LINUX下的getifaddrs()函数的内存释放问题

17fr4d477t

LINUX下的getifaddrs()函数的内存释放问题
在LINUX下获取网卡信息需要用到IOCTL或者getifaddrs
而我在用getifaddrs的时候遇到了内存方面的问题

先看相关定义:
==========

函数定义:
/* Create a linked list of `struct ifaddrs' structures, one for each
   network interface on the host machine.  If successful, store the
   list in *IFAP and return 0.  On errors, return -1 and set `errno'.

   The storage returned in *IFAP is allocated dynamically and can
   only be properly freed by passing it to `freeifaddrs'.  */
extern int getifaddrs (struct ifaddrs **__ifap) __THROW;

/* Reclaim the storage allocated by a previous `getifaddrs' call.  */
extern void freeifaddrs (struct ifaddrs *__ifa)  __THROW;
==============
此函数需要的结构体定义:

struct ifaddrs
{
  struct ifaddrs *ifa_next;     /* Pointer to the next structure.  */

  char *ifa_name;               /* Name of this network interface.  */
  unsigned int ifa_flags;       /* Flags as from SIOCGIFFLAGS ioctl.  */

  struct sockaddr *ifa_addr;    /* Network address of this interface.  */
  struct sockaddr *ifa_netmask; /* Netmask of this interface.  */
  union
  {
    /* At most one of the following two is valid.  If the IFF_BROADCAST
       bit is set in `ifa_flags', then `ifa_broadaddr' is valid.  If the
       IFF_POINTOPOINT bit is set, then `ifa_dstaddr' is valid.
       It is never the case that both these bits are set at once.  */
    struct sockaddr *ifu_broadaddr; /* Broadcast address of this interface. */
    struct sockaddr *ifu_dstaddr; /* Point-to-point destination address.  */
  } ifa_ifu;
  /* These very same macros are defined by <net/if.h> for `struct ifaddr'.
     So if they are defined already, the existing definitions will be fine.  */
# ifndef ifa_broadaddr
#  define ifa_broadaddr ifa_ifu.ifu_broadaddr
# endif
# ifndef ifa_dstaddr
#  define ifa_dstaddr   ifa_ifu.ifu_dstaddr
# endif

  void *ifa_data;               /* Address-specific data (may be unused).  */
};
=============
我在调用了getifaddrs()之后,正常地完成了需要的工作
但是最后如果用freeifaddrs,则出现运行时错误
*** glibc detected *** d: free(): invalid pointer: 0x0804a4d4 ***
======= Backtrace: =========
/lib/libc.so.6[0xb7eda911]
/lib/libc.so.6(__libc_free+0x84)[0xb7edbf84]
/lib/libc.so.6(freeifaddrs+0x1d)[0xb7f512dd]
d[0x8048989]
d[0x80486a5]
/lib/libc.so.6(__libc_start_main+0xdc)[0xb7e8c87c]
d[0x8048491]
======= Memory map: ========
08048000-08049000 r-xp 00000000 03:07 48637 /home/souldump/bin/d
08049000-0804a000 rw-p 00000000 03:07 48637 /home/souldump/bin/d
0804a000-0806b000 rw-p 0804a000 00:00 0 [heap]
b7d00000-b7d21000 rw-p b7d00000 00:00 0
b7d21000-b7e00000 ---p b7d21000 00:00 0
b7e76000-b7e77000 rw-p b7e76000 00:00 0
b7e77000-b7f90000 r-xp 00000000 03:05 16184 /lib/libc-2.4.so
b7f90000-b7f92000 r--p 00118000 03:05 16184 /lib/libc-2.4.so
b7f92000-b7f94000 rw-p 0011a000 03:05 16184 /lib/libc-2.4.so
b7f94000-b7f98000 rw-p b7f94000 00:00 0
b7fab000-b7fb5000 r-xp 00000000 03:05 20108 /lib/libgcc_s.so.1
b7fb5000-b7fb6000 rw-p 00009000 03:05 20108 /lib/libgcc_s.so.1
b7fb6000-b7fb7000 rw-p b7fb6000 00:00 0
b7fb7000-b7fd1000 r-xp 00000000 03:05 16177 /lib/ld-2.4.so
b7fd1000-b7fd3000 rw-p 00019000 03:05 16177 /lib/ld-2.4.so
bfb2b000-bfb41000 rw-p bfb2b000 00:00 0 [stack]
ffffe000-fffff000 ---p 00000000 00:00 0 [vdso]

实际上也有人出现相同问题:
http://p.g.yupoo.com/nph-proxy.cgi/000110A/http/www.linuxdby.com/bbs/viewthread.php=3ftid=3d10756
此人说:"这说明不是真正的链表，指针非法"
但是又没有进一步说明怎么解决
他干脆没有调用freeifaddrs,自然会内存泄漏.....

我去看了afaddrs.c
freeifaddrs的定义居然是:
void
freeifaddrs (struct ifaddrs *ifa)
{
free (ifa);
}
怎么样,很囧吧,明明在头文件里说"必须用freeifaddrs才能正确free..."
然后我看了一下getifaddrs的函数体

他在getifaddrs内部定义了一个结构
struct ifaddrs_storage
{
struct ifaddrs ifa;
union
{
/* Save space for the biggest of the four used sockaddr types and
avoid a lot of casts. */
struct sockaddr sa;
struct sockaddr_ll sl;
struct sockaddr_in s4;
struct sockaddr_in6 s6;
} addr, netmask, broadaddr;
char name[IF_NAMESIZE + 1];
};

然后把获取的各网卡信息一个个填充到此结构的struct ifaddrs ifa中,ifa的next值手动设置为下一个struct ifaddrs_storage中的ifa的地址...
这酒是所谓的"伪链表"吧?
这就是我无法正确free掉它的原因?
我究竟要怎么把它free掉?freeifaddrs一运行就运行时错误




LINUX取得本机IP的简单C程序
注意这里用了两个struct ifaddrs



//代码根据UNP和man手册编写
//适用于LINUX/BSD(FreeBSD, MacOS X)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#include <sys/types.h>
#include <sys/socket.h>

#include <netinet/in.h>
#include <arpa/inet.h>
#include <ifaddrs.h>

int main(void)
{
        struct ifaddrs        *ifc, *ifc1;
        char                ip[64];
        char                nm[64];

        if (0 != getifaddrs(&ifc)) return(-1);
        ifc1 = ifc;

        printf("Iface\tIP address\tNetmask\n");
        for(; NULL != ifc; ifc = (*ifc).ifa_next) {
                printf("%s", (*ifc).ifa_name);
                if (NULL != (*ifc).ifa_addr) {
                        inet_ntop(AF_INET, &(((struct sockaddr_in*)((*ifc).ifa_addr))->sin_addr), ip, 64);
                        printf("\t%s", ip);
                } else {
                        printf("\t\t");
                }
                if (NULL != (*ifc).ifa_netmask) {
                        inet_ntop(AF_INET, &(((struct sockaddr_in*)((*ifc).ifa_netmask))->sin_addr), nm, 64);
                        printf("\t%s", nm);
                } else {
                        printf("\t\t");
                }
                printf("\n");
        }

        freeifaddrs(ifc1);
        return(0);
}



--------------------------------------------------------------------------------



============原来的============
struct ifaddrs *ifap, *ifaphead, *ifaTmp;
getifaddrs(&ifap);
ifaphead = ifap;
while((ifapTmp = ifap) != NULL)
{
//实际任务代码  
ifap = ifapTmp->ifa_next;
}
freeifaddrs(ifaphead);
=========修改后========
struct ifaddrs *ifap, *ifaphead;
getifaddrs(&ifap);
ifaphead = ifap;
while(ifap != NULL)
{
//实际任务代码  
ifap = ifa_next;
}
freeifaddrs(ifaphead);
==================
仅仅是用了一个ifapTmp来代替ifap做事,区别仅此而已(而且我也忘了一开始为什么要用ifapTmp....)
但是最后都是用了freeifaddrs(ifaphead)啊,并没有传错指针啊????
中间的代码并没有对这段数据做任何修改啊.....

请指教一下,这唯一的区别为什么会造成我原先的代码freeifaddrs失败?谢谢!



ifaphead = ifap;                                //这里用ifaphead保存ifap指针地址
while((ifapTmp = ifap) != NULL)
{
ifap = ifapTmp->ifa_next;                //这里修改了ifap的地址
}
freeifaddrs(ifaphead);                       //由于ifap的地址修改，所以ifaphead已经是无效指针。