What does ulimit -s unlimited do

Question

There are understandably many related questions on stack allocation  What and where are the stack and heap   Why is there a limit on the stack size   Size of stack and heap memory  However on various  nix machines I can issue the bash command  ulimit -s unlimited   or the csh command  set stacksize unlimited   How does this change how programs are executed  Are there any impacts on program or system performance  e g   why wouldn t this be the default    In case more system details are relevant  I m mostly concerned with programs compiled with GCC on Linux running on x86 64 hardware

User · Answer

ulimit -s unlimited lets the stack grow unlimited   This may prevent your program from crashing if you write programs by recursion  especially if your programs are not tail recursive  compilers can  optimize  those   and the depth of recursion is large

User · Answer

When you call a function  a new  namespace  is allocated on the stack  That s how functions can have local variables  As functions call functions  which in turn call functions  we keep allocating more and more space on the stack to maintain this deep hierarchy of namespaces    To curb programs using massive amounts of stack space  a limit is usually put in place via ulimit -s  If we remove that limit via ulimit -s unlimited  our programs will be able to keep gobbling up RAM for their evergrowing stack until eventually the system runs out of memory entirely    int eat stack space void    return eat stack space         If we compile this with no optimization and run it  our computer could crash    Usually  using a ton of stack space is accidental or a symptom of very deep recursion that probably should not be relying so much on the stack  Thus the stack limit    Impact on performace is minor but does exist  Using the time command  I found that eliminating the stack limit increased performance by a few fractions of a second  at least on 64bit Ubuntu

User · Answer

stack size can indeed be unlimited   STK LIM is the default   STK LIM MAX is something that differs per architecture  as can be seen from include asm-generic resource h         RLIMIT STACK default maximum - some architectures override it       ifndef  STK LIM MAX   define  STK LIM MAX           RLIM INFINITY  endif   As can be seen from this example generic value is infinite  where RLIM INFINITY is  again  in generic case defined as         SuS says limits have to be unsigned     Which makes a ton more sense anyway        Some architectures override this  for compatibility reasons        ifndef RLIM INFINITY   define RLIM INFINITY            0UL   endif   So I guess the real answer is - stack size CAN be limited by some architecture  then unlimited stack trace will mean whatever  STK LIM MAX is defined to  and in case it s infinity - it is infinite  For details on what it means to set it to infinite and what implications it might have  refer to the other answer  it s way better than mine

[linux] What does "ulimit -s unlimited" do?

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