Return a struct from a function in C

Question

Today I was teaching a couple of friends how to use C structs  One of them asked if you could return a struct from a function  to which I replied   quot No  You d return pointers to dynamically malloced structs instead  quot  Coming from someone who primarily does C    I was expecting not be able to return structs by values  In C   you can overload the operator   for your objects and makes complete sense to have a function to return your object by value  In C  however  you do not have that option and so it got me thinking what the compiler is actually doing  Consider the following  struct MyObj      double x  y      struct MyObj foo        struct MyObj a           a x   10      a y   10           return a             int main           struct MyObj a           a   foo          This DOES work     struct b   a     This does not work            return 0         I understand why struct b   a  should not work -- you cannot overload operator   for your data type  How is it that a   foo    compiles fine  Does it mean something other than struct b   a   Maybe the question to ask is  What exactly does the return statement in conjunction to   sign do

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When making a call such as a   foo     the compiler might push the address of the result structure on the stack and passes it as a  hidden  pointer to the foo   function  Effectively  it could become something like   void foo MyObj  r        struct MyObj a                  r   a     foo  amp a     However  the exact implementation of this is dependent on the compiler and or platform  As Carl Norum notes  if the structure is small enough  it might even be passed back completely in a register

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There is no issue in passing back a struct  It will be passed by value But  what if the struct contains any member which has a address of a local variable struct emp       int id      char  name      struct emp get         char  name    quot John quot        struct emp e1    100  name        return  e1      int main          struct emp e2   get         printf  quot  s n quot   e2 name      Now  here e1 name contains a memory address local to the function get    Once get   returns  the local address for name would have been freed up  SO  in the caller if we try to access that address  it may cause segmentation fault  as we are trying a freed address  That is bad   Where as the e1 id will be perfectly valid as its value will be copied to e2 id So  we should always try to avoid returning local memory addresses of a function  Anything malloced can be returned as and when wanted

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You can return a structure from a function  or use the   operator  without any problems   It s a well-defined part of the language   The only problem with struct b   a is that you didn t provide a complete type   struct MyObj b   a will work just fine   You can pass structures to functions as well - a structure is exactly the same as any built-in type for purposes of parameter passing  return values  and assignment   Here s a simple demonstration program that does all three - passes a structure as a parameter  returns a structure from a function  and uses structures in assignment statements    include  lt stdio h gt   struct a      int i      struct a f struct a x       struct a r   x     return r     int main void       struct a x     12       struct a y   f x      printf   d n   y i      return 0      The next example is pretty much exactly the same  but uses the built-in int type for demonstration purposes   The two programs have the same behaviour with respect to pass-by-value for parameter passing  assignment  etc     include  lt stdio h gt   int f int x       int r   x    return r     int main void      int x   12    int y   f x     printf   d n   y     return 0

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struct var e2 address pushed as arg to callee stack and values gets assigned there  In fact  get   returns e2 s address in eax reg  This works like call by reference

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yes  it is possible we can pass structure and return structure as well  You were right but you actually did not pass the data type which should be like this struct MyObj b   a    Actually I also came to know when I was trying to find out a better solution to return more than one values for function without using pointer or global variable   Now below is the example for the same  which calculate the deviation of a student marks about average    include lt stdio h gt  struct marks      int maths      int physics      int chem      struct marks deviation struct marks student1   struct marks student2     int main         struct marks student      student maths  87      student chem   67      student physics 96       struct marks avg      avg maths  55      avg chem   45      avg physics 34        struct marks dev      struct marks dev  deviation student  avg        printf   d  d  d   dev maths dev chem dev physics        return 0      struct marks deviation struct marks student   struct marks student2        struct marks dev       dev maths   student maths-student2 maths      dev chem   student chem-student2 chem      dev physics   student physics-student2 physics        return dev

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struct emp       int id      char  name      struct emp get         char  name    John        struct emp e1    100  name        return  e1      int main          struct emp e2   get         printf   s n   e2 name       works fine with newer versions of compilers  Just like id  content of the name gets copied to the assigned structure variable

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You can assign structs in C  a   b   is valid syntax   You simply left off part of the type -- the struct tag -- in your line that doesn t work

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The struct b line doesn t work because it s a syntax error   If you expand it out to include the type it will work just fine  struct MyObj b   a      Runs fine   What C is doing here is essentially a memcpy from the source struct to the destination   This is true for both assignment and return of struct values  and really every other value in C

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As far as I can remember  the first versions of C only allowed to return a value that could fit into a processor register  which means that you could only return a pointer to a struct  The same restriction applied to function arguments   More recent versions allow to pass around larger data objects like structs  I think this feature was already common during the eighties or early nineties   Arrays  however  can still be passed and returned only as pointers

[c] Return a `struct` from a function in C

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