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Altera_Forum's avatar
Altera_Forum
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15 years ago

Passing Structures into Functions

Hi Guys,

question

I need some clarification on passing structures into functions.

in what i'll outline below, i don't understand why the function prototypes are as they are.

This is how i understand it, but if i say something wrong please correct me.

example that confuses me

What i've got is the Niche stack with the below structure. 

typedef
 
struct SSS_SOCKET { 
enum { READY, COMPLETE, CLOSE } state; 
int fd; 
int close; 
INT8U rx_buffer; 
INT8U *rx_rd_pos; /* position we've read up to */
INT8U *rx_wr_pos; /* position we've written up to */
 } SSSConn;

This says to me "define a structure type SSS_Socket, and create an instance called SSSConn.".

Now functions that use this have their prototypes as: 

void SSS_handle_receive(SSSConn* conn)

and function call appears as: 

SSS_handle_receive(&conn)

What i don't understand is why is an instance of a structure being used instead of the type itself i would have thought a prototype would be: 

void SSS_handle_receive(SSS_SOCKET* conn)

can someone please explain whats happening here?

4 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    You are correct, both forms only create a 'type'. You can exemplify this by slightly changing the first form into 

    typedef  struct { 
    enum { READY, COMPLETE, CLOSE } state; 
    int fd; 
    int close; 
    INT8U rx_buffer; 
    INT8U *rx_rd_pos; /* position we've read up to */
    INT8U *rx_wr_pos; /* position we've written up to */
    } SSSConn;

    You see that you can leave out the ' SSS_SOCKET' tag.

    Only when you instantiate a 'type' actual storage will be allocated. e.g.: 

    
SSSConn master , slaves ;
    will actually reserve storage.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Thank you very much, i now understand except for one thing,

    would this mean, that no instance is created by this code?
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    There is a typedef at the very beginning of the code making SSSConn visible as a type (alias 'struct SSS_SOCKET').

    If you reformat the source a bit it becomes beter visible 

    
typedef  struct SSS_SOCKET { 
    enum { READY, COMPLETE, CLOSE } state; 
    int fd; 
    int close; 
    INT8U rx_buffer; 
    INT8U *rx_rd_pos; /* position we've read up to */
    INT8U *rx_wr_pos; /* position we've written up to */
    } SSSConn;

    Most often we first see the declaration of the structure, followed by the typedef statement. 

    
struct SSS_SOCKET { 
    enum { READY, COMPLETE, CLOSE } state; 
    int fd; 
    int close; 
    INT8U rx_buffer; 
    INT8U *rx_rd_pos; /* position we've read up to */
    INT8U *rx_wr_pos; /* position we've written up to */
    } ;
typedef struct SSS_SOCKET SSSConn ;

    The first code snippet does the two in one go.