How do you detach an array of strings from shared memory? C

Question

I have:

int array_id;
char* records[10];

// get the shared segment
if ((array_id = shmget(IPC_PRIVATE, 1, 0666)) == -1) {
            perror("Array Creating");
}

// attach
records[0] = (char*) shmat(array_id, (void*)0, 0);
if ((int) *records == -1) {
     perror("Array Attachment");
}

which works fine, but when i try and detach i get an "invalid argument" error.

// detach
int error;
if( (error = shmdt((void*) records[0])) == -1) {
      perror(array detachment);   
}

any ideas? thank you

How do you know the shmat worked "fine"? You did not do error checking... and array_id is used uninitialized. Never post modified and incomplete code, always post complete and actual code we can compile! — Jens
– Jens, Commented Nov 9, 2012 at 4:46
@Jens Yes he did, he is checking that error is = -1. Im confused about the perror(array detachment) part, is that actually a thing in c? — Fantastic Mr Fox
– Fantastic Mr Fox, Commented Nov 9, 2012 at 4:49
sorry i left the error checking out but i did check after attaching. i know it worked fine because i didn't get any errors and i also assigned some strings and printed them out. and i've been told to use perror() instead of printf() O.o — Tim
– Tim, Commented Nov 9, 2012 at 4:51
@Ben No he didn't. I'm talking about the attach, not the detach. Detaching when the attach already failed is no surprise. — Jens
– Jens, Commented Nov 9, 2012 at 4:51
@Tim This is impossible to diagnose unless you provide a complete, self contained program we can compile. So far all we have is "Doctor, it hurts when I do something." :-) — Jens
– Jens, Commented Nov 9, 2012 at 4:53

alk · Accepted Answer · 2012-11-09 10:11:15Z

1

In shmdt(), no need to convert the pointer argument to void* it will automatically take care of this .

Remove (void*) from shmdt((void*) records[0])). It should be like this.

if ((error = shmdt(records[0]) ) == -1)
{
  perror("Array detachment");
}

And it will work.

Also in shmat() , on error it returns (void*) -1 so your comparison will give warning. so do like this

if ((char *)records[0] == (void *)-1)
{
  perror("Array Attachment");
}

edited Nov 9, 2012 at 10:11

alk

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answered Nov 9, 2012 at 6:01

Omkant

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3 Comments

alk Over a year ago

There is no need to cast records[0] when test for ==(void*)-1).

Omkant Over a year ago

@alk :I saw the man page for shmat in that its given (void*) -1..That's why i wrote. Can you elaborate a little for me here because comparison of pointer to integer will give you wanrning won't it?

alk Over a year ago

records[0] is a pointer (a char *). Any pointer can be assigned and comparted to void* without casting. This casting -1 to void* is just a dirty trick to have shmat() be able to return some error.

alk · Accepted Answer · 2012-11-09 10:46:05Z

1

Assuming attaching went well, invalid argument simply means that either the segment has already been detached, or the value of records[0] had changed since it was set by attaching.

edited Nov 9, 2012 at 10:46

answered Nov 9, 2012 at 10:15

alk

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2 Comments

alk Over a year ago

My answer implies that the problem does not lie in how you attach/detach. The code is basically Ok. If you want to polish it up go for Omkant's answer. @Tim

Tim Over a year ago

you were right, i didn't modify records[0] and the error went away. @alk

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