How do I determine if an integer is unassigned?
int i; /* no assignment */
if (/* conditional statement here to check if int i is unassigned or not */) {
printf("Integer is unassigned!\n");
} else {
printf("Integer is assigned!\n");
}
You can't. It will have "undefined" content, meaning it'll contain what ever happens to be in that memory location at that time.
. . . unless i is declared at the global scope, then it will be initialised to Zero.
C doesn't intrinsically support this - just like it doesn't intrinsically support bounds checking on arrays. It's a trade-off between speed/efficiency and safety.
In general... initialize your variables.
If i is global or static, its value will be 0, otherwise its value can be anything and there is no way to find out whether it is garbage or not.
You may be able to ask for compiler warnings if you use uninitialized values. They're not wholly reliable, however - you get the occasional false positive where the DFA isn't as clever as you'd hope, and maybe occasional false negatives (I'd hope not, but I promise nothing).
For GCC:
-Wuninitialized -O1
If you want to write conditional code:
int a = 3;
int b;
int *p = (rand() > RAND_MAX/2) ? &a : &b;
if (is_uninitialized(*p)) // blah
then you're out of luck. Unlike some dynamic languages, C has no concept of "the undefined value". If a variable is not initialized, it isn't given some special value which can be tested for later. It's not given a value at all, so it's undefined what happens when you use the variable.
As others have noted, you can't write a C program that detects if one of its own variables is uninitialized, and you should strive to make sure that variables are always initialized.
If your goal is to make sure all variables are initialized, a tool like valgrind can detect uses of uninitialized variables dynamically, through expensive run-time analysis.
If your goal is to make sure that private data is initialized exactly once, the usual method is to protect it with
int i;
static bool initialized = 0;
...
if (!initialized) {
initialized = 1;
i = ... i's initial value ...;
}
It's very simple. You know it is unassigned because you did not initialise it.
As all previous answers, there is no way to detect that at runtime.
However, almost any static code analysis tool with warn you for unassigned variables.
Using a variable before intialization (or assignment) is serious cause of errors. You can not reliably check it at runtime, but you can detect it during or before compilation.
I suggest not to check it inside the code. Because this is likely to cause compiler warnings (Variable 'i' is used before it has been assigned a value), introduce new errors and has very little chance on succes in medium to large programs.
The best method is to use static code analys tools (like QA/C or PCLint).
Using compiler at high warning sensitivity level is a free option, with much less coverage as the specialized tools.
If you perform code reviews, you can also include a check for uninitialized variables on the checklist. This is no guarantee, but it will trigger manual checks from reviewers.
If it is runtime checking you want, then you can start of by intializing variables to an out-of-range value. For instance -1 for an otherwise postive value. Then you can check for
#define UNASSIGNED_VALUE -1
static int number_of_apples = UNASSIGNED_VALUE;
if (UNASSIGNED_VALUE == number_of_apples)
{
// error handling
}
this is not a true 'unintialized' variable, but at least you can detect whether runtime assignments in legal range were done.
Usually variables are set to 0 by the C library, but not necessarily.
But basically, you can't. Assign them a default value in the definition, for instance:
int i = 0; /* Or what ever value you know won't be used elsewhere */
Then if you run some code and want to check if the value was set there, you can compare to your initial value.
In C, an integer takes on an undefined value when it is created. This means that if your first use of that integer comes from a register/memory location/device with a 5893872 in it, that's the value of that integer. (Mileage varies for debug/release compilation.)
The usual method of dealing with this is to use a nonsensical default:
int number_of_widgets = -1;
...or a flag to indicate its state:
int number_of_widgets;
int number_of_widgets_assigned = 0;
if (number_of_widgets_assigned)
do something
else
do something else
number_of_widgets_assigned = 1;
There is no other way to detect whether something has been assigned to - unless you want to get into the debugging features of your hardware and I suspect that is not what this conversation is about.
Checking whether or not a variable you're using is initialized (assigned) at runtime is notoriously difficult for C. There's no language support for it, and the information available at runtime simply is insufficient for perfect detection of uninitialized values. Dynamic analysis tools such as Valgrind/Memcheck goes through great lengths (such as keeping track of every byte of memory in your process's address space and then examining every store to mark a byte as intiialized ) to determine whether or not the value using is initialized and are still susceptible to false positives.
If you're just trying to minimize such errors in your programs, static analysis tools such as lint can do a reasonably good job of informing you of whether or not you're using uninitialized variables. In fact, I believe most compilers will do their best to tell you when you're doing this (though, they're certainly not perfect.)
In C# I'd use:
Nullable<int> i = null; /* null assignment */
if (i == null) {
printf("Integer is unassigned!\n");
} else {
printf("Integer is assigned!\n");
}
Not sure if this'd translate to C, though.
Related
I´m a bit confused with these 2.
I have a function called check that does the following:
bool check(const char *word)
{
char newWord[LENGTH + 1] = "";
for (int i = 0; word[i]; i++)
{
newWord[i] = tolower(word[i]);
}
}
Now for example if I use ="", the variable newWord will have all of it´s values as '\0' anytime I run the function check();
But when using char newWord[LENGTH + 1]; the variable seems to keep the old values even after my functions has returned, so when I do check() again, the char newWord already has values from the previous time I ran that function.
I know this is related to pointers and memory allocation but I just cannot seem to get how this works.
It's not a fancy answer, but compilers (and versions of compilers) have different opinions on whether to initialize memory before you use it. Unless things have changed recently, the only variables that get automatically initialized are variables with a static scope (global variables and those explicitly marked static).
For everything else, certain compilers might set everything to zeroes (or another value for debugging), but most won't add that small overhead to your program, when you're probably just going to assign a value of your own, soon enough. One of the biggest debugging effort of my career was because we changed C compilers from one that didn't pre-initialize variables (like most do) to one that did, exposing a bunch of errors my predecessors didn't catch (assigning to the wrong variable, looking for non-zero values and suddenly finding them), so it's an important feature to know about.
The key to your question, though is "seems to keep," because it's only an accident that the old string is still in the right position. If you call another function between check() calls, you'll start to see different scratch memory.
Moral of the story? Always initialize every one of your variables, unless you absolutely know that it's going to get a value before you use it.
The values that you find after running check() twice are still garbage values. When you allocate some memory using
char newWord[LENGTH + 1];
You are always getting "some" memory that the operating system handles you (in laymen-terms), having initially garbage values. It is just a coincidence that you're getting the same memory blocks that you got from the previous call to check().
However, when you do:
char newWord[LENGTH + 1] = "";
You are explicitly initializing those memory blocks to \0.
If you don't initialize a local variable, it has an indeterminate value. Which could be anything, including random "garbage" or left-overs sitting in RAM memory since previous execution. There are no guarantees of what value you will get - and that's it.
Accessing such an uninitialized variable's value might also invoke undefined behavior in some cases:
(Why) is using an uninitialized variable undefined behavior?
Recently I needed to compare two uint arrays (one volatile and other nonvolatile) and results were confusing, there got to be something I misunderstood about volatile arrays.
I need to read an array from an input device and write it to a local variable before comparing this array to a global volatile array. And if there is any difference i need to copy new one onto global one and publish new array to other platforms. Code is something as blow:
#define ARRAYLENGTH 30
volatile uint8 myArray[ARRAYLENGTH];
void myFunc(void){
uint8 shadow_array[ARRAYLENGTH],change=0;
readInput(shadow_array);
for(int i=0;i<ARRAYLENGTH;i++){
if(myArray[i] != shadow_array[i]){
change = 1;
myArray[i] = shadow_array[i];
}
}
if(change){
char arrayStr[ARRAYLENGTH*4];
array2String(arrayStr,myArray);
publish(arrayStr);
}
}
However, this didn't work and everytime myFunc runs, it comes out that a new message is published, mostly identical to the earlier message.
So I inserted a log line into code:
for(int i=0;i<ARRAYLENGTH;i++){
if(myArray[i] != shadow_array[i]){
change = 1;
log("old:%d,new:%d\r\n",myArray[i],shadow_array[i]);
myArray[i] = shadow_array[i];
}
}
Logs I got was as below:
old:0,new:0
old:8,new:8
old:87,new:87
...
Since solving bug was time critical I solved the issue as below:
char arrayStr[ARRAYLENGTH*4];
char arrayStr1[ARRAYLENGTH*4];
array2String(arrayStr,myArray);
array2String(arrayStr1,shadow_array);
if(strCompare(arrayStr,arrayStr1)){
publish(arrayStr1);
}
}
But, this approach is far from being efficient. If anyone have a reasonable explanation, i would like to hear.
Thank you.
[updated from comments:]
For the volatile part, global array has to be volatile, since other threads are accessing it.
If the global array is volatile, your tracing code could be inaccurate:
for(int i=0;i<ARRAYLENGTH;i++){
if(myArray[i] != shadow_array[i]){
change = 1;
log("old:%d,new:%d\r\n",myArray[i],shadow_array[i]);
myArray[i] = shadow_array[i];
}
}
The trouble is that the comparison line reads myArray[i] once, but the logging message reads it again, and since it is volatile, there's no guarantee that the two reads will give the same value. An accurate logging technique would be:
for (int i = 0; i < ARRAYLENGTH; i++)
{
uintu_t value;
if ((value = myArray[i]) != shadow_array[i])
{
change = 1;
log("old:%d,new:%d\r\n", value, shadow_array[i]);
myArray[i] = shadow_array[i];
}
}
This copies the value used in the comparison and reports that. My gut feel is it is not going to show a difference, but in theory it could.
global array has to be volatile, since other threads are accessing it
As you "nicely" observe declaring an array volatile is not the way to protect it against concurrent read/write access by different threads.
Use a mutex for this. For example by wrapping access to the "global array" into a function which locks and unlocks this mutex. Then only use this function to access the "global array".
References:
Why is volatile not considered useful in multithreaded C or C++ programming?
https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt
Also for printf()ing unsigned integers use the conversion specifier u not d.
A variable (or Array) should be declared volatile when it may Change outside the current program execution flow. This may happen by concurrent threads or an ISR.
If there is, however, only one who actually writes to it and all others are jsut Readers, then the actual writing code may treat it as being not volatile (even though there is no way to tell teh Compiler to do so).
So if the comparison function is the only Point in the Project where teh gloal Array is actually changed (updated) then there is no Problem with multiple reads. The code can be designed with the (external) knowledge that there will be no Change by an external source, despite of the volatile declaration.
The 'readers', however, do know that the variable (or the array content) may change and won't buffer it reads (e.g by storing the read vlaue in a register for further use), but still the array content may change while they are reading it and the whole information might be inconsistent.
So the suggested use of a mutex is a good idea.
It does not, however, help with the original Problem that the comparison Loop fails, even though nobody is messing with the array from outside.
Also, I wonder why myArray is declared volatile if it is only locally used and the publishing is done by sending out a pointer to ArrayStr (which is a pointer to a non-volatile char (array).
There is no reason why myArray should be volatile. Actually, there is no reason for its existence at all:
Just read in the data, create a temporary tring, and if it differes form the original one, replace the old string and publish it. Well, it's maybe less efficient to always build the string, but it makes the code much shorter and apparently works.
static char arrayStr[ARRAYLENGTH*4]={};
char tempStr[ARRAYLENGTH*4];
array2String(tempStr,shadow_array);
if(strCompare(arrayStr,tempStr)){
strCopy(arrayStr, tempStr);
publish(arrayStr);
}
}
I am calling a function that returns a variable through a pointer parameter. I do not care about the return value of this parameter nor do I want to make a dummy variable to pass to the function. For a simple example's sake, let's say the function is as follows and I don't care about, nor want to make a dummy variable for parameter "d".
void foo(int a, int b, int* c, int* d)
{
*c = a+b;
*d = a+b+*c;
}
I understand that a NULL pointer is in theory a pointer to a location that is not the address of any object or function. Would it be correct to pass NULL into "d" in this function if NULL was defined as the following? Or is this going to change whatever is at the 0'th element in memory?
#define NULL ((void *)0)
The target device is an MSP430 and I am using IAR C. No operating system is used therefore no memory management is implemented
EDIT: Please note that I do not want to create a dummy variable. Also if there was a way to fool the compiler into optimizing the "d" parameter out without altering the function definition, this is preferable.
EDIT#2: I would rather not use the & operator in the function call as it generates inefficient code that I do not want to generate
EDIT#3: For those who don't believe me when I am talking about the & operator... the compiler manual states "Avoid taking the address of local variables using the & operator. This is inefficient
for two main reasons. First, the variable must be placed in memory, and thus cannot be placed in a processor register. This results in larger and slower code. Second, the optimizer can no longer assume that the local variable is unaffected over function calls."
No, it is not correct.
The C standard does not define the behavior when you do this. On many systems, it will cause an error (some sort of memory fault) when foo attempts to store to address 0. If it does not, then you will have written data to address 0, presumably overwriting something else there that may have been needed, so your system may fail at a later time.
You should change your function a bit to allow passing NULL
void foo(int a, int b, int* c, int* d)
{
if(c != NULL)
{
*c = a+b;
if(d != NULL)
{
*d = a+b+*c;
}
}
}
Now you can safely pass NULL. Otherwise, as the other answers already state, you end up dereferencing a NULL pointer which results in undefined behavior.
In your example, if you don't care about the pointer d and you pass NULL as you defined then it'll probably crash due to dereferencing NULL.
You should pass a valid pointer even if you don't care about the result.
Why not just declare a temporary and pass?
int tempd;
foo(a,b,&c, &tempd);
There is no such thing as the 0th element in memory due to virtual memory. However, if you attempt this, your program will crash with a memory exception. I assume you want to ignore d if it's null so simply do this:
if(d != NULL)
{
*d = a+b+*c
}
Since you don't want to create a dummy variable and can't change the function you'll most likely end up scribbling at the memory position 0 on your device whatever that means. Maybe it's a memory mapped hardware register, maybe it's just normal physical memory.
If it's a register, maybe it doesn't have any effect unless you write the magical value 4711 into it which will happen once every three months and the device halts and catches fire. (has happened to me, it's fun to overwrite the boot eeprom on a device)
Or if it's memory maybe you'll send a NULL pointer to a different function later and that function will happily read the value that this function wrote there and you'll end up at 5 in the morning tearing your hair out and yelling "this can't possibly affect that!". (has happened to me on some ancient unix that used to map the NULL page)
Maybe your compiler adds a safety net for you. Maybe it doesn't. Maybe the next version will. Maybe the next hardware revision will come with memory unmapped at address 0 and the device will halt.
I'd create a dummy variable in the calling function and move on to a more interesting problem, but if you're a stress junkie, pass NULL and see what happens (today or in 10 years).
In that specific example code both passed in pointers are dereferenced. Dereferencing NULL is undefined behavior. Just go with the dummy variables.
In general: If a function accepts a pointer it should state if the null pointer is a valid value for the argument. If it doesn't say anything stay on the safe side and just assume it isn't. This will safe you a lot of grief.
Interesting question! Generally speaking, NULL is reserved as an "invalid" address. You shouldn't try to write to it, but I don't think the standard specifies what should happen if you do. On a Windows machine, this will generate an access violation exception. I don't know what will happen on your device.
In any case, passing NULL is the usual way to indicate that you're not interested in the value of an out parameter. But your function must be aware of this and act accordingly:
if( c ) {
*c = a+b;
if( d ) {
*d = a+b+*c;
}
}
Edit
If you can't change the function definition, then you're probably out of luck. You can trick the compiler into not passing d if the calling convention is cdecl. Just declare the function without the d parameter:
extern void foo( int a, int b, int * c );
However, you're definitely into dangerous shenanigans territory here. The function definition will still expect the d paramater, so it will see random garbage.
The only other thing I can think of is passing a fixed address. Since you're writing for a specific device, there might be an address range that's safe to write to. (That is, it won't cause exceptions or corrupt actual memory.)
void * SafeAddress = (void *)0x12345678;
foo( a, b, &c, SafeAddress );
Of course, the easiest thing is to just use the dummy variable. I know you've said more than once that this generates inefficient code, but does that have to be the case? Does it make a difference if the dummy is a local variable versus a global one?
The function tries to store value at the provided address. If the address is invalid, then there will be a malfunction of some sort -- whether you use an operating system or not is irrelevant.
Either you have to give the function some valid address (even if you don't care for the value in a particular case), or you have to change the function so that it does not store the value (and, probably, does not even even compute it), if the address for it is NULL (which may or may not be 0x0 on your platform, BTW).
You keep repeating, that you "don't want" to do the former and can not do the latter. Well, then you have an unsolvable dilemma. Maybe, there already exists some other address, where dummy values like this can be stored in your program (or on the platform) -- you can pass that.
If there is no OS involved, then you must be dealing with some funky programmable device, which means, there ought to be seasoned C-programmers around you. Ask them for confirmation of what you are told here -- clearly, you aren't trusting the answers given to you by several people already.
It will try to assign something at memory location 0x0, so I'd say it will crash
I was able to save code space without increasing memory usage on the stack by declaring a dummy variable as well as a pointer to the dummy variable.
int Dummy;
int* Dummy_ptr = &Dummy;
This allowed the compiler to make optimizations on the function call as the & operator was not used in the function call.
The call is now
foo(a, b, c_ptr, Dummy_ptr);
EDIT: For those of you who don't believe me.
I took a look at the assembler. The Dummy variable exists on the stack, though because it is not used later on, and because it is only a return from the function the address is never passed to the function and any use of that variable in the function is optimized out.
I know that sometimes if you don't initialize an int, you will get a random number if you print the integer.
But initializing everything to zero seems kind of silly.
I ask because I'm commenting up my C project and I'm pretty straight on the indenting and it compiles fully (90/90 thank you Stackoverflow) but I want to get 10/10 on the style points.
So, the question: when is it appropriate to initialize, and when should you just declare a variable:
int a = 0;
vs.
int a;
There are several circumstances where you should not initialize a variable:
When it has static storage duration (static keyword or global var) and you want the initial value to be zero. Most compilers will actually store zeros in the binary if you explicitly initialize, which is usually just a waste of space (possibly a huge waste for large arrays).
When you will be immediately passing the address of the variable to another function that fills its value. Here, initializing is just a waste of time and may be confusing to readers of the code who wonder why you're storing something in a variable that's about to be overwritten.
When a meaningful value for the variable can't be determined until subsequent code has completed execution. In this case, it's actively harmful to initialize the variable with a dummy value such as zero/NULL, as this prevents the compiler from warning you if you have some code paths where a meaningful value is never assigned. Compilers are good at warning you about accessing uninitialized variables, but can't warn you about "still contains dummy value" variables.
Aside from these issues, I'd say it's generally good practice to initialize your non-static variables when possible.
A rule that hasn't been mentioned yet is this: when the variable is declared inside a function it is not initialised, and when it is declared in static or global scope it's set to 0:
int a; // is set to 0
void foo() {
int b; // set to whatever happens to be in memory there
}
However - for readability I would usually initialise everything at declaration time.
If you're interested in learning this sort of thing in detail, I'd recommend this presentation and this book
I can think of a couple of reason off the top of my head:
When you're going to be initializing it later on in your code.
int x;
if(condition)
{
func();
x = 2;
}
else
{
x = 3;
}
anotherFunc(x); // x will have been set a value no matter what
When you need some memory to store a value set by a function or another piece of code:
int x; // Would be pointless to give x a value here
scanf("%d", &x);
If the variable is in the scope of of a function and not a member of a class I always initialize it because otherwise you will get warnings. Even if this variable will be used later I prefer to assign it on declaration.
As for member variables, you should initialize them in the constructor of your class.
For pointers, always initialize them to some default, particularly NULL, even if they are to be used later, they are dangerous when uninitialized.
Also it is recommended to build your code with the highest level of warnings that your compiler supports, it helps to identify bad practices and potential errors.
Static and global variables will be initialized to zero for you so you may skip initialization. Automatic variables (e.g. non-static variables defined in function body) may contain garbage and should probably always be initialized.
If there is a non-zero specific value you need at initialization then you should always initialize explicitly.
It's always good practice to initialize your variables, but sometimes it's not strictly necessary. Consider the following:
int a;
for (a = 0; a < 10; a++) { } // a is initialized later
or
void myfunc(int& num) {
num = 10;
}
int a;
myfunc(&a); // myfunc sets, but does not read, the value in a
or
char a;
cin >> a; // perhaps the most common example in code of where
// initialization isn't strictly necessary
These are just a couple of examples where it isn't strictly necessary to initialize a variable, since it's set later (but not accessed between declaration and initialization).
In general though, it doesn't hurt to always initialize your variables at declaration (and indeed, this is probably best practice).
In general, there's no need to initialize a variable, with 2 notable exceptions:
You're declaring a pointer (and not assigning it immediately) - you
should always set these to NULL as good style and defensive
programming.
If, when you declare the variable, you already know
what value is going to be assigned to it. Further assignments use up
more CPU cycles.
Beyond that, it's about getting the variables into the right state that you want them in for the operation you're going to perform. If you're not going to be reading them before an operation changes their value (and the operation doesn't care what state it is in), there's no need to initialize them.
Personally, I always like to initialize them anyway; if you forgot to assign it a value, and it's passed into a function by mistake (like a remaining buffer length) 0 is usually cleanly handled - 32532556 wouldn't be.
There is absolutely no reason why variables shouldn't be initialised, the compiler is clever enough to ignore the first assignment if a variable is being assigned twice. It is easy for code to grow in size where things you took for granted (such as assigning a variable before being used) are no longer true. Consider:
int MyVariable;
void Simplistic(int aArg){
MyVariable=aArg;
}
//Months later:
int MyVariable;
void Simplistic(int aArg){
MyVariable+=aArg; // Unsafe, since MyVariable was never initialized.
}
One is fine, the other lands you in a heap of trouble. Occasionally you'll have issues where your application will run in debug mode, but release mode will throw an exception, one reason for this is using an uninitialised variable.
As long as I have not read from a variable before writing to it, I have not had to bother with initializing it.
Reading before writing can cause serious and hard to catch bugs. I think this class of bugs is notorious enough to gain a mention in the popular SICP lecture videos.
Initializing a variable, even if it is not strictly required, is ALWAYS a good practice. The few extra characters (like "= 0") typed during development may save hours of debugging time later, particularly when it is forgotten that some variables remained uninitialized.
In passing, I feel it is good to declare a variable close to its use.
The following is bad:
int a; // line 30
...
a = 0; // line 40
The following is good:
int a = 0; // line 40
Also, if the variable is to be overwritten right after initialization, like
int a = 0;
a = foo();
it is better to write it as
int a = foo();
I have a char pointer which would be used to store a string. It is used later in the program.
I have declared and initialized like this:
char * p = NULL;
I am just wondering if this is good practice. I'm using gcc 4.3.3.
Yes, it's good idea.
Google Code Style recommends:
To initialize all your variables even if you don't need them right now.
Initialize pointers by NULL, int's by 0 and float's by 0.0 -- just for better readability.
int i = 0;
double x = 0.0;
char* c = NULL;
You cannot store a string in a pointer.
Your definition of mgt_dev_name is good, but you need to point it somewhere with space for your string. Either malloc() that space or use a previously defined array of characters.
char *mgt_dev_name = NULL;
char data[4200];
/* ... */
mgt_dev_name = data; /* use array */
/* ... */
mgt_dev_name = malloc(4200);
if (mgt_dev_name != NULL) {
/* use malloc'd space */
free(mgt_dev_name);
} else {
/* error: not enough memory */
}
It is good practice to initialize all variables.
If you're asking whether it's necessary, or whether it's a good idea to initialize the variable to NULL before you set it to something else later on: It's not necessary to initialize it to NULL, it won't make any difference for the functionality of your program.
Note that in programming, it's important to understand every line of code - why it's there and what exactly it's doing. Don't do things without knowing what they mean or without understanding why you're doing them.
Another option is to not define the variable until the place in your code where you have access to it's initial value. So rather then doing:
char *name = NULL;
...
name = initial_value;
I would change that to:
...
char *name = initial_value;
The compiler will then prevent you from referencing the variable in the part of the code where it has no value. Depending on the specifics of your code this may not always be possible (for example, the initial value is set in an inner scope but the variable has a different lifetime), moving the definition as late as possible in the code prevents errors.
That said, this is only allowed starting with the c99 standard (it's also valid C++). To enable c99 features in gcc, you'll need to either do:
gcc -std=gnu99
or if you don't want gcc extensions to the standard:
gcc -std=c99
No, it is not a good practice, if I understood your context correctly.
If your code actually depends on the mgt_dev_name having the initial value of a null-pointer, then, of course, including the initializer into the declaration is a very good idea. I.e. if you'd have to do this anyway
char *mgt_dev_name;
/* ... and soon after */
mgt_dev_name = NULL;
then it is always a better idea to use initialization instead of assignment
char *mgt_dev_name = NULL;
However, initialization is only good when you can initialize your object with a meaningful useful value. A value that you will actually need. In general case, this is only possible in languages that allow declarations at any point in the code, C99 and C++ being good examples of such languages. By the time you need your object, you usually already know the appropriate initializer for that object, and so can easily come up with an elegant declaration with a good initializer.
In C89/90 on the other hand, declarations can only be placed at the beginning of the block. At that point, in general case, you won't have meaningful initializers for all of your objects. Should you just initialize them with something, anything (like 0 or NULL) just to have them initialized? No!!! Never do meaningless things in your code. It will not improve anything, regardless of what various "style guides" might tell you. In reality, meaningless initialization might actually cover bugs in your code, making it the harder to discover and fix them.
Note, that even in C89/90 it is always beneficial to strive for better locality of declarations. I.e. a well-known good practice guideline states: always make your variables as local as they can be. Don't pile up all your local object declarations at the very beginning of the function, but rather move them to the beginning of the smallest block that envelopes the entire lifetime of the object as tightly as possible. Sometimes it might even be a good idea to introduce a fictive, otherwise unnecessary block just to improve the locality of declarations. Following this practice will help you to provide good useful initializers to your objects in many (if not most) cases. But some objects will remain uninitialized in C89/90 just because you won't have a good initializer for them at the point of declaration. Don't try to initialize them with "something" just for the sake of having them initialized. This will achieve absolutely nothing good, and might actually have negative consequences.
Note that some modern development tools (like MS Visual Studio 2005, for example) will catch run-time access to uninitialized variables in debug version of the code. I.e these tools can help you to detect situations when you access a variable before it had a chance to acquire a meaningful value, indicating a bug in the code. But performing unconditional premature initialization of your variables you essentially kill that capability of the tool and sweep these bugs under the carpet.
This topic has already been discussed here:
http://www.velocityreviews.com/forums/t282290-how-to-initialize-a-char.html
It refers to C++, but it might be useful for you, too.
There are several good answers to this question, one of them has been accepted. I'm going to answer anyway in order to expand on practicalities.
Yes, it is good practice to initialize pointers to NULL, as well as set pointers to NULL after they are no longer needed (i.e. freed).
In either case, its very practical to be able to test a pointer prior to dereferencing it. Lets say you have a structure that looks like this:
struct foo {
int counter;
unsigned char ch;
char *context;
};
You then write an application that spawns several threads, all of which operate on a single allocated foo structure (safely) through the use of mutual exclusion.
Thread A gets a lock on foo, increments counter and checks for a value in ch. It does not find one, so it does not allocate (or modify) context. Instead, it stores a value in ch so that thread B can do this work instead.
Thread B Sees that counter has been incremented, notes a value in ch but isn't sure if thread A has done anything with context. If context was initialized as NULL, thread B no longer has to care what thread A did, it knows context is safe to dereference (if not NULL) or allocate (if NULL) without leaking.
Thread B does its business, thread A reads its context, frees it, then re-initializes it to NULL.
The same reasoning applies to global variables, without the use of threads. Its good to be able to test them in various functions prior to dereferencing them (or attempting to allocate them thus causing a leak and undefined behavior in your program).
When it gets silly is when the scope of the pointer does not go beyond a single function. If you have a single function and can't keep track of the pointers within it, usually this means the function should be re-factored. However, there is nothing wrong with initializing a pointer in a single function, if only to keep uniform habits.
The only time I've ever seen an 'ugly' case of relying on an initialized pointer (before and after use) is in something like this:
void my_free(void **p)
{
if (*p != NULL) {
free(*p);
*p = NULL;
}
}
Not only is dereferencing a type punned pointer frowned upon on strict platforms, the above code makes free() even more dangerous, because callers will have some delusion of safety. You can't rely on a practice 'wholesale' unless you are sure every operation is in agreement.
Probably a lot more information than you actually wanted.
Preferred styles:
in C: char * c = NULL;
in C++: char * c = 0;
My rationale is that if you don't initialize with NULL, and then forget to initialize altogether, the kinds of bugs you will get in your code when dereferencing are much more difficult to trace due to the potential garbage held in memory at that point. On the other hand, if you do initialize to NULL, most of the time you will only get a segmentation fault, which is better, considering the alternative.
Initializing variables even when you don't need them initialized right away is a good practice. Usually, we initialize pointers to NULL, int to 0 and floats to 0.0 as a convention.
int* ptr = NULL;
int i = 0;
float r = 0.0;
It is always good to initialize pointer variables in C++ as shown below:
int *iPtr = nullptr;
char *cPtr = nullptr;
Because initializing as above will help in condition like below since nullptr is convertible to bool, else your code will end up throwing some compilation warnings or undefined behaviour:
if(iPtr){
//then do something.
}
if(cPtr){
//then do something.
}