how to use memcpy to initialize a char pointer in struct

Question

this is how I define struct.

struct urlFormat
{
      int port;
      char *host;
      char *path;
      int cat;
      char *status;
};

this is how I initialize strcut and allocate the space for the pointer.

struct urlFormat *res;
res = malloc(sizeof(struct urlFormat));

when I used memcpy() function, it reported segmentation fault.

char *ptr1 = (char *)url;
  int len = strlen(ptr1);
  memcpy(res->host, ptr1, len);

I don't know how to solve it.

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– Gaurang Dave, Commented Apr 17, 2018 at 3:08
You don't allocate any memory for host to point to; you have only allocated memory for the struct. — ad absurdum
– ad absurdum, Commented Apr 17, 2018 at 3:09

John3136 · Accepted Answer · 2018-04-17 03:14:43Z

3

res->host is just a pointer (that is not pointing to anything yet).

Until res->host is pointing to some valid memory you can't memcpy to it.

You can either malloc some memory res->host = malloc(len + 1);(+1 for the 0 terminator and sizeof(char) is always 1 so omit it) or in this case just use res->host = strdup(ptr1);

edited Apr 17, 2018 at 3:14

answered Apr 17, 2018 at 3:11

John3136

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1 Comment

John3136 Over a year ago

Comment now deleted but OP asked how to copy part of a string: You need to allocate space for as much as you want to copy and then copy that amount. strncpy would do it or you could use strdup and just put a 0 in the string after the last character that you want. This wastes a bit of memory but is completely legal. It all depends on what you really want to do.

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