How do I bit shift unsigned long in C?

Question

I've tried everything to figure this out and I don't get what I'm doing wrong. converting to signed makes everything past the numerical hex values and I know if I have an integer I can straight up use "ULL" to convert it and bitshift away but I can't figure out

basically

void   tick(unsigned long* tmp) { //input as a parameter
   signed long temp = tmp;

   printf("%x\n", temp >> 16 & 0xff);

   tmp = (signed) temp;

   return;
}

EDIT: so I'm stupid and I should've used %lu not %x, now I can see my results but I don't see any change occurring with clock, even with bitshifting the temp

First, the initialization `signed long temp = tmp` is very suspect. Secondly, for some specified input, what is the expected and actual output? And please read about [how to ask good questions](http://stackoverflow.com/help/how-to-ask), as well as [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/). Lastly learn how to create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve). — Some programmer dude, Nov 01 '18 at 07:28
Why are you mixing pointers and non pointers, do you mean `signed long temp = (signed long)*tmp;`?, if you want to work whit the address of `tmp` use `intptr_t` instead of `long` — David Ranieri, Nov 01 '18 at 07:33
THe question is not so much of what you're doing wrong, but rather what are you trying to achieve? — Jabberwocky, Nov 01 '18 at 08:26
I must've not stated everything properly, I apologize. The goal is to add one byte to the temp variable in a clock format. So the temp variable is laid out like so: 0x(hour)(minute)(second)(am/pm) with 2 bytes for each parameter. I need to add one "second" to the long parameter but I'm struggling in converting the unsigned long into a long to modify in the first place (I'll update my question as well) — Jacinth Gudetti, Nov 01 '18 at 10:32
I'm new to C, I see the problem with %x, I should've used %lu — Jacinth Gudetti, Nov 01 '18 at 10:37

score 0 · Answer 1 · answered Nov 01 '18 at 10:48

The function parameter is a pointer, so you need to dereference it to access it's value.
You need to assign the bitshifted value to your variable if you want to store it.
And if you want the value to be changed under the pointer, you need to assign the new value to the derefenced pointer
You also need to use long formating options for it to display correctly. (Here, lux -> long unsigned hexadecimal)

Here is a modified version:

#include <stdio.h>

void    tick(unsigned long* input)
{
  unsigned long temp;

  temp = ((*input) >> 16) & 0xff;
  printf("%lux\n", temp);

  *input = temp;
}

int main()
{
  long test = 1;
  tick(&test);
  test = 0b100000000000000000;
  tick(&test);
  test = 0b111111111111111111;
  tick(&test);
}

score 0 · Answer 2 · answered Nov 01 '18 at 11:03

From information in the comments, you actually have your integer laid out like with hour/minute/seconds/am-pm with 2 bytes(make sure you meant 2 bytes, and not 2 nibbles) for each field like e.g. this:

+--+--+--+--+--+--+--+--+
|H |H |M |M |S |S |AM PM|
+--+--+--+--+--+--+--+--+

And you want to extract the 2 bytes containing the seconds and add 1 to them.

You'll need 8 bytes (64 bits) to represent that value, so I'll use the uint64_t type instead of an unsigned long for that, you get the uint64_t type by #include <stdint.h>

void tick(uint64_t *value) { //input as a parameter
  //extract seconds 
  unsigned int seconds ((*value) >> 16) & 0xffff

  printf("seconds = %u\n", seconds);

 //add 1 second and pack the value back:
 seconds += 1;
 *value = (value & 0xffffffff0000ffff) | ((seconds & 0xffff) << 16);

 return; //returns void
}

(Now, you might also need to handle wrapping, e.g. increase the minute when seconds reach 60 and so on)

I need to use unsigned long unfortunately, can I convert to a uint64_t? — Jacinth Gudetti, Nov 01 '18 at 19:44
If you know that unsigned long on your platform is 64 bit, just use unsigned long instead of uint64_t. But be sure that you really meant that each field (such as a second) is 16 bits (2 bytes) and not 8 bit (1 byte). — nos, Nov 01 '18 at 20:37

How do I bit shift unsigned long in C?

2 Answers2