Significance of ios base sync with stdio false cin tie NULL

Question

What is the significance of including  ios base  sync with stdio false   cin tie NULL     in C   programs   In my tests  it speeds up the execution time  but is there a test case I should be worried about by including this    Do the 2 statements always have to be together  or is the first one sufficient  i e   ignoring cin tie NULL    Also  is it permissible to use simultaneous C and C   commands if its value has been set to false   https   www codechef com viewsolution 7316085  The above code worked fine  until I used scanf printf in a C   program with the value as true  In this case  it gave a segmentation fault  What could be the possible explanation for this

User · Answer

It s just common stuff for making cin input work faster   For a quick explanation  the first line turns off buffer synchronization between the cin stream and C-style stdio tools  like scanf or gets      so cin works faster  but you can t use it simultaneously with stdio tools   The second line unties cin from cout     by default the cout buffer flushes each time when you read something from cin  And that may be slow when you repeatedly read something small then write something small many times  So the line turns off this synchronization  by literally tying cin to null instead of cout

User · Answer

Using ios base  sync with stdio false   is sufficient to decouple the C and C   streams  You can find a discussion of this in Standard C   IOStreams and Locales  by Langer and Kreft  They note that how this works is implementation-defined   The cin tie NULL  call seems to be requesting a decoupling between the activities on cin and cout  I can t explain why using this with the other optimization should cause a crash  As noted  the link you supplied is bad  so no speculation here

User · Answer

Lot s of great answer  I just want to add a small note about decoupling the stream  cin tie NULL    I have faced an issue while decoupling the stream with CodeChef platform  When I submitted my code  the platform response was  quot Wrong Answer quot  but after tying the stream and testing the submission  It worked  So  If anyone wants to untie the stream  the output stream must be flushed  Edit  I am not familiar with all the platform but this is what I have experienced

User · Answer

The two calls have different meanings that have nothing to do with performance  the fact that it speeds up the execution time is  or might be  just a side effect  You should understand what each of them does and not blindly include them in every program because they look like an optimization   ios base  sync with stdio false     This disables the synchronization between the C and C   standard streams  By default  all standard streams are synchronized  which in practice allows you to mix C- and C  -style I O and get sensible and expected results  If you disable the synchronization  then C   streams are allowed to have their own independent buffers  which makes mixing C- and C  -style I O an adventure   Also keep in mind that synchronized C   streams are thread-safe  output from different threads may interleave  but you get no data races    cin tie NULL     This unties cin from cout  Tied streams ensure that one stream is flushed automatically before each I O operation on the other stream   By default cin is tied to cout to ensure a sensible user interaction  For example   std  cout  lt  lt   Enter name    std  cin  gt  gt  name    If cin and cout are tied  you can expect the output to be flushed  i e   visible on the console  before the program prompts input from the user  If you untie the streams  the program might block waiting for the user to enter their name but the  Enter name  message is not yet visible  because cout is buffered by default  output is flushed displayed on the console only on demand or when the buffer is full    So if you untie cin from cout  you must make sure to flush cout manually every time you want to display something before expecting input on cin   In conclusion  know what each of them does  understand the consequences  and then decide if you really want or need the possible side effect of speed improvement

User · Answer

This is to synchronize IOs from C and C   world  If you synchronize  then you have a guaranty that the orders of all IOs is exactly what you expect  In general  the problem is the buffering of IOs that causes the problem  synchronizing let both worlds to share the same buffers  For example cout  lt  lt   Hello   printf  World    cout  lt  lt   Ciao    without synchronization you ll never know if you ll get HelloCiaoWorld or HelloWorldCiao or WorldHelloCiao     tie lets you have the guaranty that IOs channels in C   world are tied one to each other  which means for example that every output have been flushed before inputs occurs  think about cout  lt  lt   What s your name     cin  gt  gt  name     You can always mix C or C   IOs  but if you want some reasonable behavior you must synchronize both worlds  Beware that in general it is not recommended to mix them  if you program in C use C stdio  and if you program in C   use streams  But you may want to mix existing C libraries into C   code  and in such a case it is needed to synchronize both

[c++] Significance of ios_base::sync_with_stdio(false); cin.tie(NULL);

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