struct as global variable

Question

so if we declare and define a struct before main and want to use this struct in other file which are in the same root as this one, do we need to declare it again in that file? Especially I want to share the memory of a array whose element is the struct so I need to use the shm_get in another file, do I need to declare those struct again?

btw, is it in C

code will be like this:

 typedef struct {
 char y1;
 char y2;
 char y3;
 int x;
 } itemB;

int main(){...
itemB* BufferB;

then i share the memory

shmem2 = shm_get(542421, (void**)&BufferB, 30*sizeof(itemB));

so if i write another file which want to share the BufferB, I know should declare one more time BufferB and call the shm_get again use the same initial key, but should I declare a struct one more time? and where?

You need to tag the language you are using.

Neil Kirk
– Neil Kirk

2015-03-02 02:21:32 +00:00
Commented Mar 2, 2015 at 2:21 — Neil Kirk
– Neil Kirk, Commented Mar 2, 2015 at 2:21
And also share a bit of the code you've tried.

Augmented Jacob
– Augmented Jacob

2015-03-02 02:23:58 +00:00
Commented Mar 2, 2015 at 2:23 — Augmented Jacob
– Augmented Jacob, Commented Mar 2, 2015 at 2:23

M.M · Accepted Answer · 2015-03-02 02:41:38Z

1

The struct declaration does not have to be visible if you are only using a pointer to the struct, but it does need to be visible for sizeof(itemB) to work, or for you to be able to access any of the struct members by name.

If the struct definition is needed in multiple files then usually the definition is placed in a common file called a header which is #included from the files which need to see the definition.

It would be possible to copy-paste the definition to wherever it is needed, but that runs the risk of one definition being updated without the other definition being kept in sync, which would violate the One Definition Rule.

answered Mar 2, 2015 at 2:41

M.M

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4 Comments

Echo Over a year ago

so it is okay if I declare the struct before the main in every files that I need it or I can just put them in the main? @Matt McNabb

M.M Over a year ago

Your program must only have one main so that comment doesn't make sense

Echo Over a year ago

I have several files which run individual, but they only share the specific memory which is the BufferB who contains the items. @Matt McNabb

M.M Over a year ago

@Echo OK, as explained in my answer, you could copy-paste the itemB definition to every file but it would be safer to put it in a common file such as itemB.h and do #include "itemB.h" before main() in each program

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