library with struct definitions

This is a discussion on library with struct definitions within the C Programming forums, part of the General Programming Boards category; How would I define some structs in a seperate c file and have them exported? I have inside c file ...

  1. #1
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    library with struct definitions

    How would I define some structs in a seperate c file and have them exported?

    I have inside c file stuff that looks like this:
    types.c
    Code:
    #include "types.h"
    static struct s_pos{
      ...
      };
    extern struct s_cell{
      ...
      };
    and in header, types.h

    Code:
    struct s_cell;
    things are declared in this
    Code:
    #include "types.h"
    extern struct s_cell map[256][256];
    When I compile, I get error
    [warning] storage size of `map` isn't known

    so they must not be exported correctly.

    Could someone explain or point me to a helpful page, thanks.

  2. #2
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    Include the file in which struct cell is defined.

  3. #3
    Registered User whiteflags's Avatar
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    storage size of map is unknown because any declaration with extern is not a definition. What itCbitC said was true:

    Code:
    /* types.h */
    
    extern struct s_cell;
    
    /* types.c */
    
    struct s_cell { /* ... */ };
    
    struct s_cell map[256][256];
    This sort of thing can be useful for hiding implementations and so forth, though.

  4. #4
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    What I want to do is have the data types defined in a seperate file that will compile to an object alone and then include into another file and set up variables using those data types.

    I want struct s_cell and struct s_cell map[x][y] to be in source different files.

  5. #5
    Registered User whiteflags's Avatar
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    Well as long as you compile types.c with the source that defines map, then you should be ok. Make sure to include the header in both types.c and the other source file. From what I've shown you, nothing needs to be different.

  6. #6
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    Should I be able to do something like this?

    Code:
     > types.c
    struct s_cell {
      int foo;
      int bar;
      };
    
     > types.h
    struct s_cell;
    
     > defs.c
    #include "types.h"
    struct s_cell map[256][256];
    map[0][0].foo = 4;
    map[0][0].bar = 3;
    
     > defs.h
    extern struct s_cell map[256][256];
    
     > main.c
    #include "defs.h"
    int main()
    {map[0][0].foo = map[0][0].bar;}
    Sorry, I must be misunderstanding someone.

  7. #7
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    Why not declare and define the struct in the header file?

  8. #8
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    That's not what I want to do. :P

  9. #9
    Registered User whiteflags's Avatar
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    I'm not really sure how more explicit I can be, but this is starting to look completely wrong.

    > defs.c
    #include "types.h"
    struct s_cell map[256][256];
    map[0][0].foo = 4;
    map[0][0].bar = 3;

    First of all, you cannot put anything but declarations or definitions outside of a function, so even if map was correct in definition, you would need to use an array initializer, which is probably not what you want. Declaration and definition also mean separate things. While a declaration can be a definition, only the definition sets aside space for a variable. Meaning that you actually have a variable in global scope, which is, more often than not, a symptom of bad design.

    Likewise, cramming all your declarations in one file does not help in any capacity, it only reaffirms that your design is poor. Learn about scope.

    Now that we have an understanding, let me assume that map indeed should be global.

    The extern keyword when applied to an identifier merely declares the identifier as a specific type of thing (for map, a 256-element array of int [256], if that makes sense). The definition occurs later, in another source file.

    The static keyword has several meanings, determine if any of them apply to your situation.

    But if map is indeed global, then defs.c should probably be scrapped and map itself not statically defined. If map is not global, then this whole thread is a nonissue, and you need to redesign your program.
    Last edited by whiteflags; 02-28-2009 at 01:12 AM.

  10. #10
    CSharpener vart's Avatar
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    Quote Originally Posted by seaking1 View Post
    Should I be able to do something like this?
    .
    yes except the contents of the types.c should be moved to types.h
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  11. #11
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    Yes, map is a global.
    Last edited by seaking1; 02-28-2009 at 01:20 AM.

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