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

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->strptr = strdup (source_struct->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

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

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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

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

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  amp  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- gt one intvar   floatvar2  NodeA- gt 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- gt txt2fi NodeA- gt 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- gt insidenode subvar  mypointer  NodeA- gt 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- gt txt2fi  amp  NodeA- gt txt2fi belong to the same typedef     A correct and simple answer to this topic I found at  In C  why can  39 t I assign a string to a char array after it  39 s declared   Arrays  also of chars  are second-class citizens in C

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

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

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