HELP!!! Shared Memory...

[zeny]

Hi everybody!

In the file "server.c" i created a shared memory segment of the size of a 1024 byte string and then i attached a string to the segment, just like this:

int key = 12345;
char *message = "hello_world";

if((shmid = shmget(key, sizeof(char)*1024, IPC_CREAT | 0666)) < 0){
perror("Error in shmget()!");
exit(1);
}

if((message = shmat(shmid, NULL, 0)) ==(char *) -1){
perror("Error in shmat!");
exit(1);
}



Then in the file "client.c" i try to get the message sent by the server.c trough the shared memory:


int key = 12345;
char *msg, *message;

if((shmid = shmget(key, sizeof(char)*1024, 0666)) < 0){
perror("Error in shmget!");
exit(1);
}

if ((msg = shmat(shmid, NULL, 0)) == (char*) -1) {
perror("Error in shmat!");
exit(1);
}

...

//after waiting for the server to put the message into the segment, it does:

message = msg;

printf("MESSAGE: %s\n", message);
fflush(stdout);



I don´t get any compiler errors, nor runtime errors (no segmentation faults), but the printing result is always empty, void. I really don´t understand the reason why that happens! I´ve tried a bunch of stuff like mallocs, but nothing worked. I´d be most thankfull for any help

Best regards

[Banfa]

int key = 12345;
char *message = "hello_world";
 
if((shmid = shmget(key, sizeof(char)*1024, IPC_CREAT | 0666)) < 0){
        perror("Error in shmget()!");
        exit(1);
}
 
if((message = shmat(shmid, NULL, 0)) ==(char *) -1){
        perror("Error in shmat!");
        exit(1);
}
 

Not that I know anything about *nix shared memory but in this section of code where you call shmat you over write the pointer message that was pointing to the string constant "hello_world" with the pointer to the shared memory segment. You then don't copy anything into the shared memory segment, so when your client reads it there is nothing there. I suggest you try

int key = 12345;
const char const *message = "hello_world";
char *pshm;
 
 
if((shmid = shmget(key, sizeof(char)*1024, IPC_CREAT | 0666)) < 0){
        perror("Error in shmget()!");
        exit(1);
}
 
if((pshm = shmat(shmid, NULL, 0)) ==(char *) -1){
        perror("Error in shmat!");
        exit(1);
}
 
strcpy(pshm, message);
 

[horace1]

take a look at the example code on
http://www.cs.cf.ac.uk/Dave/C/node27.html

works on Fedora 6 OK