C union containing only one struct












7















I have started today to program on a PIC16f88, and found that the header for its registers contains a union that only contains a struct:



extern volatile unsigned char ANSEL __at(0x09B);

typedef union {

    struct {

        unsigned ANS0       :1;

        unsigned ANS1       :1;

        unsigned ANS2       :1;

        unsigned ANS3       :1;

        unsigned ANS4       :1;

        unsigned ANS5       :1;

        unsigned ANS6       :1;

    };

} ANSELbits_t;

extern volatile ANSELbits_t ANSELbits __at(0x09B);


Does it provide any benefits to enclose the struct inside a union that only contains that struct?



Its access I guess is going to be exactly the same as if it were a simple struct (because the struct is anonymous):



ANSELbits.ANS4 = 0;






c struct unions







share|improve this question




















  • 1





    Would you mind confirming that you have or haven't omitted another member of the union?

    – Bathsheba
    3 hours ago











  • I copied exactly the code from the header. Nothing omitted.

    – Cacahuete Frito
    3 hours ago
















7















I have started today to program on a PIC16f88, and found that the header for its registers contains a union that only contains a struct:



extern volatile unsigned char ANSEL __at(0x09B);

typedef union {

    struct {

        unsigned ANS0       :1;

        unsigned ANS1       :1;

        unsigned ANS2       :1;

        unsigned ANS3       :1;

        unsigned ANS4       :1;

        unsigned ANS5       :1;

        unsigned ANS6       :1;

    };

} ANSELbits_t;

extern volatile ANSELbits_t ANSELbits __at(0x09B);


Does it provide any benefits to enclose the struct inside a union that only contains that struct?



Its access I guess is going to be exactly the same as if it were a simple struct (because the struct is anonymous):



ANSELbits.ANS4 = 0;






c struct unions







share|improve this question




















  • 1





    Would you mind confirming that you have or haven't omitted another member of the union?

    – Bathsheba
    3 hours ago











  • I copied exactly the code from the header. Nothing omitted.

    – Cacahuete Frito
    3 hours ago














7












7








7


1






I have started today to program on a PIC16f88, and found that the header for its registers contains a union that only contains a struct:



extern volatile unsigned char ANSEL __at(0x09B);

typedef union {

    struct {

        unsigned ANS0       :1;

        unsigned ANS1       :1;

        unsigned ANS2       :1;

        unsigned ANS3       :1;

        unsigned ANS4       :1;

        unsigned ANS5       :1;

        unsigned ANS6       :1;

    };

} ANSELbits_t;

extern volatile ANSELbits_t ANSELbits __at(0x09B);


Does it provide any benefits to enclose the struct inside a union that only contains that struct?



Its access I guess is going to be exactly the same as if it were a simple struct (because the struct is anonymous):



ANSELbits.ANS4 = 0;






c struct unions







share|improve this question
















I have started today to program on a PIC16f88, and found that the header for its registers contains a union that only contains a struct:



extern volatile unsigned char ANSEL __at(0x09B);

typedef union {

    struct {

        unsigned ANS0       :1;

        unsigned ANS1       :1;

        unsigned ANS2       :1;

        unsigned ANS3       :1;

        unsigned ANS4       :1;

        unsigned ANS5       :1;

        unsigned ANS6       :1;

    };

} ANSELbits_t;

extern volatile ANSELbits_t ANSELbits __at(0x09B);


Does it provide any benefits to enclose the struct inside a union that only contains that struct?



Its access I guess is going to be exactly the same as if it were a simple struct (because the struct is anonymous):



ANSELbits.ANS4 = 0;






c struct unions




c struct unions






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 3 hours ago







Cacahuete Frito

















asked 3 hours ago









Cacahuete FritoCacahuete Frito

1097




1097








  • 1





    Would you mind confirming that you have or haven't omitted another member of the union?

    – Bathsheba
    3 hours ago











  • I copied exactly the code from the header. Nothing omitted.

    – Cacahuete Frito
    3 hours ago














  • 1





    Would you mind confirming that you have or haven't omitted another member of the union?

    – Bathsheba
    3 hours ago











  • I copied exactly the code from the header. Nothing omitted.

    – Cacahuete Frito
    3 hours ago








1




1





Would you mind confirming that you have or haven't omitted another member of the union?

– Bathsheba
3 hours ago





Would you mind confirming that you have or haven't omitted another member of the union?

– Bathsheba
3 hours ago













I copied exactly the code from the header. Nothing omitted.

– Cacahuete Frito
3 hours ago





I copied exactly the code from the header. Nothing omitted.

– Cacahuete Frito
3 hours ago












2 Answers
2






active

oldest

votes


















3














There is no benefit in standard portable C.



But code like this is used to circumvent (in a non-portable way) all the type checking that your C compiler will make.



You are then empowered to set all the members of the underlying struct in one go, which is useful in this case as it contains a lot of bit fields.






share|improve this answer



















  • 2





    any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

    – P__J__
    3 hours ago








  • 1





    @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

    – Bathsheba
    3 hours ago






  • 1





    @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

    – Bathsheba
    3 hours ago






  • 2





    @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

    – P__J__
    3 hours ago






  • 3





    Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

    – Jonathan Leffler
    3 hours ago



















3














It does not make any difference if you wrap and I suppose that someone has forgoten to add another member (or did not copy-paste everything) as in the declaration below. No warnings will be suppressed. 



typedef union {

    struct {

        unsigned ANS0       :1;

        unsigned ANS1       :1;

        unsigned ANS2       :1;

        unsigned ANS3       :1;

        unsigned ANS4       :1;

        unsigned ANS5       :1;

        unsigned ANS6       :1;

    };

    uint8_t d8;

} ANSELbits_t;

extern volatile ANSELbits_t ANSELbits __at(0x09B);


BTW if the struct has to fit in 1 byte (8 bits) this declaration is wrong and uint_t type should be used instead.






share|improve this answer























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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3














    There is no benefit in standard portable C.



    But code like this is used to circumvent (in a non-portable way) all the type checking that your C compiler will make.



    You are then empowered to set all the members of the underlying struct in one go, which is useful in this case as it contains a lot of bit fields.






    share|improve this answer



















    • 2





      any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

      – P__J__
      3 hours ago








    • 1





      @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

      – Bathsheba
      3 hours ago






    • 1





      @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

      – Bathsheba
      3 hours ago






    • 2





      @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

      – P__J__
      3 hours ago






    • 3





      Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

      – Jonathan Leffler
      3 hours ago
















    3














    There is no benefit in standard portable C.



    But code like this is used to circumvent (in a non-portable way) all the type checking that your C compiler will make.



    You are then empowered to set all the members of the underlying struct in one go, which is useful in this case as it contains a lot of bit fields.






    share|improve this answer



















    • 2





      any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

      – P__J__
      3 hours ago








    • 1





      @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

      – Bathsheba
      3 hours ago






    • 1





      @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

      – Bathsheba
      3 hours ago






    • 2





      @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

      – P__J__
      3 hours ago






    • 3





      Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

      – Jonathan Leffler
      3 hours ago














    3












    3








    3







    There is no benefit in standard portable C.



    But code like this is used to circumvent (in a non-portable way) all the type checking that your C compiler will make.



    You are then empowered to set all the members of the underlying struct in one go, which is useful in this case as it contains a lot of bit fields.






    share|improve this answer













    There is no benefit in standard portable C.



    But code like this is used to circumvent (in a non-portable way) all the type checking that your C compiler will make.



    You are then empowered to set all the members of the underlying struct in one go, which is useful in this case as it contains a lot of bit fields.







    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered 3 hours ago









    BathshebaBathsheba

    177k27253376




    177k27253376








    • 2





      any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

      – P__J__
      3 hours ago








    • 1





      @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

      – Bathsheba
      3 hours ago






    • 1





      @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

      – Bathsheba
      3 hours ago






    • 2





      @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

      – P__J__
      3 hours ago






    • 3





      Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

      – Jonathan Leffler
      3 hours ago














    • 2





      any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

      – P__J__
      3 hours ago








    • 1





      @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

      – Bathsheba
      3 hours ago






    • 1





      @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

      – Bathsheba
      3 hours ago






    • 2





      @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

      – P__J__
      3 hours ago






    • 3





      Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

      – Jonathan Leffler
      3 hours ago








    2




    2





    any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

    – P__J__
    3 hours ago







    any porability considerations are voided when I see extern volatile ANSELbits_t ANSELbits __at(0x09B); which is specific to Keil compiler used by some programmers in the uC programming (expensive and IMO not very good), but supported directly by ARM

    – P__J__
    3 hours ago






    1




    1





    @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

    – Bathsheba
    3 hours ago





    @CacahueteFrito: in which case I'm pretty convinced I'm correct. Remove the union and see what happens. Compiler warnings galore I'd expect.

    – Bathsheba
    3 hours ago




    1




    1





    @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

    – Bathsheba
    3 hours ago





    @P__J__: Trust me, this is an old-fashioned idiom. Not much more I can say really!

    – Bathsheba
    3 hours ago




    2




    2





    @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

    – P__J__
    3 hours ago





    @CacahueteFrito PIC uC? - my condolences :). LOL - the original programmer did not know how to use unions and has found this "workaround" :)

    – P__J__
    3 hours ago




    3




    3





    Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

    – Jonathan Leffler
    3 hours ago





    Using bit-fields means the code is not portable, period. Almost everything about bit-fields is implementation-defined.

    – Jonathan Leffler
    3 hours ago













    3














    It does not make any difference if you wrap and I suppose that someone has forgoten to add another member (or did not copy-paste everything) as in the declaration below. No warnings will be suppressed. 



    typedef union {
    
    struct {
    
        unsigned ANS0       :1;
    
        unsigned ANS1       :1;
    
        unsigned ANS2       :1;
    
        unsigned ANS3       :1;
    
        unsigned ANS4       :1;
    
        unsigned ANS5       :1;
    
        unsigned ANS6       :1;
    
    };
    
    uint8_t d8;
    
} ANSELbits_t;
    
extern volatile ANSELbits_t ANSELbits __at(0x09B);


    BTW if the struct has to fit in 1 byte (8 bits) this declaration is wrong and uint_t type should be used instead.






    share|improve this answer




























      3














      It does not make any difference if you wrap and I suppose that someone has forgoten to add another member (or did not copy-paste everything) as in the declaration below. No warnings will be suppressed. 



      typedef union {
      
    struct {
      
        unsigned ANS0       :1;
      
        unsigned ANS1       :1;
      
        unsigned ANS2       :1;
      
        unsigned ANS3       :1;
      
        unsigned ANS4       :1;
      
        unsigned ANS5       :1;
      
        unsigned ANS6       :1;
      
    };
      
    uint8_t d8;
      
} ANSELbits_t;
      
extern volatile ANSELbits_t ANSELbits __at(0x09B);


      BTW if the struct has to fit in 1 byte (8 bits) this declaration is wrong and uint_t type should be used instead.






      share|improve this answer


























        3












        3








        3







        It does not make any difference if you wrap and I suppose that someone has forgoten to add another member (or did not copy-paste everything) as in the declaration below. No warnings will be suppressed. 



        typedef union {
        
    struct {
        
        unsigned ANS0       :1;
        
        unsigned ANS1       :1;
        
        unsigned ANS2       :1;
        
        unsigned ANS3       :1;
        
        unsigned ANS4       :1;
        
        unsigned ANS5       :1;
        
        unsigned ANS6       :1;
        
    };
        
    uint8_t d8;
        
} ANSELbits_t;
        
extern volatile ANSELbits_t ANSELbits __at(0x09B);


        BTW if the struct has to fit in 1 byte (8 bits) this declaration is wrong and uint_t type should be used instead.






        share|improve this answer













        It does not make any difference if you wrap and I suppose that someone has forgoten to add another member (or did not copy-paste everything) as in the declaration below. No warnings will be suppressed. 



        typedef union {
        
    struct {
        
        unsigned ANS0       :1;
        
        unsigned ANS1       :1;
        
        unsigned ANS2       :1;
        
        unsigned ANS3       :1;
        
        unsigned ANS4       :1;
        
        unsigned ANS5       :1;
        
        unsigned ANS6       :1;
        
    };
        
    uint8_t d8;
        
} ANSELbits_t;
        
extern volatile ANSELbits_t ANSELbits __at(0x09B);


        BTW if the struct has to fit in 1 byte (8 bits) this declaration is wrong and uint_t type should be used instead.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 3 hours ago









        P__J__P__J__

        10.1k2723




        10.1k2723






























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