Problem with gethostbyname
in [UK Linux]
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From: Martin Liddle on 7 May 2008 10:33 I am trying to update some code (that I didn't originally write) which uses gethostbyname to turn a name into an IPv4 address. This works well enough most of the time but if the internet connection fails (which it does from time to time because it might be dialup, via mobile phone or WiFi) then the program exits which is undesirable. I was hoping that I could insert a loop so that instead of exiting the program sleeps for a while and then tries again until the connection is available again. This doesn't work; once a call has failed all subsequent calls fail until the program is restarted. I am not running a local DNS server; resolv.conf just points at the nameservers provided by whatever connection happens to be in use. Any clue what I am doing wrong or how this should be coded? -- Martin Liddle, Tynemouth Computer Services, 3 Kentmere Way, Staveley, Chesterfield, Derbyshire, S43 3TW. Web site: <http://www.tynecomp.co.uk>.
From: Tony Mountifield on 7 May 2008 11:04 In article <94mptFgF3bIIFwP1(a)tynecomp.invalid>, Martin Liddle <News-reply_spam_a(a)tynecomp.co.uk> wrote: > I am trying to update some code (that I didn't originally write) which > uses gethostbyname to turn a name into an IPv4 address. This works well > enough most of the time but if the internet connection fails (which it > does from time to time because it might be dialup, via mobile phone or > WiFi) then the program exits which is undesirable. I was hoping that I > could insert a loop so that instead of exiting the program sleeps for a > while and then tries again until the connection is available again. This > doesn't work; once a call has failed all subsequent calls fail until the > program is restarted. I am not running a local DNS server; resolv.conf > just points at the nameservers provided by whatever connection happens > to be in use. Any clue what I am doing wrong or how this should be > coded? I would be very surprised if it gethostbyname() that is causing the program to exit. If gethostbyname() encounters an error, it returns NULL instead of a pointer to a struct hostent. You need to make sure you check that the return value is non-NULL before you try to dereference it, or your program will indeed exit. Actually, re-reading your description now confuses me: first you say "the program exits", but later you say "all subsequent calls fail until the program is restarted", which suggests it is still running. ??? Could you describe the relevant code in more detail or even include a listing of the relevant section? Cheers Tony -- Tony Mountifield Work: tony(a)softins.co.uk - http://www.softins.co.uk Play: tony(a)mountifield.org - http://tony.mountifield.org
From: Martin Liddle on 7 May 2008 12:44 In message <fvsger$j3f$1(a)softins.clara.co.uk>, Tony Mountifield <tony(a)softins.clara.co.uk> writes > >I would be very surprised if it gethostbyname() that is causing the program >to exit. > Agreed; it terminates slightly later. >If gethostbyname() encounters an error, it returns NULL instead of a >pointer to a struct hostent. You need to make sure you check that the >return value is non-NULL before you try to dereference it, or your program >will indeed exit. > Agreed and in my modified version this is exactly what I check. >Actually, re-reading your description now confuses me: first you say "the >program exits", but later you say "all subsequent calls fail until the >program is restarted", which suggests it is still running. ??? > The first description refers to how the program was; the second to my modified version where it loops (the intention being it loops until it succeeds with a 30 second sleep in each loop). -- Martin Liddle, Tynemouth Computer Services, 3 Kentmere Way, Staveley, Chesterfield, Derbyshire, S43 3TW. Web site: <http://www.tynecomp.co.uk>.
From: Peter Benie on 7 May 2008 12:57 In article <94mptFgF3bIIFwP1(a)tynecomp.invalid>, Martin Liddle <News-reply_spam_a(a)tynecomp.co.uk> wrote: >This doesn't work; once a call has failed all subsequent calls fail until the >program is restarted. I am not running a local DNS server; resolv.conf >just points at the nameservers provided by whatever connection happens >to be in use. Could this be negative caching by 'nscd'? Kill it and see if your problems go away. Peter
From: Martin Liddle on 7 May 2008 13:52
In message <s3e*DFhcs(a)news.chiark.greenend.org.uk>, Peter Benie <peterb(a)chiark.greenend.org.uk> writes >Could this be negative caching by 'nscd'? Kill it and see if your >problems go away. > Nice idea but nscd isn't running on this Fedora 8 system (although it is installed). Is there anything else that could be doing something similar? -- Martin Liddle, Tynemouth Computer Services, 3 Kentmere Way, Staveley, Chesterfield, Derbyshire, S43 3TW. Web site: <http://www.tynecomp.co.uk>. |