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Local, automatic variables assume unexpected values if they are read before they are initialized. The C Standard, 6.7.9, paragraph 10, specifies [ISO/IEC 9899:2011]

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

See undefined behavior 11.

When local, automatic variables are stored on the program stack, for example, their values default to whichever values are currently stored in stack memory.

Additionally, some dynamic memory allocation functions do not initialize the contents of the memory they allocate.

Function

Initialization

aligned_alloc()

Does not perform initialization

calloc()

Zero-initializes allocated memory

malloc()

Does not perform initialization

realloc()

Copies contents from original pointer; may not initialize all memory

Uninitialized automatic variables or dynamically allocated memory has indeterminate values, which for objects of some types, can be a trap representation. Reading such trap representations is undefined behavior; it can cause a program to behave in an unexpected manner and provide an avenue for attack. (See undefined behavior 10 and undefined behavior 12.)  In many cases, compilers issue a warning diagnostic message when reading uninitialized variables. (See MSC00-C. Compile cleanly at high warning levels for more information.)

Noncompliant Code Example (Return-by-Reference)

In this noncompliant code example, the set_flag() function is intended to set the parameter, sign_flag, to the sign of number. However, the programmer neglected to account for the case where number is equal to 0. Because the local variable sign is uninitialized when calling set_flag() and is never written to by set_flag(), the comparison operation exhibits undefined behavior when reading sign.

void set_flag(int number, int *sign_flag) {
  if (NULL == sign_flag) {
    return;
  }

  if (number > 0) {
    *sign_flag = 1;
  } else if (number < 0) {
    *sign_flag = -1;
  }
}

int is_negative(int number) {
  int sign;
  set_flag(number, &sign);
  return sign < 0;
}

Some compilers assume that when the address of an uninitialized variable is passed to a function, the variable is initialized within that function. Because compilers frequently fail to diagnose any resulting failure to initialize the variable, the programmer must apply additional scrutiny to ensure the correctness of the code.

This defect results from a failure to consider all possible data states. (See MSC01-C. Strive for logical completeness for more information.)

Compliant Solution (Return-by-Reference)

This compliant solution trivially repairs the problem by accounting for the possibility that number can be equal to 0.

Although compilers and static analysis tools often detect uses of uninitialized variables when they have access to the source code, diagnosing the problem is difficult or impossible when either the initialization or the use takes place in object code for which the source code is inaccessible. Unless doing so is prohibitive for performance reasons, an additional defense-in-depth practice worth considering is to initialize local variables immediately after declaration.

void set_flag(int number, int *sign_flag) {
  if (NULL == sign_flag) {
    return;
  }

  /* Account for number being 0 */
  if (number >= 0) { 
    *sign_flag = 1;
  } else {
    *sign_flag = -1;
  }
}

int is_negative(int number) {
  int sign = 0; /* Initialize for defense-in-depth */
  set_flag(number, &sign);
  return sign < 0;
}

Noncompliant Code Example (Uninitialized Local)

In this noncompliant code example, the programmer mistakenly fails to set the local variable error_log to the msg argument in the report_error() function [Mercy 2006]. Because error_log has not been initialized, an indeterminate value is read. The sprintf() call copies data from the arbitrary location pointed to by the indeterminate error_log variable until a null byte is reached, which can result in a buffer overflow.

#include <stdio.h>

/* Get username and password from user, return -1 on error */
extern int do_auth(void);
enum { BUFFERSIZE = 24 }; 
void report_error(const char *msg) {
  const char *error_log;
  char buffer[BUFFERSIZE];

  sprintf(buffer, "Error: %s", error_log);
  printf("%s\n", buffer);
}

int main(void) {
  if (do_auth() == -1) {
    report_error("Unable to login");
  }
  return 0;
}

Noncompliant Code Example (Uninitialized Local)

In this noncompliant code example, the report_error() function has been modified so that error_log is properly initialized:

#include <stdio.h>
enum { BUFFERSIZE = 24 }; 
void report_error(const char *msg) {
  const char *error_log = msg;
  char buffer[BUFFERSIZE];

  sprintf(buffer, "Error: %s", error_log);
  printf("%s\n", buffer);
}

This example remains problematic because a buffer overflow will occur if the null-terminated byte string referenced by msg is greater than 17 characters, including the null terminator. (See STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator for more information.)

Compliant Solution (Uninitialized Local)

In this compliant solution, the buffer overflow is eliminated by calling the snprintf() function:

#include <stdio.h>
enum { BUFFERSIZE = 24 };
void report_error(const char *msg) {
  char buffer[BUFFERSIZE];

  if (0 < snprintf(buffer, BUFFERSIZE, "Error: %s", msg))
    printf("%s\n", buffer);
  else
    puts("Unknown error");
}

Compliant Solution (Uninitialized Local)

A less error-prone compliant solution is to simply print the error message directly instead of using an intermediate buffer:

#include <stdio.h>
 
void report_error(const char *msg) {
  printf("Error: %s\n", msg);
}

Noncompliant Code Example (mbstate_t)

In this noncompliant code example, the function mbrlen() is passed the address of an automatic mbstate_t object that has not been properly initialized. This is undefined behavior 200 because mbrlen() dereferences and reads its third argument.

#include <string.h> 
#include <wchar.h>
 
void func(const char *mbs) {
  size_t len;
  mbstate_t state;

  len = mbrlen(mbs, strlen(mbs), &state);
}

Compliant Solution (mbstate_t)

Before being passed to a multibyte conversion function, an mbstate_t object must be either initialized to the initial conversion state or set to a value that corresponds to the most recent shift state by a prior call to a multibyte conversion function. This compliant solution sets the mbstate_t object to the initial conversion state by setting it to all zeros:

#include <string.h> 
#include <wchar.h>
 
void func(const char *mbs) {
  size_t len;
  mbstate_t state;

  memset(&state, 0, sizeof(state));
  len = mbrlen(mbs, strlen(mbs), &state);
}

Noncompliant Code Example (POSIX, Entropy)

In this noncompliant code example described in "More Randomness or Less" [Wang 2012], the process ID, time of day, and uninitialized memory junk is used to seed a random number generator. This behavior is characteristic of some distributions derived from Debian Linux that use uninitialized memory as a source of entropy because the value stored in junk is indeterminate. However, because accessing an indeterminate value is undefined behavior, compilers may optimize out the uninitialized variable access completely, leaving only the time and process ID and resulting in a loss of desired entropy.

#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/time.h>
  
void func(void) {
  struct timeval tv;
  unsigned long junk;

  gettimeofday(&tv, NULL);
  srandom((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
}

In security protocols that rely on unpredictability, such as RSA encryption, a loss in entropy results in a less secure system.

Compliant Solution (POSIX, Entropy)

This compliant solution seeds the random number generator by using the CPU clock and the real-time clock instead of reading uninitialized memory:

#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/time.h>

void func(void) {     
  double cpu_time;
  struct timeval tv;

  cpu_time = ((double) clock()) / CLOCKS_PER_SEC;
  gettimeofday(&tv, NULL);
  srandom((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ cpu_time);
}

Noncompliant Code Example (realloc())

The realloc() function changes the size of a dynamically allocated memory object. The initial size bytes of the returned memory object are unchanged, but any newly added space is uninitialized, and its value is indeterminate. As in the case of malloc(), accessing memory beyond the size of the original object is undefined behavior 181.

It is the programmer's responsibility to ensure that any memory allocated with malloc() and realloc() is properly initialized before it is used.

In this noncompliant code example, an array is allocated with malloc() and properly initialized. At a later point, the array is grown to a larger size but not initialized beyond what the original array contained. Subsequently accessing the uninitialized bytes in the new array is undefined behavior.

#include <stdlib.h>
#include <stdio.h>
enum { OLD_SIZE = 10, NEW_SIZE = 20 };
 
int *resize_array(int *array, size_t count) {
  if (0 == count) {
    return 0;
  }
 
  int *ret = (int *)realloc(array, count * sizeof(int));
  if (!ret) {
    free(array);
    return 0;
  }
 
  return ret;
}
 
void func(void) {
 
  int *array = (int *)malloc(OLD_SIZE * sizeof(int));
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < OLD_SIZE; ++i) {
    array[i] = i;
  }
 
  array = resize_array(array, NEW_SIZE);
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < NEW_SIZE; ++i) {
    printf("%d ", array[i]);
  }
}

Compliant Solution (realloc())

In this compliant solution, the resize_array() helper function takes a second parameter for the old size of the array so that it can initialize any newly allocated elements:

#include <stdlib.h>
#include <stdio.h> 
#include <string.h>

enum { OLD_SIZE = 10, NEW_SIZE = 20 };
 
int *resize_array(int *array, size_t old_count, size_t new_count) {
  if (0 == new_count) {
    return 0;
  }
 
  int *ret = (int *)realloc(array, new_count * sizeof(int));
  if (!ret) {
    free(array);
    return 0;
  }
 
  if (new_count > old_count) {
    memset(ret + old_count, 0, (new_count - old_count) * sizeof(int));
  }
 
  return ret;
}
 
void func(void) {
 
  int *array = (int *)malloc(OLD_SIZE * sizeof(int));
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < OLD_SIZE; ++i) {
    array[i] = i;
  }
 
  array = resize_array(array, OLD_SIZE, NEW_SIZE);
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < NEW_SIZE; ++i) {
    printf("%d ", array[i]);
  }
}

Exceptions

EXP33-C-EX1: Reading uninitialized memory by an lvalue of type unsigned char does not trigger undefined behavior. The unsigned char type is defined to not have a trap representation, which allows for moving bytes without knowing if they are initialized. (See the C Standard, 6.2.6.1, paragraph 3.) However, on some architectures, such as the Intel Itanium, registers have a bit to indicate whether or not they have been initialized. The C Standard, 6.3.2.1, paragraph 2, allows such implementations to cause a trap for an object that never had its address taken and is stored in a register if such an object is referred to in any way.

Risk Assessment

Reading uninitialized variables is undefined behavior and can result in unexpected program behavior. In some cases, these security flaws may allow the execution of arbitrary code.

Reading uninitialized variables for creating entropy is problematic because these memory accesses can be removed by compiler optimization. VU#925211 is an example of a vulnerability caused by this coding error.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP33-C

High

Probable

Medium

P12

L1

Automated Detection

ToolVersionCheckerDescription
Astrée
18.10

uninitialized-local-read

uninitialized-variable-use

Fully checked
Axivion Bauhaus Suite

6.9.0

CertC-EXP33
CodeSonar
5.0p0
LANG.MEM.UVARUninitialized variable
Compass/ROSE

Automatically detects simple violations of this rule, although it may return some false positives. It may not catch more complex violations, such as initialization within functions taking uninitialized variables as arguments. It does catch the second noncompliant code example, and can be extended to catch the first as well

Coverity
2017.07

UNINIT

Implemented
Cppcheck
1.66

uninitvar
uninitdata
uninitstring
uninitMemberVar
uninitStructMember

Detects uninitialized variables, uninitialized pointers, uninitialized struct members, and uninitialized array elements (However, if one element is initialized, then cppcheck assumes the array is initialized.)
There are FN compared to some other tools because Cppcheck tries to avoid FP in impossible paths.

GCC4.3.5

Can detect some violations of this rule when the -Wuninitialized flag is used

Klocwork
2018

UNINIT.HEAP.MIGHT
UNINIT.HEAP.MUST
UNINIT.STACK.ARRAY.MIGHT
UNINIT.STACK.ARRAY.MUST UNINIT.STACK.ARRAY.PARTIAL.MUST
UNINIT.STACK.MIGHT
UNINIT.STACK.MUST


LDRA tool suite
9.7.1

53 D, 69 D, 631 S, 652 S

Fully implemented

Parasoft C/C++test

10.4.1

CERT_C-EXP33-a

Avoid use before initialization

Parasoft Insure++

10.4.1


Runtime analysis
Polyspace Bug Finder

R2018a

Non-initialized pointer

Non-initialized variable

Pointer not initialized before dereference

Variable not initialized before use

PRQA QA-C
9.5

2726, 2727, 2728, 2961,

2962, 2963, 2966, 2967,

2968, 2971, 2972, 2973,

2976, 2977, 2978

Fully implemented
PRQA QA-C++
4.3

2961, 2962, 2963, 2966,

2967, 2968, 2971, 2972,

2973, 2976, 2977, 2978


PVS-Studio

6.23

V573, V614, V670, V679
RuleChecker
18.10

uninitialized-local-read

Partially checked
Splint3.1.1

Related Vulnerabilities

CVE-2009-1888 results from a violation of this rule. Some versions of SAMBA (up to 3.3.5) call a function that takes in two potentially uninitialized variables involving access rights. An attacker can exploit these coding errors to bypass the access control list and gain access to protected files [xorl 2009].

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardMSC00-C. Compile cleanly at high warning levelsPrior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding StandardMSC01-C. Strive for logical completenessPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CEXP53-CPP. Do not read uninitialized memoryPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013Initialization of Variables [LAV]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961Referencing uninitialized memory [uninitref]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-4562017-07-05: CERT: Exact
CWE 2.11CWE-4572017-07-05: CERT: Exact
CWE 2.11CWE-7582017-07-05: CERT: Rule subset of CWE
CWE 2.11CWE-9082017-07-05: CERT: Rule subset of CWE

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and EXP33-C


  • Intersection( CWE-119, EXP33-C) = Ø



  • EXP33-C is about reading uninitialized memory, but this memory is considered part of a valid buffer (on the stack, or returned by a heap function). No buffer overflow is involved.


CWE-676 and EXP33-C


  • Intersection( CWE-676, EXP33-C) = Ø



  • EXP33-C implies that memory allocation functions (e.g., malloc()) are dangerous because they do not initialize the memory they reserve. However, the danger is not in their invocation, but rather reading their returned memory without initializing it.


CWE-758 and EXP33-C

Independent( INT34-C, INT36-C, MSC37-C, FLP32-C, EXP33-C, EXP30-C, ERR34-C, ARR32-C)

CWE-758 = Union( EXP33-C, list) where list =


  • Undefined behavior that results from anything other than reading uninitialized memory


CWE-665 and EXP33-C

Intersection( CWE-665, EXP33-C) = Ø

CWE-665 is about correctly initializing items (usually objects), not reading them later. EXP33-C is about reading memory later (that has not been initialized).

CWE-908 and EXP33-C

CWE-908 = Union( EXP33-C, list) where list =


  • Use of uninitialized items besides raw memory (objects, disk space, etc)


New CWE-CERT mappings:

CWE-123 and EXP33-C

Intersection( CWE-123, EXP33-C) = Ø

EXP33-C is only about reading uninitialized memory, not writing, whereas CWE-123 is about writing.

CWE-824 and EXP33-C

EXP33-C = Union( CWE-824, list) where list =


  • Read of uninitialized memory that does not represent a pointer


Bibliography

[Flake 2006]
[ISO/IEC 9899:2011]Subclause 6.7.9, "Initialization"
Subclause 6.2.6.1, "General"
Subclause 6.3.2.1, "Lvalues, Arrays, and Function Designators"
[Mercy 2006]
[VU#925211]
[Wang 2012]"More Randomness or Less"
[xorl 2009]"CVE-2009-1888: SAMBA ACLs Uninitialized Memory Read"



18 Comments

  1. If programmers ignore compiler warnings about uninitialized values, that is the insecure coding practice.

    Compilers can catch this error quite well.  Adding an irrelevant initialization simply makes that level of compiler analysis unavailable.  I've seen too many cases where a programmer was told to clean up the warning and responded by simply adding an initialization - changing code that used an unpredictable value inappropriately to code that used a predictable value inappropriately.  Or, looking at it another way, this change converts a bug that can easily be found by static analysis to one that cannot be found by any reasonable automatic analysis.

    If you're using a C99 compiler, you can usually put off declaring variables until you are in a position to give them an appropriate value.  Most C++ writers recommend a style in which variable declaration is put off until an actual value is available.  Personally, I don't like that style, since it makes it hard to find the declaration of a variable - and thus the most fundamental property of the variable, its type.  (Of course, if you assume you always read your code in an IDE, this may be a non-issue for you.)

    The hard cases - which I've found to be the source of many bugs - is a variable that is supposed to be set by, say, each arm of a switch or if/then/else; or by the first iteration of a loop; and is then used after the body of the switch or loop.  Then it turns out that one arm of the switch or if/then/else forgets to set the value, or the loop unexpectedly has a zero trip count.  Not only are these risky; I would contend that they account for a large fraction of actual bugs due to uninitialized variables.  Most compilers these days will catch these.  But requiring that the value be given some arbitrary initial value disables the compiler analysis - at which point you can have a very subtle failure.

                                                                                                                                               -- Jerry

    1. There are static analysis techniques that can catch the failure even when an inappropriate initialization is present.

      The variable can be in one of the following states:

      Undefined   ( U ) - The value is indeterminate;
      Referenced ( R ) - The value is used in some way (e.g. an expression);
      Defined      ( D ) - The variable is explicitly initialized (not default initialized) or assigned a value.

      Given the above, the following data-flow anomalies can be detected.

      UR - Variable is not assigned a value before use;
      DU - Variable is assigned a value that is never used;
      DD - Variable is assigned a value twice with no intermediate use.

      So, if a variable is initialized to hide a warning (UR anomaly) that exists on one path, DD anomaly(s) with then exist in the other path(s).

  2. Anonymous

    In my opinion -- and in my experience -- the advice to initialize variables reflexively is bad advice.  Often, it merely replaces an "unpredictable" value with a "wrong" value, which does not improve the program's characteristics.  True, a consistent wrong value may be easier to debug than a Heisenbuggly unpredictable value.  But (as the article itself admits) compilers are pretty good at dataflow analysis, and many if not most will squawk if the code uses a variable that is not provably initialized on all paths leading to the reference.  Blanket initialization completely defeats such analysis, suppressing a compile-time diagnostic in favor of a run-time error and late nights with the debugger.  This is a step in the wrong direction, a denial of one of the major themes of software enginerring ("Bug cost increases with time to removal"), and an initiative that should be resisted steadfastly.

    Rescind this rotten rule.

     --

    Eric Sosman

    esosman AT ieee (star) org

  3. I've narrowed this rule down to "do not reference uninitialized variables" which makes more sense to me. Next, I'll probably have to move it to the Expressions section as it is no longer focused on declarations.

    This is a much less drastic fix than you suggested, but I think it does the trick. Plus, I like the non-compliant example because it shows the security risk. Let me know if you are happy with this solution.

  4. This presentation isolates an interesting pattern that could lead to uninitialized variable use without compiler warnings. Halvar says that basically, since the compiler doesn't do interprocedural dataflow analysis, if you have a variable x, and you pass &x to another function, then x is considered to be initialized after the function call. If the function didn't actually touch x, you could be in trouble. This seems like a reasonably plausible idiom, but I haven't thought too hard about how it would manifest itself in real software.

    Anyway, I know this doesn't help much with revising this rule, but I thought it was interesting.

    http://www.blackhat.com/presentations/bh-europe-06/bh-eu-06-Flake.pdf

    1. I added an example to capture this idiom.

  5. See: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35534 for an interesting example of inter-procedural data flow analysis in GCC 4.3.0 on the source of GDB 6.7.1.  The hack fix I used to get GDB to compile with -Werror was basically the 'initialize variable' hack that the other correspondents maligned.  The fix chosen by GDB in the 6.8 development code was to set the variable in the called function.  The original code in GDB was safe - the 'unset' variable was not referenced when not initialized because of the return status; but it would be pretty hard for a compiler to determine that.  The fix avoids the issue by ensuring that the variable is initialized even though it was benign.  (Note: GDB compiles with -Werror, which converts warnings into errors.)

    This is a valid reason for initializing a variable when it isn't strictly necessary - the compiler complains even though you can see that the compiler's complaint is not actually valid.  I've had this sort of problem more than once.  Granted, there are times when the 'fix' conceals a real problem; there are, in my experience, more occasions when the 'fix' stops an incorrect warning by the compiler. 

  6. I have just added a reference to the Debian/OpenSSL vul, stating facts without interpretations.

    One can argue that unitialized memory can be securely used as a source of entropy, as illustrated by OpenSSL. But I feel that the vul does not constitute a valid exception to this rule. This is far too narrow of a scope to warrant invalidating this rule. One may also argue that the unitialized memory usage was unnecessary, as random keys were already generated by other means. Furthermore, one can argue that since code validators and other programmers treat unitialized memory usage as a bug, doing so is likely to lead to misunderstandings about the code intentions and subsequent vulnerabilities, again as illustreated by OpenSSL.

    1. Typo: "OpenSSL code utilized initialized memory" should be "uninitialized".

      I've looked a bit closer at that issue now - check this writeup and the Debian bug report it refers to before assigning blame.  Fine demonstration of how unclean code was "enhanced" by procedural problems and carelessness into a huge bug.  As far as the OpenSSL codebase is concerned it might have been avoided if they had isolated the unclean code, included strong comments, and a valid if(don't be clever) code path instead of a debugging path which wasn't that important since it after all was just for debugging.  In short, deliberate unclean code requires discipline.

      As for whether this should be a rule or not: Seems simple enough.  If this can't be a rule then probably quite a lot of other rules can't be either.  (Several of which I disagree with, but that's another matter.  I won't be following this standard anyway.)  Another case I know of is that Berkeley DB for the sake of performance does not initialize uninteresting bytes before writing them to disk.  I haven't checked what kind of data exactly.  Padding bytes in structs, maybe.  But if so it doesn't actually break this rule since the structs themselves would be initialized.

      Which reminds me - it's not just uninitialized variables which should not be used.  Uninitialized memory is more correct, like memory from malloc().  But then padding bytes must be an exception.  Not sure if it's worth the wordsmithing to get it quite right. 

      1. fixed the typo and added a comment about malloc()

  7. However, on some architectures, such as the Intel Itanium, registers have a bit to indicate whether or not they have been initialized. The C Standard, 6.3.2.1, paragraph 2, allows such implementations to cause a trap for an object that never had its address taken and is stored in a register if such an object is referred to in any way.

    Is it forbidden only on such platforms or is it forbidden on all platforms?

    According to my reading of the standard, it is always undefined.

    1. Reading uninitialized values through an unsigned char is allowed, except in the circumstances where that value has never had its address taken and the value is stored in a register, for some architectures. Eg)

      register unsigned char c; // Presume the compiler honors the register keyword
      if (c) {} // Possible UB
       
      register unsigned char c2; // Similar
      (void)&c2;
      if (c2) {} // Okay
      memcpy(&c, &c2, 1); // Also okay

      So I would say this is only narrowly forbidden on some platforms, and only when working with a nonpointer value. Effectively, this exception is what allows you to implement memcpy().

      1. Yes, I know the intent.

        It seems perfect legal in C99 wording, and suddenly becomes UB in C11.

        What I want to say is that, the standard doesn't say "it may be put in a register which may contain special trap representation", instead it says something like "read if never initialized is UB". So it seems perfect legal for the compiler to exploit the fact that the read may never happen.

        1. There are several issues here that need to be addressed.

          1.  "Is it forbidden only on such platforms or is it forbidden on all platforms?"
            No, it is never forbidden.  That is not what undefined behavior means.
          2. "According to my reading of the standard, it is always undefined."
            That is correct.
          3. "It seems perfect legal in C99 wording, and suddenly becomes UB in C11."
            a.  The standard never addresses legality.  That would affect the sovereignty of countries that adopt the standard.  The standard does address validity.
            b.  Undefined behavior is not invalid code.  It is valid code for which the standard does not impose requirements.
            c.  Reading through uninitialized register-capable lvalues was not called out as undefined behavior in C99.  This was an oversight that was corrected in C11.  It has always resulted in something you didn't want; it just happens to be even more dangerous on architectures that trap.

          This is one of the reasons undefined behavior exists.  It serves as a warning that different hardware behaves differently, possibly with dangerous results.  It is never useful to read uninitialized register-capable variables, so nothing has been lost.

          1. If the C standard doesn't impose any reqirements for some code, why should it be called valid code?

            It has always resulted in something you didn't want; it just happens to be even more dangerous on architectures that trap.

            The standard doesn't impose any reqirements, so it is equally dangerous everywhere.

            Why care the hardware? The standard is some contract between a programmer and an implementation, and the implementation is not hardware alone.

            A sufficiently advanced compiler is indistinguishable from an adversary.

            1. nytsdd,

              I'm fairly sympathetic to your comments.  The bottom line for the C Committee is that there is no useful purpose for reading uninitialized registers in the C language, so this was made undefined behavior to allow for optimization opportunities.  This is a logical and consistent decision for WG14 to reach.  My experience has also been that hardware drives the requirements of the C Standard more than anything else. 

              In this coding standard, we recommend MSC15-C. Do not depend on undefined behavior, and in this particular rule, we require that code not read uninitialized memory, which eliminates the undefined behavior.  We allow an exception for reading uninitialized memory by an lvalue of type unsigned char, and then we have an exception to the exception for an object that never had its address taken and is stored in a register.  If you follow our coding standard, even an advanced compiler cannot defeat you. ;^)

              I do like your comment, however.  OK if I quote it in my "Dangerous Optimizations" lecture?

              1. A sufficiently advanced compiler is indistinguishable from an adversary.

                I don't mind if you quote it, but I myself quote it from http://blog.regehr.org/archives/970 , and I don't know the original source.

        2. Note in C90 reading from an indeterminate value was in the definition of undefined behavior, it became more nuanced in C99 and then stricter in C11. We can see the history laid out well in this article: Reading indeterminate contents might as well be undefined

          Also interesting to note that in C++ prior to C++14 the standard used indeterminate value without actually defining the term and then in C++14 made using an indeterminate value undefined behavior except in the case of unsigned narrow types see this Stackoverflow question