dump route table on macos

test.c

// stolen from https://stackoverflow.com/a/11265543

#include <stdio.h>
#include <stdlib.h>
#include <err.h>

#include <sys/param.h>
#include <sys/sysctl.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <net/route.h>

/* Darwin doesn't define this for some very odd reason */
#ifndef SA_SIZE
# define SA_SIZE(sa)                        \
    (  (!(sa) || ((struct sockaddr *)(sa))->sa_len == 0) ?  \
           sizeof(long)     :               \
           1 + ( (((struct sockaddr *)(sa))->sa_len - 1) | (sizeof(long) - 1) ) )
#endif


static void
ntreestuff(void)
{
    size_t needed;
    int mib[6];
    char *buf, *next, *lim;
    struct rt_msghdr *rtm;
    struct sockaddr *sa;
    struct sockaddr_in *sockin;
    char line[MAXHOSTNAMELEN];

    mib[0] = CTL_NET;
    mib[1] = PF_ROUTE;
    mib[2] = 0;
    mib[3] = 0;
    mib[4] = NET_RT_DUMP;
    mib[5] = 0;
    if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0) {
        err(1, "sysctl: net.route.0.0.dump estimate");
    }

    if ((buf = (char *)malloc(needed)) == NULL) {
        errx(2, "malloc(%lu)", (unsigned long)needed);
    }   
    if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0) {
        err(1, "sysctl: net.route.0.0.dump");
    }
    lim  = buf + needed;
    for (next = buf; next < lim; next += rtm->rtm_msglen) {
        rtm = (struct rt_msghdr *)next;
        sa = (struct sockaddr *)(rtm + 1);
        sa = (struct sockaddr *)(SA_SIZE(sa) + (char *)sa);
        sockin = (struct sockaddr_in *)sa;
        inet_ntop(AF_INET, &sockin->sin_addr.s_addr, line, sizeof(line) - 1);
        printf("defaultrouter=%s\n", line);
    }

    free(buf);
}

int
main(int argc __unused, char *argv[] __unused)
{
    ntreestuff();
    return (0);
}

test2.c

// stolen from https://stackoverflow.com/a/8095530

#include <stdio.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <sys/sysctl.h>
#include "getgateway.h"
#include "route.h"
#include <net/if.h>
#include <string.h>

#define CTL_NET         4               /* network, see socket.h */


#if defined(BSD) || defined(__APPLE__)

#define ROUNDUP(a) \
((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))

int getdefaultgateway(in_addr_t * addr)
{
    int mib[] = {CTL_NET, PF_ROUTE, 0, AF_INET,
        NET_RT_FLAGS, RTF_GATEWAY};
    size_t l;
    char * buf, * p;
    struct rt_msghdr * rt;
    struct sockaddr * sa;
    struct sockaddr * sa_tab[RTAX_MAX];
    int i;
    int r = -1;
    if(sysctl(mib, sizeof(mib)/sizeof(int), 0, &l, 0, 0) < 0) {
        return -1;
    }
    if(l>0) {
        buf = malloc(l);
        if(sysctl(mib, sizeof(mib)/sizeof(int), buf, &l, 0, 0) < 0) {
            return -1;
        }
        for(p=buf; p<buf+l; p+=rt->rtm_msglen) {
            rt = (struct rt_msghdr *)p;
            sa = (struct sockaddr *)(rt + 1);
            for(i=0; i<RTAX_MAX; i++) {
                if(rt->rtm_addrs & (1 << i)) {
                    sa_tab[i] = sa;
                    sa = (struct sockaddr *)((char *)sa + ROUNDUP(sa->sa_len));
                } else {
                    sa_tab[i] = NULL;
                }
            }

            if( ((rt->rtm_addrs & (RTA_DST|RTA_GATEWAY)) == (RTA_DST|RTA_GATEWAY))
               && sa_tab[RTAX_DST]->sa_family == AF_INET
               && sa_tab[RTAX_GATEWAY]->sa_family == AF_INET) {


                if(((struct sockaddr_in *)sa_tab[RTAX_DST])->sin_addr.s_addr == 0) {
                        char ifName[128];
                        if_indextoname(rt->rtm_index,ifName);

                        if(strcmp("en0",ifName)==0){

                                *addr = ((struct sockaddr_in *)(sa_tab[RTAX_GATEWAY]))->sin_addr.s_addr;
                                r = 0;                        
                        }
                }
            }
        }
        free(buf);
    }
    return r;
}
#endif

test3.c

#include <stdio.h>
#include <stdlib.h>

#include <sys/param.h>
#include <sys/sysctl.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <net/route.h>

/* Darwin doesn't define this for some very odd reason */
#ifndef SA_SIZE
# define SA_SIZE(sa)                        \
    (  (!(sa) || ((struct sockaddr *)(sa))->sa_len == 0) ?  \
           sizeof(long)     :               \
           1 + ( (((struct sockaddr *)(sa))->sa_len - 1) | (sizeof(long) - 1) ) )
#endif

static int ntreestuff(void)
{
    int mib[6];
    mib[0] = CTL_NET;
    mib[1] = PF_ROUTE;
    mib[2] = 0;
    mib[3] = 0;
    mib[4] = NET_RT_DUMP;
    mib[5] = 0;

    size_t needed;
    if ( sysctl( mib, 6, NULL, &needed, NULL, 0 ) < 0 )
    {
        return -1;
    }
    char * buffer;
    if ( ( buffer = (char *)( malloc( needed ) ) ) == NULL )
    {
        return -1;
    }
    if ( sysctl( mib, 6, buffer, &needed, NULL, 0 ) < 0 )
    {
        return -1;
    }

    struct rt_msghdr * rtm;
    int count = 0;
    for ( char * i = buffer; i < buffer + needed; i += rtm->rtm_msglen )
    {
        rtm = (struct rt_msghdr *)( i );

        if ( rtm->rtm_type != RTM_GET )
            continue;
        if ( !( rtm->rtm_flags & RTF_UP ) )
            continue;
        if ( !( rtm->rtm_flags & RTF_GATEWAY ) )
            continue;
        if ( !( rtm->rtm_addrs & ( 1 << RTAX_DST ) ) )
            continue;
        if ( !( rtm->rtm_addrs & ( 1 << RTAX_NETMASK ) ) )
            continue;
        printf( "flags: %x\n", rtm->rtm_flags );
        struct sockaddr * sa = (struct sockaddr *)( (char *)( rtm ) + sizeof( struct rt_msghdr ) );
        struct sockaddr_in * sockin;

        char line[MAXHOSTNAMELEN];

        if ( rtm->rtm_addrs & ( 1 << RTAX_DST ) )
        {
            sockin = (struct sockaddr_in *)( sa + RTAX_DST );
            inet_ntop( AF_INET, &sockin->sin_addr.s_addr, line, sizeof( line ) - 1 );
            printf( "dest: %s\n", line );
        }

        if ( rtm->rtm_addrs & ( 1 << RTAX_NETMASK ) )
        {
            sockin = (struct sockaddr_in *)( sa + RTAX_NETMASK );
            inet_ntop( AF_INET, &sockin->sin_addr.s_addr, line, sizeof( line ) - 1 );
            printf( "netmask: %s\n", line );
        }

        if ( rtm->rtm_addrs & ( 1 << RTAX_GATEWAY ) )
        {
            sockin = (struct sockaddr_in *)( sa + RTAX_GATEWAY );
            inet_ntop( AF_INET, &sockin->sin_addr.s_addr, line, sizeof( line ) - 1 );
            printf( "gateway: %s\n", line );
        }

        printf( "iface: %hu\n", rtm->rtm_index );
        count++;
    }

    printf( "count: %d\n", count );

    free( buffer );

    return 0;
}

int main( int argc, char ** argv )
{
    (void) argc;
    (void) argv;
    ntreestuff();
    return 0;
}

@Wangzihaooooo 0.0.0.0 is the default value to my understanding. Mac has many network devices these days and not all of them will have a gateway configured. I think it can be assumed that 0.0.0.0 is not a valid gateway address, and if that value is encountered in the sockaddr struct, then it is essentially unset.