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cmallocfree

Is casting free() argument to void * neccessary?

Is it neccessary to cast the value passed to free() to a void pointer in this code snippet?

free((void *) np->defn);

np is a struct in a linked list and defn is a char *.

Solution

  • C11 : 7.22.3.3 The free function

    Synopsis 

    #include <stdlib.h>
void free(void *ptr);

    It means that it can take pointer to any type (void * is a generic pointer type). In general no need to cast the argument to void *, but in cases like 

    int const *a = malloc(sizeof(int));
free(a);

    compiler will generate a waring 

    test.c: In function 'main':
test.c:33:7: warning: passing argument 1 of 'free' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  free(a);
       ^
In file included from test.c:2:0:
/usr/include/stdlib.h:151:7: note: expected 'void *' but argument is of type 'const int *'
 void  free(void *);
       ^

    To suppress this warning a cast is needed 

     free((void *)a);