C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Segmentation fault with STL list class - PLEASE HELP          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message cpptutor2000@yahoo.com Guest Posted: Fri Jul 22, 2005 8:40 pm    Post subject: Segmentation fault with STL list class - PLEASE HELP I am using an STL list to store data packets in a network simulator. The data packets are really C structs, which have other C structs inside them. These structs contain unsigned char arrays of various sizes, and some unsigned int values. There are no pointers values stored in these structs. I have a function, from inside of which I have a loop as follows: Please note that dataPacketStore is the name of the struct being stored. if(!dlist.empty()){ list<dataPacketStore>::iterator iter; list<dataPacketStore>::iterator dbegin = dlist.begin(); list<dataPacketStore>::iterator dend = dlist.end(); for(iter = dbegin; iter != dend; iter++) if((*iter).disabled == true) dlist.erase(iter); //'disabled' is a boolean variable in the struct dataPacketStore } Calling this erase method causes a segmentation fault. Is this because of the fact that nested structs are there? Any help or suggestions would be greatly appreciated. Thanks in advance for your help. Back to top Thomas Tutone Guest Posted: Fri Jul 22, 2005 9:36 pm    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP [email]cpptutor2000 (AT) yahoo (DOT) com[/email] wrote: Quote: I am using an STL list to store data packets in a network simulator. [snip] Quote: I have a function, from inside of which I have a loop as follows: Please note that dataPacketStore is the name of the struct being stored. if(!dlist.empty()){ list<dataPacketStore>::iterator iter; list<dataPacketStore>::iterator dbegin = dlist.begin(); list<dataPacketStore>::iterator dend = dlist.end(); for(iter = dbegin; iter != dend; iter++) if((*iter).disabled == true) dlist.erase(iter); Calling list's erase member function invalidates all iterators that point to the value that was removed. So, in your example, you erase the element pointed to by iter, which invalidates iter (think about it - it points to an element that has been removed from the list). Then you try to increment iter. That leads to undefined behavior (which can include seg faults). But erase actually returns a valid iterator. Try this instead (untested): if (iter->disabled) iter = dlist.erase(iter); Best regards, Tom Back to top Greg Guest Posted: Mon Jul 25, 2005 10:30 am    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP Thomas Tutone wrote: Quote: cpptutor2000 (AT) yahoo (DOT) com wrote: I am using an STL list to store data packets in a network simulator. [snip] I have a function, from inside of which I have a loop as follows: Please note that dataPacketStore is the name of the struct being stored. if(!dlist.empty()){ list<dataPacketStore>::iterator iter; list<dataPacketStore>::iterator dbegin = dlist.begin(); list<dataPacketStore>::iterator dend = dlist.end(); for(iter = dbegin; iter != dend; iter++) if((*iter).disabled == true) dlist.erase(iter); Calling list's erase member function invalidates all iterators that point to the value that was removed. So, in your example, you erase the element pointed to by iter, which invalidates iter (think about it - it points to an element that has been removed from the list). Then you try to increment iter. That leads to undefined behavior (which can include seg faults). But erase actually returns a valid iterator. Try this instead (untested): if (iter->disabled) iter = dlist.erase(iter); Best regards, Tom Your suggestion will fix the crash, but it has introduced another, more subtle problem. The loop will now skip over the iterator that erase() returned, by incrementing it before the test is next evaluated. For every item deleted (unless it's the last in the list) another item in the list will be skipped. Greg Back to top Earl Purple Guest Posted: Mon Jul 25, 2005 11:00 am    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP which is why you should probably prefer to use the algorithmic dlist.erase( std::remove_if( dbegin, dend, isDisabled ) ); where isDisabled is defined as: bool isDisabled ( const dataPacketStore & dps ) { return dps.disabled; } and possibly you could use a function object: struct IsDisabled { bool operator() ( const dataPacketStore & dps ) const { return dps.disabled; } }; in which case change the algorithmic call to dlist.erase( std::remove_if( dbegin, dend, IsDisabled() ) ); Back to top Earl Purple Guest Posted: Mon Jul 25, 2005 11:02 am    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP oops too late I can't go back and retrieve my message. But you'll need another dend in the erase thus: dlist.erase( std::remove_if( dbegin, dend, isDisabled ), dend ); and dlist.erase( std::remove_if( dbegin, dend, IsDisabled() ), dend ); Back to top Earl Purple Guest Posted: Mon Jul 25, 2005 11:23 am    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP actually no need for that at all as list has its own remove_if function thus: dlist.remove_if( dbegin, dend, isDisabled ); or dlist.remove_if( dbegin, dend, IsDisabled() ); The other method will work but using list's own remove_if is preferred. Back to top Clark S. Cox III Guest Posted: Mon Jul 25, 2005 6:52 pm    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP On 2005-07-25 07:02:06 -0400, "Earl Purple" <earlpurple0 (AT) yahoo (DOT) com> said: Quote: oops too late I can't go back and retrieve my message. But you'll need another dend in the erase thus: dlist.erase( std::remove_if( dbegin, dend, isDisabled ), dend ); and dlist.erase( std::remove_if( dbegin, dend, IsDisabled() ), dend ); Or just: dlist.remove_if(isDisabled) and dlist.remove_if(IsDisabled()); -- Clark S. Cox, III [email]clarkcox3 (AT) gmail (DOT) com[/email] Back to top Thomas Tutone Guest Posted: Mon Jul 25, 2005 7:01 pm    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP Greg wrote: Quote: Calling list's erase member function invalidates all iterators that point to the value that was removed. So, in your example, you erase the element pointed to by iter, which invalidates iter (think about it - it points to an element that has been removed from the list). Then you try to increment iter. That leads to undefined behavior (which can include seg faults). But erase actually returns a valid iterator. Try this instead (untested): if (iter->disabled) iter = dlist.erase(iter); Your suggestion will fix the crash, but it has introduced another, more subtle problem. The loop will now skip over the iterator that erase() returned, by incrementing it before the test is next evaluated. For every item deleted (unless it's the last in the list) another item in the list will be skipped. Absolutely right. Thanks for the correction. I suppose that one could do something like: while (iter!=dlist.end() && iter->disabled) iter = dlist.erase(iter); But I agree with another poster's suggestion that one is better off using an algorithm-like member function instead. Best regards, Tom Back to top Clark S. Cox III Guest Posted: Wed Jul 27, 2005 7:13 pm    Post subject: Re: Segmentation fault with STL list class - PLEASE HELP On 2005-07-25 15:01:09 -0400, "Thomas Tutone" <Thomas8675309 (AT) yahoo (DOT) com> said: Quote: Greg wrote: Calling list's erase member function invalidates all iterators that point to the value that was removed. So, in your example, you erase the element pointed to by iter, which invalidates iter (think about it - it points to an element that has been removed from the list). Then you try to increment iter. That leads to undefined behavior (which can include seg faults). But erase actually returns a valid iterator. Try this instead (untested): if (iter->disabled) iter = dlist.erase(iter); Your suggestion will fix the crash, but it has introduced another, more subtle problem. The loop will now skip over the iterator that erase() returned, by incrementing it before the test is next evaluated. For every item deleted (unless it's the last in the list) another item in the list will be skipped. Absolutely right. Thanks for the correction. I suppose that one could do something like: while (iter!=dlist.end() && iter->disabled) iter = dlist.erase(iter); This, of course, would stop at the first enabled object: (D=Disabled, E=Enabled) Before: (DDDDEDED) After: (EDED) Quote: But I agree with another poster's suggestion that one is better off using an algorithm-like member function instead. -- Clark S. Cox, III [email]clarkcox3 (AT) gmail (DOT) com[/email] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.