[SOLVED] C - How can you read a string into shmget and shmat?

golmschenk · 03-08-2010, 11:18 PM

I want to read a string into shared memory using shmget and shmat. I'm using something like:

Code:

char *bookmark_strings, *s;

Book_Key = ftok(SHM_LOC,SHM_KEY);
shared_bookmarks = shmget(Book_Key, MAX_BOOKMARKS * sizeof(bookmarks), (0644 | IPC_CREAT));

bookmark_strings = shmat(shared_bookmarks, NULL, 0);
s = bookmark_strings;

*s = "hello";

I know that since it's a string I should be using something other then just char, but with something different shmat fails to be assigned. Can anyone help? Thanks!

paulsm4 · 03-08-2010, 11:23 PM

It's simple, but you'd really be doing yourself a favor by taking a look at "Beej's Guide to IPC":

http://beej.us/guide/bgipc/

theNbomr · 03-09-2010, 08:51 AM

You also might want to brush up on the use of pointers in C. In particular, the statement

Code:

*s = "hello";

probably isn't doing what you want it to do.
I imagine you intended to make a copy of the literal string "hello" in the shared memory, but that would require something more like

Code:

 strcpy( s, "hello");

Your code writes the address of the literal string "hello" into shared memory, and I'm guessing, probably evokes one or more compiler errors or warnings.
--- rod.

golmschenk · 03-09-2010, 05:16 PM

I'm having trouble getting the shared memory to act in the form of an array of strings (an array of char arrays).

Code:

char *bookmark_strings[10], *s[10], input_s[100];

Book_Key = ftok(SHM_LOC,SHM_KEY);
shared_bookmarks = shmget(Book_Key, MAX_BOOKMARKS * sizeof(bookmarks), (0644 | IPC_CREAT));

bookmark_strings = shmat(shared_bookmarks, (void *)0, 0);
s = bookmark_strings;

When I try to do something like this I get

Code:

error: incompatible types in assignment

Any suggestions? Thanks!

crts · 03-09-2010, 05:52 PM

Quote:

Originally Posted by golmschenk

I'm having trouble getting the shared memory to act in the form of an array of strings (an array of char arrays).

Code:

char *bookmark_strings[10], *s[10], input_s[100];

Book_Key = ftok(SHM_LOC,SHM_KEY);
shared_bookmarks = shmget(Book_Key, MAX_BOOKMARKS * sizeof(bookmarks), (0644 | IPC_CREAT));

bookmark_strings = shmat(shared_bookmarks, (void *)0, 0);
s = bookmark_strings;

When I try to do something like this I get

Code:

error: incompatible types in assignment

Any suggestions? Thanks!

Code:

void *shmat(int shmid, void *shmaddr, int shmflg);

shmat returns a pointer and not an array of pointers.
Try the following:

Code:

bookmark_strings[0] = shmat(shared_bookmarks, (void *)0, 0);

This will not raise an error during compilation. Of course, this will only assign just one value to the first array element. You might want to rewrite your code to use a loop.

theNbomr · 03-09-2010, 06:09 PM

Okay, where to start? At the beginning, I suppose.

Code:

char *bookmark_strings[10]

Ten pointers to characters. Uninitialized.

Code:

bookmark_strings = shmat(shared_bookmarks, (void *)0, 0);

A pointer to void returned by a function. The pointer to void is assigned to ... the address of the zeroeth element of an array (of pointers to char), which is not an lvalue. Something wrong with that. Maybe you actually meant

Code:

bookmark_strings[0] = (char *)shmat(shared_bookmarks, (void *)0, 0);

Code:

s = bookmark_strings;

Once again, assignment to non-lvalue. Could you possibly have meant

Code:

s[0] = bookmark_strings[0]

? It might make sense if the previous line was per my suggestion.

I think very little of your problem has to do with shared memory, per se, just some missing understanding of the use of arrays and pointers in the C language. If I am wrong about that, then paulsm4's suggestion of reading Beej's Guide was already an outstanding pointer (pun intended).

--- rod.

golmschenk · 03-09-2010, 06:50 PM

Haha, great. Thanks everybody. I got everything working. Thanks!