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Simulation of following code, compiled with V6.12 and optimzation level -O2
#include <stdio.h> typedef struct { char data[23]; } device_t; static device_t deviceList[100]; static device_t* rotate(device_t *device) { int index; index = (device - &deviceList[0]) + 1; if (index < 0 || index > 30) { index = 0; } printf("index = %d\n", index); return &deviceList[index]; } int main(void) { unsigned int n = 0; while (1) { printf("n = %d: ", n); rotate((device_t*)(n)); n += 1000; } }
prints n = 0: index = 0 n = 1000: index = 0 n = 2000: index = 0 n = 3000: index = 0 n = 4000: index = 0 n = 5000: index = 0 n = 6000: index = 163395741 n = 7000: index = 350133493 n = 8000: index = 536871245 n = 9000: index = 723608997 n = 10000: index = 910346749 n = 11000: index = 1097084501 n = 12000: index = 1283822253 n = 13000: index = 1470560005 n = 14000: index = 1657297757 n = 15000: index = 1844035509 into Debug (printf) Viewer, which is obviously incorrect. Maybe a bug, or do I missunderstand something?
Thanks Tamir fo replying,
cropping index to 0.. 30 fails with following code, too. Printf output is
addr = 0X2000016C index = 373474418 addr = 0X20000183 index = 373474419 addr = 0X2000019A index = 373474420 addr = 0X200001B1 index = 373474421 addr = 0X200001C8 index = 373474422 addr = 0X200001DF index = 373474423 addr = 0X200001F6 index = 373474424
#include <stdio.h> #include <stdint.h> typedef struct { char data[23]; } device_t; struct { device_t list1[1000]; char dummy; device_t deviceList[31]; } list; static device_t* rotate(device_t *device) { int index; index = (device - &list.deviceList[0]) + 1; if (index < 0 || index > 30) { index = 0; } printf("index = %d\n", index); return &list.deviceList[index]; } int main(void) { device_t *device; device = &list.list1[0]; while (1) { printf("addr = %#8X ", (uint32_t)device); rotate(device); device++; } }
Again, you're subtracting the addresses of pieces of memory that are (presumably) not logically related. Without knowing what you attempt to achieve, this code
index = (device - &list.deviceList[0]) + 1;
would make sense if it were written instead as:
index = (device - &list.deviceList[0]) ;
, in conjunction with
for (uint32_t x = 0 ; x < sizeof(list.deviceList) / sizeof(device_t) ; x++) { device = &list.deviceList[x]; rotate(device); }
Perhaps start by saying what you expect the code to do (show your working), and why you think the actual results are "wrong".
What analysis / investigation / debugging have you done to see where the "unexpected" behaviour originates ?
you are rigth, I am violating the C Standard, 6.5.6 ISO/IEC 9899:2011
Thank you very much
For clarification.
The function
device_t *rotate(device_t *device);
returns next element to device. If device is last element, the first element is returned. If device does not point to an valid object in list, any list element could be returned.
If device does not point to an valid object in list, any list element could be returned.
Except that that part of the intended function is just impossible to implement without causing undefined behaviour via C99 6.5.6 or 6.5.8 along the way. In other words: forget about it.
If that's what you have to achieve, you have to change the signature from handling pointers to handling indices into the array.
In short: in C, there's no such thing as deriving a trustworthy pointer from garbage input. The only checks you can do on untrustworthy pointers are trivial variations on
(ptr != 0) (ptr == 0)
Thanks for your advice.