Date: Wed, 18 Jul 2001 09:45:31 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Anand Franklin J <anandfranklin@hotmail.com> Cc: freebsd-net@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG Subject: Re: Hi(some clarifcation about sysctl usage in ifconfig) Message-ID: <3B55BD2B.7CB85DF1@mindspring.com> References: <F197ZDgC81TbdkSmLAz000000d2@hotmail.com>
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Anand Franklin J wrote: > The below code is part of ifconfig which use sysctl: > 1:mib[0] = CTL_NET; > 2:mib[1] = PF_ROUTE; > 3:mib[2] = 0; > 4:mib[3] = 0; /* address family */ > 5:mib[4] = NET_RT_IFLIST; > 6:mib[5] = 0; > > /* if particular family specified, only ask about it */ > 7:if (afp) > 8: mib[3] = afp->af_af; > > 9:if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0) > 10: errx(1, "iflist-sysctl-estimate"); > 11:if ((buf = malloc(needed)) == NULL) > errx(1, "malloc"); > 12:if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0) > errx(1, "actual retrieval of interface table"); > the doubts are: > 1) what is the purpose of the above c statements. > 2)how are the mib values stored before finding its value > line from 1->8 In the kernel. In other words, they are _not_ writable, they are readable from the kernel. > 3)line from 9->11 is clear, that is it finds the length of the buffer to be > allocated. > 4)what is the significance of line 12 and what will the buf variable will > contain. 9->11 calculate the space and allocate a region for storing the interface list. 12 actually retrieves the list into the allocation space provided (buf). After it runs, if it runs successfully, it will contain a list of interfaces. Probably you want to look at using getifaddrs(3) instead; see the manual page for this library function. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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