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How to detect integer overflow in C [duplicate]


How do I detect unsigned integer multiply overflow?Detecting signed overflow in C/C++How do I check if A+B exceed long long? (both A and B is long long)Test for overflow in integer additionHow do you set, clear, and toggle a single bit?How do I detect unsigned integer multiply overflow?Improve INSERT-per-second performance of SQLite?Catch and compute overflow during multiplication of two large integersWhen to address integer overflow in CDetecting signed overflow in C/C++How to find (all) integer overflows in a C program?Speed comparison with Project Euler: C vs Python vs Erlang vs HaskellHow to check if overflow occured?automatic overflow detection in C++?






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;








16
















This question already has an answer here:



  • Detecting signed overflow in C/C++

    12 answers



  • How do I detect unsigned integer multiply overflow?

    31 answers



We know CPython promotes integers to long integers (which allow arbitrary-precision arithmetic) silently when the number gets bigger.



How can we detect overflow of int and long long in pure C?










share|improve this question















marked as duplicate by sleske, ead, phuclv, Cody Gray Apr 2 at 16:38


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.













  • 3





    It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

    – Some programmer dude
    Apr 2 at 7:11







  • 1





    Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

    – Antti Haapala
    Apr 2 at 12:49






  • 1





    there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

    – phuclv
    Apr 2 at 14:00












  • and add 1 more codereview.stackexchange.com/questions/37177/…

    – NoChance
    Apr 2 at 14:36

















16
















This question already has an answer here:



  • Detecting signed overflow in C/C++

    12 answers



  • How do I detect unsigned integer multiply overflow?

    31 answers



We know CPython promotes integers to long integers (which allow arbitrary-precision arithmetic) silently when the number gets bigger.



How can we detect overflow of int and long long in pure C?










share|improve this question















marked as duplicate by sleske, ead, phuclv, Cody Gray Apr 2 at 16:38


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.













  • 3





    It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

    – Some programmer dude
    Apr 2 at 7:11







  • 1





    Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

    – Antti Haapala
    Apr 2 at 12:49






  • 1





    there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

    – phuclv
    Apr 2 at 14:00












  • and add 1 more codereview.stackexchange.com/questions/37177/…

    – NoChance
    Apr 2 at 14:36













16












16








16


1







This question already has an answer here:



  • Detecting signed overflow in C/C++

    12 answers



  • How do I detect unsigned integer multiply overflow?

    31 answers



We know CPython promotes integers to long integers (which allow arbitrary-precision arithmetic) silently when the number gets bigger.



How can we detect overflow of int and long long in pure C?










share|improve this question

















This question already has an answer here:



  • Detecting signed overflow in C/C++

    12 answers



  • How do I detect unsigned integer multiply overflow?

    31 answers



We know CPython promotes integers to long integers (which allow arbitrary-precision arithmetic) silently when the number gets bigger.



How can we detect overflow of int and long long in pure C?





This question already has an answer here:



  • Detecting signed overflow in C/C++

    12 answers



  • How do I detect unsigned integer multiply overflow?

    31 answers







c overflow






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Apr 2 at 22:45









Peter Mortensen

13.9k1987113




13.9k1987113










asked Apr 2 at 7:07









DeanDean

1054




1054




marked as duplicate by sleske, ead, phuclv, Cody Gray Apr 2 at 16:38


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.









marked as duplicate by sleske, ead, phuclv, Cody Gray Apr 2 at 16:38


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.









  • 3





    It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

    – Some programmer dude
    Apr 2 at 7:11







  • 1





    Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

    – Antti Haapala
    Apr 2 at 12:49






  • 1





    there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

    – phuclv
    Apr 2 at 14:00












  • and add 1 more codereview.stackexchange.com/questions/37177/…

    – NoChance
    Apr 2 at 14:36












  • 3





    It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

    – Some programmer dude
    Apr 2 at 7:11







  • 1





    Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

    – Antti Haapala
    Apr 2 at 12:49






  • 1





    there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

    – phuclv
    Apr 2 at 14:00












  • and add 1 more codereview.stackexchange.com/questions/37177/…

    – NoChance
    Apr 2 at 14:36







3




3





It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

– Some programmer dude
Apr 2 at 7:11






It's very tricky since you just can't add two numbers and check if the value is above some threshold (because signed integer arithmetic overflow and such). A simple solution might be to check if x (the value you want to check) is above a specific threshold, or if adding one goes above a threshold. If it does and the other number you want to add is larger than one, then you have an overflow situation.

– Some programmer dude
Apr 2 at 7:11





1




1





Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

– Antti Haapala
Apr 2 at 12:49





Nitpick, but, it was CPython 2.7 that did this. CPython 3 doesn't "promote" anything, even internally there is just one type.

– Antti Haapala
Apr 2 at 12:49




1




1





there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

– phuclv
Apr 2 at 14:00






there are a lot of duplicates depending on what you want to do with the values (add/sub/mul/div/...?) How to check if A+B exceed long long? (both A and B is long long), Detecting signed overflow in C/C++, Test for overflow in integer addition

– phuclv
Apr 2 at 14:00














and add 1 more codereview.stackexchange.com/questions/37177/…

– NoChance
Apr 2 at 14:36





and add 1 more codereview.stackexchange.com/questions/37177/…

– NoChance
Apr 2 at 14:36












3 Answers
3






active

oldest

votes


















29














You cannot detect signed int overflow. You have to write your code to avoid it.



Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.






share|improve this answer


















  • 3





    You can check you input values before doing a calculation to prevent overflow.

    – A.R.C.
    Apr 2 at 7:12






  • 7





    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

    – hetepeperfan
    Apr 2 at 7:17







  • 6





    @hetepeperfan It's because that's what the language standard says.

    – Sneftel
    Apr 2 at 8:40







  • 6





    @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

    – hetepeperfan
    Apr 2 at 9:07






  • 5





    @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

    – Antti Haapala
    Apr 2 at 12:51


















16














You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.



It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.



If it were me, I'd do something like this:



#include <limits.h>

int safe_add(int a, int b)

if (a >= 0)
if (b > (INT_MAX - a))
/* handle overflow */

else
if (b < (INT_MIN - a))
/* handle underflow */


return a + b;



Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.



EDIT:



GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.



bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
bool __builtin_sadd_overflow (int a, int b, int *res)
bool __builtin_saddl_overflow (long int a, long int b, long int *res)
bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)


Visit this link.



EDIT:



Regarding the question asked by someone




I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..




The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.



In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).






share|improve this answer

























  • Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

    – chqrlie
    Apr 2 at 7:28












  • @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

    – Antti Haapala
    Apr 2 at 7:30











  • It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

    – chqrlie
    Apr 2 at 7:32






  • 8





    Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

    – Antti Haapala
    Apr 2 at 7:32






  • 3





    @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

    – chqrlie
    Apr 2 at 7:37


















8














Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:



#include <limits.h>

int safe_add(int a, int b)
if (a >= 0)
if (b > INT_MAX - a)
/* handle overflow */
else
return a + b;

else
if (b < INT_MIN - a)
/* handle negative overflow */
else
return a + b;





If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:



#include <limits.h>

int safe_add(int a, int b) res < INT_MIN)
/* handle overflow */
else
return (int)res;







share|improve this answer































    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    29














    You cannot detect signed int overflow. You have to write your code to avoid it.



    Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.






    share|improve this answer


















    • 3





      You can check you input values before doing a calculation to prevent overflow.

      – A.R.C.
      Apr 2 at 7:12






    • 7





      I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

      – hetepeperfan
      Apr 2 at 7:17







    • 6





      @hetepeperfan It's because that's what the language standard says.

      – Sneftel
      Apr 2 at 8:40







    • 6





      @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

      – hetepeperfan
      Apr 2 at 9:07






    • 5





      @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

      – Antti Haapala
      Apr 2 at 12:51















    29














    You cannot detect signed int overflow. You have to write your code to avoid it.



    Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.






    share|improve this answer


















    • 3





      You can check you input values before doing a calculation to prevent overflow.

      – A.R.C.
      Apr 2 at 7:12






    • 7





      I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

      – hetepeperfan
      Apr 2 at 7:17







    • 6





      @hetepeperfan It's because that's what the language standard says.

      – Sneftel
      Apr 2 at 8:40







    • 6





      @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

      – hetepeperfan
      Apr 2 at 9:07






    • 5





      @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

      – Antti Haapala
      Apr 2 at 12:51













    29












    29








    29







    You cannot detect signed int overflow. You have to write your code to avoid it.



    Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.






    share|improve this answer













    You cannot detect signed int overflow. You have to write your code to avoid it.



    Signed int overflow is Undefined Behaviour and if it is present in your program, the program is invalid and the compiler is not required to generate any specific behaviour.







    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered Apr 2 at 7:10









    Jesper JuhlJesper Juhl

    17.5k32647




    17.5k32647







    • 3





      You can check you input values before doing a calculation to prevent overflow.

      – A.R.C.
      Apr 2 at 7:12






    • 7





      I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

      – hetepeperfan
      Apr 2 at 7:17







    • 6





      @hetepeperfan It's because that's what the language standard says.

      – Sneftel
      Apr 2 at 8:40







    • 6





      @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

      – hetepeperfan
      Apr 2 at 9:07






    • 5





      @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

      – Antti Haapala
      Apr 2 at 12:51












    • 3





      You can check you input values before doing a calculation to prevent overflow.

      – A.R.C.
      Apr 2 at 7:12






    • 7





      I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

      – hetepeperfan
      Apr 2 at 7:17







    • 6





      @hetepeperfan It's because that's what the language standard says.

      – Sneftel
      Apr 2 at 8:40







    • 6





      @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

      – hetepeperfan
      Apr 2 at 9:07






    • 5





      @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

      – Antti Haapala
      Apr 2 at 12:51







    3




    3





    You can check you input values before doing a calculation to prevent overflow.

    – A.R.C.
    Apr 2 at 7:12





    You can check you input values before doing a calculation to prevent overflow.

    – A.R.C.
    Apr 2 at 7:12




    7




    7





    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

    – hetepeperfan
    Apr 2 at 7:17






    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..

    – hetepeperfan
    Apr 2 at 7:17





    6




    6





    @hetepeperfan It's because that's what the language standard says.

    – Sneftel
    Apr 2 at 8:40






    @hetepeperfan It's because that's what the language standard says.

    – Sneftel
    Apr 2 at 8:40





    6




    6





    @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

    – hetepeperfan
    Apr 2 at 9:07





    @sneftel thats an authoritative argument lacking an authoritative source, despise it is probably correct. On top of that, standards make more sense to people, once they start to understand the language, which is perhaps a reason they visit stackoverflow in the first place.

    – hetepeperfan
    Apr 2 at 9:07




    5




    5





    @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

    – Antti Haapala
    Apr 2 at 12:51





    @hetepeperfan the reason for why the standard is written as it is, is for the most part outside the scope of Stack Overflow.

    – Antti Haapala
    Apr 2 at 12:51













    16














    You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.



    It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.



    If it were me, I'd do something like this:



    #include <limits.h>

    int safe_add(int a, int b)

    if (a >= 0)
    if (b > (INT_MAX - a))
    /* handle overflow */

    else
    if (b < (INT_MIN - a))
    /* handle underflow */


    return a + b;



    Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.



    EDIT:



    GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.



    bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
    bool __builtin_sadd_overflow (int a, int b, int *res)
    bool __builtin_saddl_overflow (long int a, long int b, long int *res)
    bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
    bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
    bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
    bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)


    Visit this link.



    EDIT:



    Regarding the question asked by someone




    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..




    The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.



    In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).






    share|improve this answer

























    • Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

      – chqrlie
      Apr 2 at 7:28












    • @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

      – Antti Haapala
      Apr 2 at 7:30











    • It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

      – chqrlie
      Apr 2 at 7:32






    • 8





      Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

      – Antti Haapala
      Apr 2 at 7:32






    • 3





      @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

      – chqrlie
      Apr 2 at 7:37















    16














    You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.



    It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.



    If it were me, I'd do something like this:



    #include <limits.h>

    int safe_add(int a, int b)

    if (a >= 0)
    if (b > (INT_MAX - a))
    /* handle overflow */

    else
    if (b < (INT_MIN - a))
    /* handle underflow */


    return a + b;



    Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.



    EDIT:



    GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.



    bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
    bool __builtin_sadd_overflow (int a, int b, int *res)
    bool __builtin_saddl_overflow (long int a, long int b, long int *res)
    bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
    bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
    bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
    bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)


    Visit this link.



    EDIT:



    Regarding the question asked by someone




    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..




    The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.



    In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).






    share|improve this answer

























    • Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

      – chqrlie
      Apr 2 at 7:28












    • @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

      – Antti Haapala
      Apr 2 at 7:30











    • It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

      – chqrlie
      Apr 2 at 7:32






    • 8





      Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

      – Antti Haapala
      Apr 2 at 7:32






    • 3





      @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

      – chqrlie
      Apr 2 at 7:37













    16












    16








    16







    You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.



    It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.



    If it were me, I'd do something like this:



    #include <limits.h>

    int safe_add(int a, int b)

    if (a >= 0)
    if (b > (INT_MAX - a))
    /* handle overflow */

    else
    if (b < (INT_MIN - a))
    /* handle underflow */


    return a + b;



    Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.



    EDIT:



    GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.



    bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
    bool __builtin_sadd_overflow (int a, int b, int *res)
    bool __builtin_saddl_overflow (long int a, long int b, long int *res)
    bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
    bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
    bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
    bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)


    Visit this link.



    EDIT:



    Regarding the question asked by someone




    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..




    The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.



    In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).






    share|improve this answer















    You can predict signed int overflow but attempting to detect it after the summation is too late. You have to test for possible overflow before you do a signed addition.



    It's not possible to avoid undefined behaviour by testing for it after the summation. If the addition overflows then there is already undefined behaviour.



    If it were me, I'd do something like this:



    #include <limits.h>

    int safe_add(int a, int b)

    if (a >= 0)
    if (b > (INT_MAX - a))
    /* handle overflow */

    else
    if (b < (INT_MIN - a))
    /* handle underflow */


    return a + b;



    Refer this paper for more information. You can also find why unsigned integer overflow is not undefined behaviour and what could be portability issues in the same paper.



    EDIT:



    GCC and other compilers have some provisions to detect the overflow. For example, GCC has following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.



    bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
    bool __builtin_sadd_overflow (int a, int b, int *res)
    bool __builtin_saddl_overflow (long int a, long int b, long int *res)
    bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
    bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
    bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
    bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)


    Visit this link.



    EDIT:



    Regarding the question asked by someone




    I think, it would be nice and informative to explain why signed int overflow undefined, whereas unsigned apperantly isn't..




    The answer depends upon the implementation of the compiler. Most C implementations (compilers) just used whatever overflow behaviour was easiest to implement with the integer representation it used.



    In practice, the representations for signed values may differ (according to the implementation): one's complement, two's complement, sign-magnitude. For an unsigned type there is no reason for the standard to allow variation because there is only one obvious binary representation (the standard only allows binary representation).







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 2 days ago

























    answered Apr 2 at 7:17









    abhiaroraabhiarora

    2,49931533




    2,49931533












    • Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

      – chqrlie
      Apr 2 at 7:28












    • @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

      – Antti Haapala
      Apr 2 at 7:30











    • It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

      – chqrlie
      Apr 2 at 7:32






    • 8





      Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

      – Antti Haapala
      Apr 2 at 7:32






    • 3





      @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

      – chqrlie
      Apr 2 at 7:37

















    • Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

      – chqrlie
      Apr 2 at 7:28












    • @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

      – Antti Haapala
      Apr 2 at 7:30











    • It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

      – chqrlie
      Apr 2 at 7:32






    • 8





      Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

      – Antti Haapala
      Apr 2 at 7:32






    • 3





      @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

      – chqrlie
      Apr 2 at 7:37
















    Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

    – chqrlie
    Apr 2 at 7:28






    Why extra parentheses? Also you could save one test on average with if (a >= 0) test overflow else test underflow return a + b;

    – chqrlie
    Apr 2 at 7:28














    @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

    – Antti Haapala
    Apr 2 at 7:30





    @chqrlie that is not sufficient because there is no possibility of overflow when a == 0.

    – Antti Haapala
    Apr 2 at 7:30













    It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

    – chqrlie
    Apr 2 at 7:32





    It is not necessary to test overflow if a == 0 but testing a just once saves one comparison if a < 0, which is half the cases.

    – chqrlie
    Apr 2 at 7:32




    8




    8





    Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

    – Antti Haapala
    Apr 2 at 7:32





    Also, both are technically called overflow. Underflow means that the value is too small in magnitude to be representable in a floating point variable.

    – Antti Haapala
    Apr 2 at 7:32




    3




    3





    @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

    – chqrlie
    Apr 2 at 7:37





    @AnttiHaapala it does not ignore the case a == 0 where there is no possible overflow, it just handles it differently.

    – chqrlie
    Apr 2 at 7:37











    8














    Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:



    #include <limits.h>

    int safe_add(int a, int b)
    if (a >= 0)
    if (b > INT_MAX - a)
    /* handle overflow */
    else
    return a + b;

    else
    if (b < INT_MIN - a)
    /* handle negative overflow */
    else
    return a + b;





    If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:



    #include <limits.h>

    int safe_add(int a, int b) res < INT_MIN)
    /* handle overflow */
    else
    return (int)res;







    share|improve this answer





























      8














      Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:



      #include <limits.h>

      int safe_add(int a, int b)
      if (a >= 0)
      if (b > INT_MAX - a)
      /* handle overflow */
      else
      return a + b;

      else
      if (b < INT_MIN - a)
      /* handle negative overflow */
      else
      return a + b;





      If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:



      #include <limits.h>

      int safe_add(int a, int b) res < INT_MIN)
      /* handle overflow */
      else
      return (int)res;







      share|improve this answer



























        8












        8








        8







        Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:



        #include <limits.h>

        int safe_add(int a, int b)
        if (a >= 0)
        if (b > INT_MAX - a)
        /* handle overflow */
        else
        return a + b;

        else
        if (b < INT_MIN - a)
        /* handle negative overflow */
        else
        return a + b;





        If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:



        #include <limits.h>

        int safe_add(int a, int b) res < INT_MIN)
        /* handle overflow */
        else
        return (int)res;







        share|improve this answer















        Signed operands must be tested before the addition is performed. Here is a safe addition function with 2 comparisons in all cases:



        #include <limits.h>

        int safe_add(int a, int b)
        if (a >= 0)
        if (b > INT_MAX - a)
        /* handle overflow */
        else
        return a + b;

        else
        if (b < INT_MIN - a)
        /* handle negative overflow */
        else
        return a + b;





        If the type long long is known to have a larger range than type int, you could use this approach, which might prove faster:



        #include <limits.h>

        int safe_add(int a, int b) res < INT_MIN)
        /* handle overflow */
        else
        return (int)res;








        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited Apr 2 at 7:55

























        answered Apr 2 at 7:40









        chqrliechqrlie

        62.5k848107




        62.5k848107













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