gcc struct assignment warning missing braces around initializer

Question

The following

#include <net/route.h>
#include <netinet/in.h>
int main(){
  struct rtentry e={};
  *((struct sockaddr_in*)(&(e.rt_genmask)))=(struct sockaddr_in){.sin_family=AF_INET,.sin_port=0,.sin_addr=INADDR_ANY};
  return 0;
}

gives the warning (gcc -std=c11 -Wall test.c)

test.c:5:45: warning: missing braces around initializer [-Wmissing-braces]
    5 |   *((struct sockaddr_in*)(&(e.rt_genmask)))=(struct sockaddr_in){.sin_family=AF_INET,.sin_port=0,.sin_addr=INADDR_ANY};
      |                                             ^
                                         ^

Why? How may I fix the code?

It copmiles if I use C compiler. godbolt.org/z/xKETe3 . But if I switch to C++ compiler, there is error not warning. — Louis Go
– Louis Go, Commented Aug 21, 2020 at 5:07
.sin_addr={INADDR_ANY} - sin_addr is a composite. This will also make it c++ friendly. — oakad
– oakad, Commented Aug 21, 2020 at 6:45

Louis Go · Accepted Answer · 2020-08-21 07:23:50Z

1

INADDR_ANY is macro defined.
in_addr is a struct for wrapping value. You need to use .sin_addr={INADDR_ANY} to make sure it's initialized by value INADDR_ANY.

           struct sockaddr_in {
               sa_family_t    sin_family; /* address family: AF_INET */
               in_port_t      sin_port;   /* port in network byte order */
               struct in_addr sin_addr;   /* internet address */
           };

           /* Internet address. */
           struct in_addr {
               uint32_t       s_addr;     /* address in network byte order */
           };

Fixed: https://godbolt.org/z/xM8r1E

#include <net/route.h>

int main(void) {
    struct rtentry e={};
    *(struct sockaddr_in*)(&(e.rt_dst))=(struct sockaddr_in){
    .sin_family=AF_INET,
    .sin_port=0,
    .sin_addr={INADDR_ANY}
    };
    return 0;
}

edited Aug 21, 2020 at 7:23

answered Aug 21, 2020 at 6:49

Louis Go

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