Passing structures in C functions

This is a discussion on Passing structures in C functions within the C Programming forums, part of the General Programming Boards category; Hi All. I'm struggling to get back to grips with C following a 15 year absence... I have a structure, ...

  1. #1
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    Passing structures in C functions

    Hi All.

    I'm struggling to get back to grips with C following a 15 year absence...

    I have a structure, as follows:-

    Code:
    typedef struct  {
            long mtype;
            int useragent;
            time_t mdatetime;
            char mtext[4096];
    } busMessage;
    And I want to populate it through a function/procedure by passing it to the function along with some variables as follows:-

    Code:
    void buildMessage( busMessage BM, long MT, int UA ) {
            BM.mtype     = MT;
            BM.useragent = UA;
            BM.mdatetime  = 1005454;
            strcpy(BM.mtext, "A test");
    }
    But when I use this function in the following way:-

    Code:
    int main(void) {
            busMessage myMessage;
    
            buildMessage (myMessage, 4, 4);
            printMessage (myMessage);
            exit(0);
    }
    .... all the fields in myMessage are zero.

    The function printMessage just does this:-

    Code:
    void printMessage( busMessage BM ) {
    
    	printf(ctime(&BM.mdatetime)); // Prints 1st Jan 1970 (correct, as mdatetime must be 0)
    
            printf("Time as int : %d\n", BM.mdatetime); // Zero
    
            printf("UA as int : %d\n", BM.useragent); // Zero
    
            printf("MT as int : %d\n", BM.mtype); // Zero
    
            printf("Payload :  %s\n", BM.mtext); // Blank
    }
    I have a funny feeling this may be a scope issue(?) but can't quite get my head around it.

    Any help would be gratefully received!

    Thanks.

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    If you want the function to modify it, you must pass it via a pointer.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #3
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    Many thanks Elysia.

    Something like (been a long time since I did this):-

    Code:
    busMessage myMessage;
    
    doSomething( &myMessage );
    
    void doSomething( *busMessage BM ) {
     (*BM).field1 = 123;
     // etc....
    }
    ?

    Thanks!

  4. #4
    Woof, woof! zacs7's Avatar
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    Close,
    Code:
    void doSomething( busMessage * BM ) {
     (*BM).field1 = 123;
     // etc....
    }
    BTW,
    Code:
    (*BM).field1 = 123;
    is the same as
    Code:
    BM->field1 = 123;
    The latter is easier on the eyes .
    Don't forget if you dereference a NULL pointer bad things happen.

    Perhaps check against NULL
    Code:
    void doSomething( busMessage * BM ) {
        if(BM == NULL)
            return;            /* don't continue */
        BM->field1 = 123;
        // etc....
    }

  5. #5
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    Fantastic - thanks for your help.

    It's all coming back to me slowly...

  6. #6
    C++まいる!Cをこわせ! Elysia's Avatar
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    I would rather think an assert is better, since it's clearly a programmer error:
    Code:
    void doSomething( busMessage * BM ) {
        assert(BM);
        BM->field1 = 123;
        // etc....
    }
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
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    Shouldnt the structure be defined like this:

    Code:
    typedef struct busMessage {
            long mtype;
            int useragent;
            time_t mdatetime;
            char mtext[4096];
    };
    Doing it the way you did it would create an instance of the structure...?

  8. #8
    C++まいる!Cをこわせ! Elysia's Avatar
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    No, that would generate a compile error - the struct is a typedef, so you must specify a new name for the type (it's not creating an instance).
    Otherwise, loose the "typedef" keyword.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  9. #9
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    nvm...............

    EDIT: Sorry im used to C++ where adding typedef in front of a struct has no effect/not the same effect. In C++ doing this will create a new instance:

    Code:
    struct  {
            long mtype;
            int useragent;
            time_t mdatetime;
            char mtext[4096];
    }busMessage;
    
    busMessage.mtype = 1;
    Last edited by 39ster; 05-26-2008 at 12:18 AM.

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by 39ster View Post
    EDIT: Sorry im used to C++ where adding typedef in front of a struct has no effect/not the same effect.
    No, it does not:

    Code:
    	typedef struct {
    		long mtype;
    		int useragent;
    		time_t mdatetime;
    		char mtext[4096];
    	} a;
    
    	a busMessage;
    	busMessage.mtype = 1;
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  11. #11
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    Quote Originally Posted by Elysia View Post
    No, it does not:

    Code:
    	typedef struct {
    		long mtype;
    		int useragent;
    		time_t mdatetime;
    		char mtext[4096];
    	} a;
    
    	a busMessage;
    	busMessage.mtype = 1;
    ahuh...Well i use structures in C++ like this:

    Defining a structure:
    Code:
    struct SomeStruct
    {
      int a, b, c;
    };
    Create an instance of a struct:
    Code:
    struct
    {
      int a, b, c;
    } blah;
    blah.a = blah.b = blah.c = 32;
    Do both:
    Code:
    struct SomeStruct
    {
      int a, b, c;
    } blah;
    blah.a = blah.b = blah.c = 32;

  12. #12
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yes, this is more correct.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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