOBJ_PATH, O_RDWR, S_IRWXU | S_IRWXG);
if (shmfd < 0) {
perror("In shm_open()");
exit(1);
}
printf("Created shared memory object %s\n", SHMOBJ_PATH);
/* requesting the shared segment -- mmap() */
shared_msg = (struct msg_s *)mmap(NULL, shared_seg_size, PROT_READ | PROT_WRITE, MAP_SHARED, shmfd, 0);
if (shared_msg == NULL) {
perror("In mmap()");
exit(1);
}
printf("Shared memory segment allocated correctly (%d bytes).\n", shared_seg_size);
rt_file = shared_msg->pt;
printf("rt_file : %s\n",rt_file);
rt_file = fopen ("res","a+");
printf("rt_file : %s\n", rt_file);
char x[10]="ABCDEFGHIJ";
fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), rt_file);
printf("Message type is %d, content is: %s\n", shared_msg->type, shared_msg->content);
if(shm_unlink(SHMOBJ_PATH) == -1){
printf("%s is used by another Process\n",SHMOBJ_PATH);
}
else{
printf("Memory is freed\n");
}
return 0;
}

Answer 1

FILE* pointers are valid only in the process where they were created.
While you can transfer the pointer value, or even the structure it points to, to another process, using it in the other process will invoke undefined behaviour.
If you want another process on the same machine to access a certain file I suggest that you send the file name, mode and offset to the other process and let it create it's own FILE* structure by calling fopen.