Undefined behavior and sequence points

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What are  quot sequence points quot   What is the relation between undefined behaviour and sequence points  I often use funny and convoluted expressions like a   i    i   to make myself feel better  Why should I stop using them  If you ve read this  be sure to visit the follow-up question Undefined behavior and sequence points reloaded    Note  This is meant to be an entry to Stack Overflow s C   FAQ  If you want to critique the idea of providing an FAQ in this form  then the posting on meta that started all this would be the place to do that  Answers to that question are monitored in the C   chatroom  where the FAQ idea started out in the first place  so your answer is very likely to get read by those who came up with the idea

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In C99 ISO IEC 9899 TC3  which seems absent from this discussion thus far the following steteents are made regarding order of evaluaiton           the order of evaluation of subexpressions and the order in which   side effects take place are both unspecified   Section 6 5 pp 67       The order of evaluation of the operands is unspecified  If an attempt   is made to modify the result of an assignment operator or to access it   after the next sequence point  the behavior sic  is undefined  Section   6 5 16 pp 91

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C  98 and C  03  This answer is for the older versions of the C   standard   The C  11 and C  14 versions of the standard do not formally contain  sequence points   operations are  sequenced before  or  unsequenced  or  indeterminately sequenced  instead   The net effect is essentially the same  but the terminology is different     Disclaimer   Okay  This answer is a bit long  So have patience while reading it  If you already know these things  reading them again won t make you crazy    Pre-requisites   An elementary knowledge of C   Standard     What are Sequence Points   The Standard says      At  certain specified points in the execution sequence called sequence points  all side effects of previous evaluations    shall be complete and no side effects of subsequent evaluations shall have taken place     1 9 7    Side effects  What are side effects   Evaluation  of  an  expression produces something and if in addition there is a change in the state of the execution environment it is said that the expression  its evaluation  has some side effect s    For example   int x   y      where y is also an int   In addition to the initialization operation the value of y gets changed due to the side effect of    operator    So far so good  Moving on to sequence points  An alternation definition of seq-points given by the comp lang c author Steve Summit      Sequence point is a point in time at which the dust has settled and all side effects which have been seen so far are guaranteed to be complete      What are the common sequence points listed in the C   Standard    Those are    at the end of the evaluation of full expression    1 9 16   A full-expression is an expression that is not a subexpression of another expression  1  Example    int a   5       is a sequence point here  in the evaluation of each of the following expressions after the evaluation of the first expression    1 9 18  2   a  amp  amp  b    5 14   a    b    5 15  a   b   c    5 16  a   b    5 18   here a   b is a comma operator  in func a a      is not a comma operator  it s merely a separator between the arguments a and a    Thus the behaviour is undefined in that case  if a is considered to be a primitive type     at a function call  whether or not the function is inline   after the evaluation of all function arguments  if any  which  takes place before execution of any expressions or statements in the function body    1 9 17     1   Note   the evaluation of a full-expression can include the evaluation of subexpressions that are not lexically part of the full-expression   For example  subexpressions involved in evaluating default argument expressions  8 3 6  are considered to be created in the expression that calls the function  not the expression that defines the default argument  2   The operators indicated are the built-in operators  as described in clause 5   When one of these operators is overloaded  clause 13  in a valid context  thus designating a user-defined operator function  the expression designates a function invocation and the operands form an argument list  without an implied sequence point between them     What is Undefined Behaviour   The Standard defines Undefined Behaviour in Section   1 3 12 as     behavior  such as might arise upon use of an erroneous program construct or erroneous data  for which this International Standard imposes no  requirements 3       Undefined  behavior  may  also  be  expected  when  this   International Standard omits the description of any explicit definition of behavior     3   permissible undefined behavior ranges from ignoring the situation completely with unpredictable results  to behaving during translation or program execution in a documented manner characteristic of the environment  with or with- out the issuance of a diagnostic message   to terminating a translation or execution  with the issuance of a diagnostic message    In short  undefined behaviour means anything can happen from daemons flying out of your nose to  your girlfriend getting pregnant     What is the relation between Undefined Behaviour and Sequence Points   Before I get into that you must know the difference s  between Undefined Behaviour  Unspecified Behaviour and Implementation Defined Behaviour   You must also know that the order of evaluation of operands of individual operators and subexpressions of individual expressions  and the order in which side effects take place  is unspecified   For example   int x   5  y   6   int z   x     y      it is unspecified whether x   or y   will be evaluated first    Another example here     Now the Standard in   5 4 says   1  Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression      What does it mean   Informally it means that between two sequence points a variable must not be modified more than once  In an expression statement  the next sequence point is usually at the terminating semicolon  and the previous sequence point is at the end of the previous statement  An expression may also contain intermediate sequence points   From the above sentence the following expressions invoke Undefined Behaviour   i       i       UB  i is modified more than once btw two SPs i     i         UB  same as above   i   2         UB  same as above i     i   1     UB  same as above       i         UB  parsed as          i     i    i    i    i      UB  there s no SP between    i   right most  and assignment to  i    i  is modified more than once btw two SPs    But the following expressions are fine   i    i    i  1    1     well defined  AFAIK  i      i  i    i        well defined  int j   i  j      i  i    j i      well defined      2  Furthermore  the prior value shall be accessed only to determine the value to be stored    What does it mean  It means if an object is written to within a full expression  any and all accesses to it within the same expression must be directly involved in the computation of the value to be written    For example in i   i   1 all the access of i  in L H S and in R H S  are directly involved in computation of the value to be written  So it is fine   This rule effectively constrains legal expressions to those in which the accesses demonstrably precede the modification   Example 1   std  printf   d  d   i   i      invokes Undefined Behaviour because of Rule no 2   Example 2   a i    i      or a   i    i or a i        i etc   is disallowed because one of the accesses of i  the one in a i   has nothing to do with the value which ends up being stored in i  which happens over in i     and so there s no good way to define--either for our understanding or the compiler s--whether the access should take place before or after the incremented value is stored  So the behaviour is undefined   Example 3    int x   i   i       Similar to above     Follow up answer for C  11 here

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This is a follow up to my previous answer and contains C  11 related material      Pre-requisites   An elementary knowledge of Relations  Mathematics      Is it true that there are no Sequence Points in C  11   Yes  This is very true    Sequence Points have been replaced by Sequenced Before and Sequenced After  and Unsequenced and Indeterminately Sequenced  relations in C  11     What exactly is this  Sequenced before  thing   Sequenced Before   1 9 13  is a relation which is     Asymmetric  Transitive   between evaluations executed by a single thread and induces a strict partial order1  Formally it means given any two evaluations See below  A and B  if A is sequenced before B  then the execution of A shall precede the execution of B  If A is not sequenced before B and B is not sequenced before A  then A and B are unsequenced 2   Evaluations A and B are indeterminately sequenced when either A is sequenced before B or B is sequenced before A  but it is unspecified which3    NOTES     1    A strict partial order is a binary relation   lt   over a set P which is asymmetric  and transitive  i e   for all a  b  and c in P  we have that                i   if a  lt  b then     b  lt  a   asymmetry              ii   if a  lt  b and b  lt  c then a  lt  c  transitivity     2   The execution of unsequenced evaluations can overlap    3   Indeterminately sequenced evaluations cannot overlap  but either could be executed first       What is the meaning of the word  evaluation  in context of C  11    In C  11  evaluation of an expression  or a sub-expression  in general includes     value computations  including determining the identity of an object for glvalue evaluation and fetching a value previously assigned to an object for prvalue evaluation  and initiation of side effects    Now     1 9 14  says      Every value computation and side effect associated with a full-expression is sequenced before every value computation and side effect associated with the next full-expression to be evaluated     Trivial example    int x   x   10     x   Value computation and side effect associated with   x is sequenced after the value computation and side effect of x   10       So there must be some relation between Undefined Behaviour and the above-mentioned things  right    Yes  Right   In    1 9 15  it has been mentioned that      Except where noted  evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced4    For example    int main          int num   19        num    num  lt  lt  3     num  gt  gt  3         Evaluation of operands of   operator are unsequenced relative to each other  Evaluation of operands of  lt  lt  and  gt  gt  operators are unsequenced relative to each other     4  In an expression that is evaluated more than once during the execution of a program  unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed consistently in different evaluations          1 9 15    The value computations of the operands of an   operator are sequenced before the value computation of the result of the operator    That means in x   y the value computation of x and y are sequenced before the value computation of  x   y    More importantly        1 9 15  If a side effect on a scalar object is unsequenced relative to either       a  another side effect on the same scalar object       or        b  a value computation using the value of the same scalar object       the behaviour is undefined    Examples   int i   5  v 10         void  f int   int      i   i       i      Undefined Behaviour i     i   i        Undefined Behaviour  i     i     i      Undefined Behaviour i   v i            Undefined Behaviour i   v   i          Well-defined Behavior i   i     1        Undefined Behaviour i     i   1        Well-defined Behaviour     i              Well-defined Behaviour f i   -1  i   -1      Undefined Behaviour  see below       When calling a function  whether or not the function is inline   every value computation and side effect associated with any argument expression  or with the postfix expression designating the called function  is sequenced before execution of every expression or statement in the body of the called function   Note  Value computations and side effects associated with different argument expressions are unsequenced       end note    Expressions  5    7  and  8  do not invoke undefined behaviour  Check out the following answers for a more detailed explanation    Multiple preincrement operations on a variable in C  0x Unsequenced Value Computations     Final Note     If you find any flaw in the post please leave a comment  Power-users  With rep  20000  please do not hesitate to edit the post for correcting typos and other mistakes

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C  17  N4659  includes a proposal Refining Expression Evaluation Order for Idiomatic C   which defines a stricter order of expression evaluation   In particular  the following sentence     8 18 Assignment and compound assignment operators           In all cases  the assignment is sequenced after the value   computation of the right and left operands  and before the value computation of the assignment expression    The right operand is sequenced before the left operand    together with the following clarification     An expression X is said to be sequenced before an expression Y if every   value computation and every side effect associated with the expression X is sequenced before every value   computation and every side effect associated with the expression Y    make several cases of previously undefined behavior valid  including the one in question   a   i    i    However several other similar cases still lead to undefined behavior   In N4140   i   i     1     the behavior is undefined   But in N4659  i   i     1     the value of i is incremented i   i     i     the behavior is undefined   Of course  using a C  17 compliant compiler does not necessarily mean that one should start writing such expressions

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I am guessing there is a fundamental reason for the change  it isn t merely cosmetic to make the old interpretation clearer  that reason is concurrency  Unspecified order of elaboration is merely selection of one of several possible serial orderings  this is quite different to before and after orderings  because if there is no specified ordering  concurrent evaluation is possible  not so with the old rules  For example in   f  a b    previously either a then b  or  b then a  Now  a and b can be evaluated with instructions interleaved or even on different cores

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