R – Objective-C: variable optimized away by compiler

cocoacompiler-constructiongdbobjective cxcode

I am trying to run the following code:

1. NSURL *checkLicenseURL = [NSURL URLWithString:@"check_license.php?accesskey=&license_key="];
// call server API
2. NSError *err = nil;
3. NSXMLDocument *xmlResult = [[NSXMLDocument alloc] initWithContentsOfURL:checkLicenseURL options:NSXMLDocumentTidyXML error:&err];

But when looking at variables in gdb, after line 1 was executed, doing

p checkLicenseURL

returns

$1 = <variable optimized away by compiler>

It also causes line 3 to crash. Why is this happening and how do I fix this?

Best Answer

Just compile without optimizations turned on, or select a "debug" build if you used a wizard of some sort to build your project. I'm not sure where to turn off optimizations in XCode but you probably want these GCC command line options for debugging:

-O0 -fno-inline

Related Solutions

What does a just-in-time (JIT) compiler do

A JIT compiler runs after the program has started and compiles the code (usually bytecode or some kind of VM instructions) on the fly (or just-in-time, as it's called) into a form that's usually faster, typically the host CPU's native instruction set. A JIT has access to dynamic runtime information whereas a standard compiler doesn't and can make better optimizations like inlining functions that are used frequently.

This is in contrast to a traditional compiler that compiles all the code to machine language before the program is first run.

To paraphrase, conventional compilers build the whole program as an EXE file BEFORE the first time you run it. For newer style programs, an assembly is generated with pseudocode (p-code). Only AFTER you execute the program on the OS (e.g., by double-clicking on its icon) will the (JIT) compiler kick in and generate machine code (m-code) that the Intel-based processor or whatever will understand.

Objective-c – Generating random numbers in Objective-C

You should use the arc4random_uniform() function. It uses a superior algorithm to rand. You don't even need to set a seed.

#include <stdlib.h>
// ...
// ...
int r = arc4random_uniform(74);

The arc4random man page:

NAME
     arc4random, arc4random_stir, arc4random_addrandom -- arc4 random number generator

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     u_int32_t
     arc4random(void);

     void
     arc4random_stir(void);

     void
     arc4random_addrandom(unsigned char *dat, int datlen);

DESCRIPTION
     The arc4random() function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8
     bit S-Boxes.  The S-Boxes can be in about (2**1700) states.  The arc4random() function returns pseudo-
     random numbers in the range of 0 to (2**32)-1, and therefore has twice the range of rand(3) and
     random(3).

     The arc4random_stir() function reads data from /dev/urandom and uses it to permute the S-Boxes via
     arc4random_addrandom().

     There is no need to call arc4random_stir() before using arc4random(), since arc4random() automatically
     initializes itself.

EXAMPLES
     The following produces a drop-in replacement for the traditional rand() and random() functions using
     arc4random():

           #define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))

Best Answer

Related Solutions

What does a just-in-time (JIT) compiler do

Objective-c – Generating random numbers in Objective-C

Related Topic