Copying one structure to another

Question

I know that I can copy the structure member by member  instead of that can I do a memcpy on structures   Is it advisable to do so   In my structure  I have a string also as member which I have to copy to another structure having the same member  How do I do that

User · Answer

You can use a struct to read write into a file. You do not need to cast it as a `char*. Struct size will also be preserved. (This point is not closest to the topic but guess it: behaving on hard memory is often similar to RAM one.)
To move (to & from) a single string field you must use strncpy and a transient string buffer '\0' terminating. Somewhere you must remember the length of the record string field.

To move other fields you can use the dot notation, ex.: NodeB->one=intvar; floatvar2=(NodeA->insidebisnode_subvar).myfl;

struct mynode {
    int one;
    int two;
    char txt3[3];
    struct{char txt2[6];}txt2fi;
    struct insidenode{
        char txt[8];
        long int myl;
        void * mypointer;
        size_t myst;
        long long myll;
         } insidenode_subvar;
    struct insidebisnode{
        float myfl;
         } insidebisnode_subvar;
} mynode_subvar;

typedef struct mynode* Node;

...(main)
Node NodeA=malloc...
Node NodeB=malloc...

You can embed each string into a structs that fit it, to evade point-2 and behave like Cobol: NodeB->txt2fi=NodeA->txt2fi ...but you will still need of a transient string plus one strncpy as mentioned at point-2 for scanf, printf otherwise an operator longer input (shorter), would have not be truncated (by spaces padded).
(NodeB->insidenode_subvar).mypointer=(NodeA->insidenode_subvar).mypointer will create a pointer alias.
NodeB.txt3=NodeA.txt3 causes the compiler to reject: error: incompatible types when assigning to type ‘char[3]’ from type ‘char *’
point-4 works only because NodeB->txt2fi & NodeA->txt2fi belong to the same typedef !!

A correct and simple answer to this topic I found at In C, why can't I assign a string to a char array after it's declared? "Arrays (also of chars) are second-class citizens in C"!!!

User · Answer

In C  memcpy is only foolishly risky  As long as you get all three parameters exactly right  none of the struct members are pointers  or  you explicitly intend to do a shallow copy  and there aren t large alignment gaps in the struct that memcpy is going to waste time looping through  or performance never matters   then by all means  memcpy  You gain nothing except code that is harder to read  fragile to future changes and has to be hand-verified in code reviews  because the compiler can t   but hey yeah sure why not   In C    we advance to the ludicrously risky  You may have members of types which are not safely memcpyable  like std  string  which will cause your receiving struct to become a dangerous weapon  randomly corrupting memory whenever used  You may get surprises involving virtual functions when emulating slice-copies  The optimizer  which can do wondrous things for you because it has a guarantee of full type knowledge when it compiles    can do nothing for your memcpy call   In C   there s a rule of thumb - if you see memcpy or memset  something s wrong  There are rare cases when this is not true  but they do not involve structs  You use memcpy when  and only when  you have reason to blindly copy bytes    Assignment on the other hand is simple to read  checks correctness at compile time and then intelligently moves values at runtime  There is no downside

User · Answer

You can memcpy structs  or you can just assign them like any other value   struct  int a  b   c  d  c a   c b   10  d   c

User · Answer

Since C90  you can simply use   dest struct   source struct    as long as the string is memorized inside an array   struct xxx       char theString 100        Otherwise  if it s a pointer  you ll need to copy it by hand   struct xxx       char  theString      dest struct   source struct  dest struct theString   malloc strlen source struct theString    1   strcpy dest struct theString  source struct theString

User · Answer

Copying by plain assignment is best  since it s shorter  easier to read  and has a higher level of abstraction  Instead of saying  to the human reader of the code   copy these bits from here to there   and requiring the reader to think about the size argument to the copy  you re just doing a plain assignment   copy this value from here to here    There can be no hesitation about whether or not the size is correct   Also  if the structure is heavily padded  assignment might make the compiler emit something more efficient  since it doesn t have to copy the padding  and it knows where it is   but mempcy   doesn t so it will always copy the exact number of bytes you tell it to copy   If your string is an actual array  i e    struct     char string 32     size t len    a  b   strcpy a string   hello    a len   strlen a string     Then you can still use plain assignment   b   a    To get a complete copy  For variable-length data modelled like this though  this is not the most efficient way to do the copy since the entire array will always be copied   Beware though  that copying structs that contain pointers to heap-allocated memory can be a bit dangerous  since by doing so you re aliasing the pointer  and typically making it ambiguous who owns the pointer after the copying operation   For these situations a  deep copy  is really the only choice  and that needs to go in a function

User · Answer

If the structures are of compatible types  yes  you can  with something like   memcpy  dest struct  source struct  sizeof   dest struct      The only thing you need to be aware of is that this is a shallow copy  In other words  if you have a char   pointing to a specific string  both structures will point to the same string   And changing the contents of one of those string fields  the data that the char   points to  not the char   itself  will change the other as well   If you want a easy copy without having to manually do each field but with the added bonus of non-shallow string copies  use strdup   memcpy  dest struct  source struct  sizeof   dest struct    dest struct- gt strptr   strdup  source struct- gt strptr     This will copy the entire contents of the structure  then deep-copy the string  effectively giving a separate string to each structure   And  if your C implementation doesn t have a strdup  it s not part of the ISO standard   get one from here

User · Answer

You can use the following solution to accomplish your goal   struct student        char name 20       char country 20      void main         struct student S   Wolverine   America        struct student X      X S      printf   s s  X name X country

[c] Copying one structure to another

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