How do I return multiple values from a function in C

Question

If I have a function that produces a result int and a result string  how do I return them both from a function   As far as I can tell I can only return one thing  as determined by the type preceding the function name

User · Answer

Since one of your result types is a string (and you're using C, not C++), I recommend passing pointers as output parameters. Use:

void foo(int *a, char *s, int size);

and call it like this:

int a;
char *s = (char *)malloc(100); /* I never know how much to allocate :) */
foo(&a, s, 100);

In general, prefer to do the allocation in the calling function, not inside the function itself, so that you can be as open as possible for different allocation strategies.

User · Answer

By passing parameters by reference to function   Examples    void incInt int  y            y         Increase the value of  x   in main  by one       Also by using global variables  but it is not recommended   Example   int a 0   void main void          Anything you want to code

User · Answer

Create a struct and set two values inside and return the struct variable   struct result       int a      char  string      You have to allocate space for the char   in your program

User · Answer

One approach is to use macros  Place this in a header file multitype h   include  lt stdlib h gt                                    HELPER MACROS                                        typeof   V  abbreviation      define TOF V    typeof   V      Expand variables list to list of typeof and variable names      define TO3  0  1  2  3  TOF  0  v0  TOF  1  v1  TOF  2  v2  TOF  3  v3   define TO2  0  1  2     TOF  0  v0  TOF  1  v1  TOF  2  v2   define TO1  0  1        TOF  0  v0  TOF  1  v1   define TO0  0           TOF  0  v0    define TO   0  1  2  3 TO MACRO      TO MACRO   define TO      TO    VA ARGS   TO3 TO2 TO1 TO0    VA ARGS        Assign to multitype      define MTA3  0  1  2  3   0   mtr v0   1   mtr v1   2   mtr v2   3   mtr v3   define MTA2  0  1  2      0   mtr v0   1   mtr v1   2   mtr v2   define MTA1  0  1         0   mtr v0   1   mtr v1   define MTA0  0            0   mtr v0    define MTA   0  1  2  3 MTA MACRO      MTA MACRO   define MTA      MTA    VA ARGS   MTA3 MTA2 MTA1 MTA0    VA ARGS        Return multitype if multiple arguments  return normally if only one      define MTR1                                                                        typedef struct mtr s                                                              TO   VA ARGS                                                                    mtr t                                                                         mtr t  mtr   malloc sizeof mtr t                                                 mtr    mtr t    VA ARGS                                                        return mtr                                                                        define MTR0  0  return  0    define MTR   0  1  2  3 MTR MACRO      MTR MACRO                                    API MACROS                                        Declare return type before function     typedef void  multitype    define multitype      multitype     Assign return values to variables      define let                                                                       for int mti   0   mti                                                             for multitype mt  mti  lt  2  mti                                                    if mti                                                                            typedef struct mtr s                                                              TO   VA ARGS                                                                    mtr t                                                                         mtr t mtr     mtr t  mt                                                         MTA   VA ARGS                                                                   free mt                                                                         else                                                                            mt     Return      define RETURN      MTR    VA ARGS   MTR1 MTR1 MTR1 MTR0    VA ARGS      This makes it possible to return up to four variables from a function and assign them to up to four variables  As an example  you can use them like this   multitype  int float double  fun         int a   55      float b   3 9      double c   24 15       RETURN  a b c      int main int argc  char  argv          int x      float y      double z       let  x y z    fun         printf    d   f   g n    x  y  z        return 0      This is what it prints    55  3 9  24 15    The solution may not be as portable because it requires C99 or later for variadic macros and for-statement variable declarations  But I think it was interesting enough to post here  Another issue is that the compiler will not warn you if you assign them the wrong values  so you have to be careful   Additional examples  and a stack-based version of the code using unions  are available at my github repository

User · Answer

Option 1  Declare a struct with an int and string and return a struct variable   struct foo        int bar1   char bar2 MAX       struct foo fun      struct foo fooObj        return fooObj      Option 2  You can pass one of the two via pointer and make changes to the actual parameter through the pointer and return the other as usual   int fun char   param     int bar        strcpy  param           return bar      or   char  fun int  param     char  str      malloc suitably          strcpy str            param      some value     return str      Option 3  Similar to the option 2  You can pass both via pointer and return nothing from the function   void fun char   param1 int  param2     strcpy  param1            param2      some calculated value

User · Answer

Use pointers as your function parameters  Then use them to return multiple value

User · Answer

Two different approaches    Pass in your return values by pointer  and modify them inside the function  You declare your function as void  but it s returning via the values passed in as pointers   Define a struct that aggregates your return values     I think that  1 is a little more obvious about what s going on  although it can get tedious if you have too many return values  In that case  option  2 works fairly well  although there s some mental overhead involved in making specialized structs for this purpose

User · Answer

I don t know what your string is  but I m going to assume that it manages its own memory   You have two solutions   1  Return a struct which contains all the types you need   struct Tuple       int a      string b      struct Tuple getPair         Tuple r     1  getString          return r     void foo         struct Tuple t   getPair        2  Use pointers to pass out values   void getPair int  a  string  b           Check that these are not pointing to NULL     assert a       assert b        a   1       b   getString       void foo         int a  b      getPair  amp a   amp b       Which one you choose to use depends largely on personal preference as to whatever semantics you like more

[c] How do I return multiple values from a function in C?

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