stm32 - C Union to copy memory -

i have following legacy code:

#define uint8 unsigned char #define uint16 unsigned short #define uint32 unsigned long typedef union {     struct     {         uint16  r1: 1;         uint16  s1: 1;         uint16  s2: 1;         uint16  d1: 1;         uint16  i1: 1;         uint16  s3: 1;         uint16  s4: 1;         uint16  s5: 1;         uint16  s6: 1;         uint16  p1: 1;         uint16  f1: 1;         uint16  i2: 1;         uint16  r2: 4;     }bb;     uint16  v16; }test_union11;  typedef struct                    /* memory index */    {                                 /* ++++++++++++ */      // write config 01-32   uint16       i1;          /*    0   */            uint16       i2;          /*    2   */            // faults   uint16       o3;          /*    4   */           uint16       o4;          /*    6   */          uint16       o5;          /*    8   */            uint16       o6;          /*   10   */            // values 01-08   uint8        o7;          /*   12   */            uint8        o8;          /*   13   */           uint8        o9;          /*   14   */            uint8        o10;         /*   15   */            uint8        o11;         /*   16   */            uint8        o12;         /*   17   */            uint8        o13;         /*   18   */            uint8        o14;         /*   19   */           // values 09-12   uint16       o15;         /*   20   */            uint16       o16;         /*   22   */            uint16       o17;         /*   24   */           uint16       o18;         /*   26   */           // values 13-14   uint32       o19;        /*   28   */           uint32       o20;        /*   32   */            uint8        octstr[8]; } test_str11;  test_str11 test_str1; test_union11 test_union; int test_memcpy(void) {       test_str1.o15 = 1500;     test_union.v16 = test_str1.i1;     memcpy(&test_union,&test_str1,sizeof(test_union11)); }  int main(void) {     test_memcpy();     printf("test pass!\n");     return 0; } 

is idea behind line test_union.v16 = test_str1.i1 test_union.bb nested structure filled data structure "test_str1"? reason neither after line nor after memcpy happened. why? , meaning of numbers in nested structure inside union?

i tested case when there no such numbers smaller union , structure , memcpy worked out, though trick first element assignment didn't. syntax that:

un = (*(test_union1*)&st); 

also worked out second test, , didn't code presented. difference between 2 expressions? , test_union.v16 = test_str1.i1 copy i1 variable or many bytes test_union.bb can hold?

the code should work on stm32f10x embedded platform, though it's not working in visual studio on pc @ moment. therefore pure c problem. thanks!

you have not assigned test_str.i1. if do, think program work. idea of union provide easy way @ 16 bits in unsigned int v16. however, platform dependant code, because int may not 16 bits (it 32 bits).

the size of union size of largest member. however, not sure alignment rules size mis-matches.

finally, think best way @ bits using bit-wise operators &, | etc.