What does request for member in something not a structure or union mean

Question

Is there an easy explanation for what this error means   request for member           in something not a structure or union   I ve encountered it several times in the time that I ve been learning C  but I haven t got a clue as to what it means

User · Answer

I saw this when I was trying to access the members.

My struct was this:

struct test { 
    int a;
    int b;
};
    
struct test testvar;

Normally we access structure members as

testvar.a;
testvar.b;

I mistook testvar to be a pointer and did this.

testvar->a;

That's when I saw this error.

request for member ‘a’ in something not a structure or union

User · Answer

You are trying to access a member of a structure  but in something that is not a structure  For example   struct       int a      int b    foo  int fum  fum d   5

User · Answer

I have enumerated possibly all cases where this error may occur in code and its comments below  Please add to it  if you come across more cases    include lt stdio h gt   include lt malloc h gt   typedef struct AStruct TypedefedStruct   struct AStruct       int member      void main             Case 1                                                                                              Use  - gt   operator to access structure member with structure pointer  instead         of dot     operator              struct AStruct  aStructObjPtr    struct AStruct   malloc sizeof struct AStruct          aStructObjPtr member   1         Error  request for member    member    in something not                                          a structure or union                                           It should be as below      aStructObjPtr- gt member   1      printf   d  aStructObjPtr- gt member     1           Case 2                                                                                              We can use dot     operator with struct variable to access its members  but          not with with struct pointer  But we have to ensure we dont forget to wrap          pointer variable inside brackets                aStructObjPtr member   2        Error  should be as below        aStructObjPtr  member   2      printf   d    aStructObjPtr  member     2          Case 3                                                                                             Use  - gt   operator to access structure member with typedefed structure pointer          instead of dot     operator              TypedefedStruct  typedefStructObjPtr    TypedefedStruct   malloc sizeof TypedefedStruct          typedefStructObjPtr member 3     Error  should be as below      typedefStructObjPtr- gt member 3      printf   d  typedefStructObjPtr- gt member      3           Case 4                                                                                              We can use dot     operator with struct variable to access its members  but          not with with struct pointer  But we have to ensure we dont forget to wrap          pointer variable inside brackets                typedefStructObjPtr member   4     Error  should be as below            typedefStructObjPtr  member 4      printf   d    typedefStructObjPtr  member      4          Case 5                                                                                            We have to be extra carefull when dealing with pointer to pointers to         ensure that we follow all above rules         We need to be double carefull while putting brackets around pointers                5 1  Access via struct ptrptr and  - gt      struct AStruct   aStructObjPtrPtr    amp aStructObjPtr         aStructObjPtrPtr- gt member   5     Error  should be as below        aStructObjPtrPtr - gt member   5      printf   d    aStructObjPtrPtr - gt member     5        5 2  Access via struct ptrptr and           aStructObjPtrPtr member   6     Error  should be as below         aStructObjPtrPtr  member   6      printf   d     aStructObjPtrPtr  member     6        5 3  Access via typedefed strct ptrptr and - gt      TypedefedStruct   typedefStructObjPtrPtr    amp typedefStructObjPtr         typedefStructObjPtrPtr- gt member   7     Error  should be as below        typedefStructObjPtrPtr - gt member   7      printf   d    typedefStructObjPtrPtr - gt member     7        5 4  Access via typedefed strct ptrptr and           typedefStructObjPtrPtr- gt member   8     Error  should be as below         typedefStructObjPtrPtr  member   8      printf   d     typedefStructObjPtrPtr  member     8        5 5  All cases 5 1 to 5 4 will fail if you include incorrect number of              Below are examples of such usage of incorrect number    correspnding            to int values assigned to them         aStructObjPtrPtr - gt member   5    Error         aStructObjPtrPtr  member   6    Error         typedefStructObjPtrPtr - gt member   7    Error          typedefStructObjPtrPtr  member   8    Error     The underlying ideas are straight    Use   with structure variable    Cases 2 and 4  Use - gt  with pointer to structure   Cases 1 and 3  If you reach structure variable or pointer to structure variable by following pointer  then wrap the pointer inside bracket    ptr   and   ptr - gt  vs  ptr  and  ptr- gt    All cases except case 1  If you are reaching by following pointers  ensure you have correctly reached pointer to struct or struct whichever is desired   Case 5  especially 5 5

User · Answer

It may means that you forgot include a header file that define this struct union  For example   foo h file   typedef union       struct               uint8 t FIFO BYTES AVAILABLE      4          uint8 t STATE                     3          uint8 t CHIP RDY                  1             uint8 t status    RF CHIP STATUS t   RF CHIP STATUS t getStatus      main c file         if  getStatus   CHIP RDY     This will generate the error  you must add the   include  foo h

User · Answer

It may also happen in the following case   eg  if we consider the push function of a stack   typedef struct stack       int a 20       int head   stack   void push stack   s        int data      printf  Enter data         scanf   d   amp   s- gt a    s- gt head        this is where the error is      main         stack  s      s  stack   calloc 1 sizeof stack        s- gt head -1      push  amp s       return 0      The error is in the push function and in the commented line  The pointer s has to be included within the parentheses  The correct code   scanf   d   amp     s - gt a     s - gt head

User · Answer

It also happens if you re trying to access an instance when you have a pointer  and vice versa   struct foo     int x  y  z      struct foo a   b    amp a   b x   12      This will generate the error  should be b- gt x or   b  x      As pointed out in a comment  this can be made excruciating if someone goes and typedefs a pointer  i e  includes the   in a typedef  like so   typedef struct foo  Foo    Because then you get code that looks like it s dealing with instances  when in fact it s dealing with pointers   Foo a foo   get a brand new foo    a foo- gt field   FANTASTIC VALUE    Note how the above looks as if it should be written a foo field  but that would fail since Foo is a pointer to struct  I strongly recommend against typedef ed pointers in C  Pointers are important  don t hide your asterisks  Let them shine

User · Answer

can also appear if   struct foo     int x  int y  int z  foo    foo x 12   instead of   struct foo     int x  int y  int z   foo    foo x 12

[c] What does "request for member '*******' in something not a structure or union" mean?

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