Why are integer types promoted during addition in C? -


so had field issue, , after days of debugging, narrowed down problem particular bit of code, processing in while loop wasn't happening :

// heavily redacted code // numbera , numberb both of uint16_t // important stuff happens in while loop  while ( numbera + 1 == numberb ) {     // processing }

this had run fine, until hit uint16 limit of 65535. bunch of print statements later, discovered numbera + 1 had value of 65536, while numberb wrapped 0. failed check , no processing done.

this got me curious, put quick c program (compiled gcc 4.9.2) check this:

#include <stdio.h> #include <stdint.h>  int main() {      uint16_t numbera, numberb;     numbera = 65535;     numberb = numbera + 1;      uint32_t numberc, numberd;     numberc = 4294967295;     numberd = numberc + 1;      printf("numbera = %d\n", numbera + 1);     printf("numberb = %d\n", numberb);      printf("numberc = %d\n", numberc + 1);     printf("numberd = %d\n", numberd);      return 0; }

and result :

numbera = 65536 numberb = 0 numberc = 0 numberd = 0

so appears result of numbera + 1 promoted uint32_t. intended c language ? or compiler / hardware oddity?

so appears result of numbera + 1 promoted uint32_t

the operands of addition promoted int before addition took place, , result of addition of same type effective operands (int).

indeed, if int 32-bit wide on compilation platform (meaning type represents uint16_t has lower “conversion rank” int), numbera + 1 computed int addition between 1 , promoted numbera part of integer promotion rules, 6.3.1.1:2 in c11 standard:

the following may used in expression wherever int or unsigned int may used: […] object or expression integer type (other int or unsigned int) integer conversion rank less or equal rank of int , unsigned int.

[…]

if int can represent values of original type […], value converted int

in case, unsigned short in likelihood uint16_t defined on platform, has values representable elements of int, unsigned short value numbera gets promoted int when occurs in arithmetic operation.


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