Endless loop in C C

Question

There are several possibilities to do an endless loop  here are a few I would choose    for        while 1       while true     do    while 1    do    while true    Is there a certain form which one should choose  And do modern compilers make a difference between the middle and the last statement or does it realize that it is an endless loop and skips the checking part entirely   Edit  as it has been mentioned I forgot goto  but this was done out of the reason that I don t like it as a command at all   Edit2  I made some grep on the latest versions taken from kernel org  I does seems as nothing much changed over time  within the Kernel at least

User · Answer

They probably compile down to nearly the same machine code, so it is a matter of taste.

Personally, I would chose the one that is the clearest (i.e. very clear that it is supposed to be an infinite loop).

I would lean towards while(true){}.

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The idiom designed into the C language  and inherited into C    for infinite looping is for      the omission of a test form  The do while and while loops do not have this special feature  their test expressions are mandatory   for     does not express  loop while some condition is true that happens to always be true   It expresses  loop endlessly   No superfluous condition is present   Therefore  the for     construct is the canonical endless loop  This is a fact   All that is left to opinion is whether or not to write the canonical endless loop  or to choose something baroque which involves extra identifiers and constants  to build a superfluous expression   Even if the test expression of while were optional  which it isn t  while    would be strange  while what   By contrast  the answer to the question for what  is  why  ever---for ever   As a joke some programmers of days past have defined blank macros  so they could write for ev e r     while true  is superior to while 1  because at least it doesn t involve the kludge that 1 represents truth  However  while true  didn t enter into C until C99  for     exists in every version of C going back to the language described in the 1978 book K amp R1  and in every dialect of C    and even related languages  If you re coding in a code base written in C90  you have to define your own true for while  true    while true  reads badly  While what is true   We don t really want to see the identifier true in code  except when we are initializing boolean variables or assigning to them  true need not ever appear in conditional tests  Good coding style avoids cruft like this   if  condition    true        in favor of   if  condition        For this reason while  0    0  is superior to while  true   it uses an actual condition that tests something  which turns into a sentence   loop while zero is equal to zero   We need a predicate to go nicely with  while   the word  true  isn t a predicate  but the relational operator    is

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Is there a certain form which one should choose      You can choose either  Its matter of choice  All are equivalent  while 1     while true    is frequently used for infinite loop by programmers

User · Answer

I would recommend while  1      or while  true       It s what most programmers would write  and for readability reasons you should follow the common idioms    Ok  so there is an obvious  citation needed  for the claim about most programmers  But from the code I ve seen  in C since 1984  I believe it is true    Any reasonable compiler would compile all of them to the same code  with an unconditional jump  but I wouldn t be surprised if there are some unreasonable compilers out there  for embedded or other specialized systems

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Everyone seems to like while  true    https   stackoverflow com a 224142 1508519  https   stackoverflow com a 1401169 1508519  https   stackoverflow com a 1401165 1508519  https   stackoverflow com a 1401164 1508519  https   stackoverflow com a 1401176 1508519  According to SLaks  they compile identically   Ben Zotto also says it doesn t matter      It s not faster    If you really care  compile with assembler output for your platform and look to see    It doesn t matter  This never matters  Write your infinite loops however you like    In response to user1216838  here s my attempt to reproduce his results   Here s my machine   cat  etc  -release CentOS release 6 4  Final    gcc version   Target  x86 64-unknown-linux-gnu Thread model  posix gcc version 4 8 2  GCC    And test files      testing cpp  include  lt iostream gt   int main         do   break    while 1         testing2 cpp  include  lt iostream gt   int main         while 1    break          testing3 cpp  include  lt iostream gt   int main         while true    break        The commands   gcc -S -o test1 asm testing cpp gcc -S -o test2 asm testing2 cpp gcc -S -o test3 asm testing3 cpp  cmp test1 asm test2 asm   The only difference is the first line  aka the filename   test1 asm test2 asm differ  byte 16  line 1   Output            file    testing2 cpp           local   ZStL8  ioinit          comm    ZStL8  ioinit 1 1          text          globl  main          type   main   function main   LFB969           cfi startproc         pushq    rbp          cfi def cfa offset 16          cfi offset 6  -16         movq     rsp   rbp          cfi def cfa register 6         nop         movl     0   eax         popq     rbp          cfi def cfa 7  8         ret          cfi endproc  LFE969           size   main   -main          type    Z41  static initialization and destruction 0ii   function  Z41  static initialization and destruction 0ii   LFB970           cfi startproc         pushq    rbp          cfi def cfa offset 16          cfi offset 6  -16         movq     rsp   rbp          cfi def cfa register 6         subq     16   rsp         movl     edi  -4  rbp          movl     esi  -8  rbp          cmpl     1  -4  rbp          jne      L3         cmpl     65535  -8  rbp          jne      L3         movl      ZStL8  ioinit   edi         call     ZNSt8ios base4InitC1Ev         movl       dso handle   edx         movl      ZStL8  ioinit   esi         movl      ZNSt8ios base4InitD1Ev   edi         call      cxa atexit  L3          leave          cfi def cfa 7  8         ret          cfi endproc  LFE970           size    Z41  static initialization and destruction 0ii   - Z41  static initialization and destruction 0ii          type    GLOBAL  sub I main   function  GLOBAL  sub I main   LFB971           cfi startproc         pushq    rbp          cfi def cfa offset 16          cfi offset 6  -16         movq     rsp   rbp          cfi def cfa register 6         movl     65535   esi         movl     1   edi         call     Z41  static initialization and destruction 0ii         popq     rbp          cfi def cfa 7  8         ret          cfi endproc  LFE971           size    GLOBAL  sub I main   - GLOBAL  sub I main          section         ctors  aw   progbits          align 8          quad    GLOBAL  sub I main          hidden   dso handle          ident   GCC   GNU  4 8 2           section         note GNU-stack     progbits   With -O3  the output is considerably smaller of course  but still no difference

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In an ultimate act of boredom  I actually wrote a few versions of these loops and compiled it with GCC on my mac mini   the      while 1    and for        produced same assembly results  while the do   while 1   produced similar but a different assembly code  heres the one for while for loop       section      TEXT   text regular pure instructions      globl   main      align  4  0x90  main                                       main      cfi startproc    BB 0      pushq    rbp Ltmp2       cfi def cfa offset 16 Ltmp3       cfi offset  rbp  -16     movq     rsp   rbp Ltmp4       cfi def cfa register  rbp     movl     0  -4  rbp  LBB0 1                                       gt This Inner Loop Header  Depth 1     jmp LBB0 1      cfi endproc   and the do while loop            section      TEXT   text regular pure instructions      globl   main      align  4  0x90  main                                       main      cfi startproc    BB 0      pushq    rbp Ltmp2       cfi def cfa offset 16 Ltmp3       cfi offset  rbp  -16     movq     rsp   rbp Ltmp4       cfi def cfa register  rbp     movl     0  -4  rbp  LBB0 1                                       gt This Inner Loop Header  Depth 1     jmp LBB0 2 LBB0 2                                       in Loop  Header BB0 1 Depth 1     movb     1   al     testb    1   al     jne LBB0 1     jmp LBB0 3 LBB0 3      movl     0   eax     popq     rbp     ret      cfi endproc

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It is very subjective  I write this   while true       in C     Because its intent is very much clear and it is also readable  you look at it and you know  infinite loop is intended    One might say for     is also clear  But I would argue that because of its convoluted syntax  this option requires extra knowledge to reach the conclusion that it is an infinite loop  hence it is relatively less clear  I would even say there are more number of programmers who don t know what for     does  even if they know usual for loop   but almost all programmers who knows while loop would immediately figure out what while true  does   To me  writing for     to mean infinite loop  is like writing while   to mean infinite loop  mdash  while the former works  the latter does NOT  In the former case  empty condition turns out to be true implicitly  but in the latter case  it is an error  I personally didn t like it   Now while 1  is also there in the competition  I would ask  why while 1   Why not while 2   while 3  or while 0 1   Well  whatever you write  you actually mean while true   mdash  if so  then why not write it instead   In C  if I ever write   I would probably write this   while 1       in C   While while 2   while 3  and while 0 1  would equally make sense  But just to be conformant with other C programmers  I would write while 1   because lots of C programmers write this and I find no reason to deviate from the norm

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I use for    ever       It is easy to read and it takes a bit longer to type  due to the shifts for the asterisks   indicating I should be really careful when using this type of loop  The green text that shows up in the conditional is also a pretty odd sight   another indication this construct is frowned upon unless absolutely necessary

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The problem with asking this question is that you ll get so many subjective answers that simply state  I prefer this       Instead of making such pointless statements  I ll try to answer this question with facts and references  rather than personal opinions   Through experience  we can probably start by excluding the do-while alternatives  and the goto   as they are not commonly used  I can t recall ever seeing them in live production code  written by professionals   The while 1   while true  and for     are the 3 different versions commonly existing in real code  They are of course completely equivalent and results in the same machine code     for       This is the original  canonical example of an eternal loop  In the ancient C bible The C Programming Language by Kernighan and Ritchie  we can read that   K amp R 2nd ed 3 5   for                   is an  infinite  loop  presumably to be broken by other means  such   as a break or return  Whether to use while or for is largely a matter   of personal preference    For a long while  but not forever   this book was regarded as canon and the very definition of the C language  Since K amp R decided to show an example of for      this would have been regarded as the most correct form at least up until the C standardization in 1990   However  K amp R themselves already stated that it was a matter of preference   And today  K amp R is a very questionable source to use as a canonical C reference  Not only is it outdated several times over  not addressing C99 nor C11   it also preaches programming practices that are often regarded as bad or blatantly dangerous in modern C programming    But despite K amp R being a questionable source  this historical aspect seems to be the strongest argument in favour of the for      The argument against the for     loop is that it is somewhat obscure and unreadable  To understand what the code does  you must know the following rule from the standard   ISO 9899 2011 6 8 5 3   for   clause-1   expression-2   expression-3   statement    --      Both clause-1 and expression-3 can be omitted  An omitted expression-2   is replaced by a nonzero constant    Based on this text from the standard  I think most will agree that it is not only obscure  it is subtle as well  since the 1st and 3rd part of the for loop are treated differently than the 2nd  when omitted      while 1    This is supposedly a more readable form than for      However  it relies on another  obscure  although well-known rule  namely that C treats all non-zero expressions as boolean logical true  Every C programmer is aware of that  so it is not likely a big issue  There is one big  practical problem with this form  namely that compilers tend to give a warning for it   condition is always true  or similar  That is a good warning  of a kind which you really don t want to disable  because it is useful for finding various bugs  For example a bug such as while i   1   when the programmer intended to write while i    1     Also  external static code analysers are likely to whine about  condition is always true       while true    To make while 1  even more readable  some use while true  instead  The consensus among programmers seem to be that this is the most readable form  However  this form has the same problem as while 1   as described above   condition is always true  warnings  When it comes to C  this form has another disadvantage  namely that it uses the macro true from stdbool h  So in order to make this compile  we need to include a header file  which may or may not be inconvenient  In C   this isn t an issue  since bool exists as a primitive data type and true is a language keyword  Yet another disadvantage of this form is that it uses the C99 bool type  which is only available on modern compilers and not backwards compatible  Again  this is only an issue in C and not in C        So which form to use  Neither seems perfect  It is  as K amp R already said back in the dark ages  a matter of personal preference   Personally  I always use for     just to avoid the compiler analyser warnings frequently generated by the other forms  But perhaps more importantly because of this   If even a C beginner knows that for     means an eternal loop  then who are you trying to make the code more readable for   I guess that s what it all really boils down to  If you find yourself trying to make your source code readable for non-programmers  who don t even know the fundamental parts of the programming language  then you are only wasting time  They should not be reading your code   And since everyone who should  be reading your code already knows what for     means  there is no point in making it further readable -  it is already as readable as it gets

User · Answer

All are going to perform same function  and it is true to choose what you prefer   i might think  while 1  or while true   is good practice to use

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Well  there is a lot of taste in this one   I think people from a C background are more likely to prefer for      which reads as  forever   If its for work  do what the locals do  if its for yourself  do the one that you can most easily read   But in my experience  do     while  1   is almost never used

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They are the same  But I suggest  while ture   which has best representation

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