offsetof() - stddef.h

ifconfig.c를 들여다 보고 있자니 신기한 녀석들이 나타나시었다.

struct arg1opt {     const char *name;     unsigned short selector;     unsigned short ifr_offset; }; #define ifreq_offsetof(x)  offsetof(struct ifreq, x) static const struct arg1opt Arg1Opt[] = {     {"SIOCSIFMETRIC",  SIOCSIFMETRIC,  ifreq_offsetof(ifr_metric)},     {"SIOCSIFMTU",     SIOCSIFMTU,     ifreq_offsetof(ifr_mtu)},     {"SIOCSIFTXQLEN",  SIOCSIFTXQLEN,  ifreq_offsetof(ifr_qlen)},     {"SIOCSIFDSTADDR", SIOCSIFDSTADDR, ifreq_offsetof(ifr_dstaddr)},     {"SIOCSIFNETMASK", SIOCSIFNETMASK, ifreq_offsetof(ifr_netmask)},     {"SIOCSIFBRDADDR", SIOCSIFBRDADDR, ifreq_offsetof(ifr_broadaddr)}, #ifdef BB_FEATURE_IFCONFIG_HW     {"SIOCSIFHWADDR",  SIOCSIFHWADDR,  ifreq_offsetof(ifr_hwaddr)}, #endif     {"SIOCSIFDSTADDR", SIOCSIFDSTADDR, ifreq_offsetof(ifr_dstaddr)}, #ifdef SIOCSKEEPALIVE     {"SIOCSKEEPALIVE", SIOCSKEEPALIVE, ifreq_offsetof(ifr_data)}, #endif #ifdef SIOCSOUTFILL     {"SIOCSOUTFILL",   SIOCSOUTFILL,   ifreq_offsetof(ifr_data)}, #endif #ifdef BB_FEATURE_IFCONFIG_MEMSTART_IOADDR_IRQ     {"SIOCSIFMAP",     SIOCSIFMAP,     ifreq_offsetof(ifr_map.mem_start)},     {"SIOCSIFMAP",     SIOCSIFMAP,     ifreq_offsetof(ifr_map.base_addr)},     {"SIOCSIFMAP",     SIOCSIFMAP,     ifreq_offsetof(ifr_map.irq)}, #endif     /* Last entry if for unmatched (possibly hostname) arg. */     {"SIOCSIFADDR",    SIOCSIFADDR,    ifreq_offsetof(ifr_addr)}, };

$ man offsetof #include <stddef.h> size_t offsetof(type, member);

머. 매크로로 해도 될텐데 왜 굳이 함수형으로 했을까는 조금 의문이지만,
간단하게 표현하자면 . 연산자를 붙여주는 역활을 한다.

#include <stddef.h>        #include <stdio.h>        #include <stdlib.h>        int main()        {            struct s {                int i;                char c;                double d;                char a[];            };            /* Output is compiler dependent */            printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",                    (long) offsetof(struct s, i),    // s.i ?                    (long) offsetof(struct s, c),    // s.c ?                    (long) offsetof(struct s, d),    // s.d ?                    (long) offsetof(struct s, a));    // s.a ?            printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));            exit(EXIT_SUCCESS);        }

생각을 해보니..

$ vi /usr/inclue/net/if.h 152 # define ifr_name       ifr_ifrn.ifrn_name      /* interface name       */ 153 # define ifr_hwaddr     ifr_ifru.ifru_hwaddr    /* MAC address          */ 154 # define ifr_addr       ifr_ifru.ifru_addr      /* address              */ 155 # define ifr_dstaddr    ifr_ifru.ifru_dstaddr   /* other end of p-p lnk */ 156 # define ifr_broadaddr  ifr_ifru.ifru_broadaddr /* broadcast address    */ 157 # define ifr_netmask    ifr_ifru.ifru_netmask   /* interface net mask   */ 158 # define ifr_flags      ifr_ifru.ifru_flags     /* flags                */ 159 # define ifr_metric     ifr_ifru.ifru_ivalue    /* metric               */ 160 # define ifr_mtu        ifr_ifru.ifru_mtu       /* mtu                  */ 161 # define ifr_map        ifr_ifru.ifru_map       /* device map           */ 162 # define ifr_slave      ifr_ifru.ifru_slave     /* slave device         */ 163 # define ifr_data       ifr_ifru.ifru_data      /* for use by interface */ 164 # define ifr_ifindex    ifr_ifru.ifru_ivalue    /* interface index      */ 165 # define ifr_bandwidth  ifr_ifru.ifru_ivalue    /* link bandwidth       */ 166 # define ifr_qlen       ifr_ifru.ifru_ivalue    /* queue length         */ 167 # define ifr_newname    ifr_ifru.ifru_newname   /* New name             */

이런식으로도 매크로 정의가 되어 있는데, 여러가지 방법으로 사용하는 이유가 있을까 라는 생각도 든다.