c - Discards qualifiers with memcpy -


why following code give me "discards qualifiers" warning?

double* const a[7]; memcpy(a,b,sizeof(double*)*7);

the error i'm getting apple llvm version 6.1.0 (clang-602.0.53) (based on llvm 3.6.0svn) 

warning: passing 'double *const [7]' parameter of type 'void *' discards qualifiers

edit:

bonus question. why restrict keyword not work either?

double* restrict a[7]; memcpy(a,b,sizeof(double*)*7);

edit 2:

i'm asking question, because want a const restrict pointer. can result code:

double* const restrict a[7] = {b[0], b[2], ... b[7]};

is stupid thing do?

you're trying pass const pointer function expects non const pointer.

edit:

more specifically, a array contents const pointers doubles. if attempted a[0] = b[0] (assuming b defined double *b[7] or similar) compiler error. results same if had const char a[7] , attempted a[0] = 'x'.

calling memcpy allows perform operation above otherwise prohibited.

in case of restrict, tells compiler memory pointed given pointer addressed given pointer. allows compiler perform optimizations.

from wikipedia:

it says lifetime of pointer, or value directly derived (such pointer + 1) used access object points.

since memcpy not expecting restrict * guarantee lost, why warning. in general, bypassing restrict can lead unpredictable behavior.


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