Comparing a volatile array to a non-volatile array - c

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);
}
}

Related

Testing lockless buffer copy in C using memory barriers

I have a few questions regarding memory barriers.
Say I have the following C code (it will be run both from C++ and C code, so atomics are not possible) that writes an array into another one. Multiple threads may call thread_func(), and I want to make sure that my_str is returned only after it was initialized fully. In this case, it is a given that the last byte of the buffer can't be 0. As such, checking for the last byte as not 0, should suffice.
Due to reordering by compiler/CPU, this can be a problem as the last byte might get written before previous bytes, causing my_str to be returned with a partially copied buffer. So to get around this, I want to use a memory barrier. A mutex will work of course, but would be too heavy for my uses.
Keep in mind that all threads will call thread_func() with the same input, so even if multiple threads call init() a couple of times, it's OK as long as in the end, thread_func() returns a valid my_str, and that all subsequent calls after initialization return my_str directly.
Please tell me if all the following different code approaches work, or if there could be issues in some scenarios as aside from getting the solution to the problem, I'd like to get some more information regarding memory barriers.
__sync_bool_compare_and_swap on last byte. If I understand correctly, any memory store/load would not be reordered, not just the one for the particular variable that is sent to the command. Is that correct? if so, I would expect this to work as all previous writes of the previous bytes should be made before the barrier moves on.
#define STR_LEN 100
static uint8_t my_str[STR_LEN] = {0};
static void init(uint8_t input_buf[STR_LEN])
{
for (int i = 0; i < STR_LEN - 1; ++i) {
my_str[i] = input_buf[i];
}
__sync_bool_compare_and_swap(my_str, 0, input_buf[STR_LEN - 1]);
}
const char * thread_func(char input_buf[STR_LEN])
{
if (my_str[STR_LEN - 1] == 0) {
init(input_buf);
}
return my_str;
}
__sync_bool_compare_and_swap on each write. I would expect this to work as well, but to be slower than the first one.
static void init(char input_buf[STR_LEN])
{
for (int i = 0; i < STR_LEN; ++i) {
__sync_bool_compare_and_swap(my_str + i, 0, input_buf[i]);
}
}
__sync_synchronize before each byte copy. I would expect this to work as well, but is this slower or faster than (2)? __sync_bool_compare_and_swap is supposed to be a full barrier as well, so which would be preferable?
static void init(char input_buf[STR_LEN])
{
for (int i = 0; i < STR_LEN; ++i) {
__sync_synchronize();
my_str[i] = input_buf[i];
}
}
__sync_synchronize by condition. As I understand it, __sync_synchronize is both a HW and SW memory barrier. As such, since the compiler can't tell the value of use_sync it shouldn't reorder. And the HW reordering will be done only if use_sync is true. is that correct?
static void init(char input_buf[STR_LEN], bool use_sync)
{
for (int i = 0; i < STR_LEN; ++i) {
if (use_sync) {
__sync_synchronize();
}
my_str[i] = input_buf[i];
}
}
GNU C legacy __sync builtins are not recommended for new code, as the manual says.
Use the __atomic builtins which can take a memory-order parameter like C11 stdatomic. But they're still builtins and still work on plain types not declared _Atomic, so using them is like C++20 std::atomic_ref. In C++20, use std::atomic_ref<unsigned char>(my_str[STR_LEN - 1]), but C doesn't provide an equivalent so you'd have to use compiler builtins to hand-roll it.
Just do the last store separately with a release store in the writer, not an RMW, and definitely not a full memory barrier (__sync_synchronize()) between every byte!!! That's way slower than necessary, and defeats any optimization to use memcpy. Also, you need the store of the final byte to be at least RELEASE, not a plain store, so readers can synchronize with it. See also Who's afraid of a big bad optimizing compiler? re: how exactly compilers can break your code if you try to hand-roll lockless code with just barriers, not atomic loads or stores. (It's written for Linux kernel code, where a macro would use *(volatile char*) to hand-roll something close to __atomic_store_n with __ATOMIC_RELAXED`)
So something like
__atomic_store_n(&my_str[STR_LEN - 1], input_buf[STR_LEN - 1], __ATOMIC_RELEASE);
The if (my_str[STR_LEN - 1] == 0) load in thread_func is of course data-race UB when there are concurrent writers.
For safety it needs to be an acquire load, like __atomic_load_n(&my_str[STR_LEN - 1], __ATOMIC_ACQUIRE) == 0, since you need a thread that loads a non-0 value to also see all other stores by another thread that ran init(). (Which did a release-store to that location, creating acquire/release synchronization and guaranteeing a happens-before relationship between these threads.)
See https://preshing.com/20120913/acquire-and-release-semantics/
Writing the same value non-atomically is also UB in ISO C and ISO C++. See Race Condition with writing same value in C++? and others.
But in practice it should be fine except with clang -fsanitize=thread. In theory a DeathStation9000 could implement non-atomic stores by storing value+1 and then subtracting 1, so temporarily there's be a different value in memory. But AFAIK there aren't real compilers that do that. I'd have a look at the generated asm on any new compiler / ISA combination you're trying, just to make sure.
It would be hard to test; the init stuff can only race once per program invocation. But there's no fully safe way to do it that doesn't totally suck for performance, AFAIK. Perhaps doing the init with a cast to _Atomic unsigned char* or typedef _Atomic unsigned long __attribute__((may_alias)) aliasing_atomic_ulong; as a building block for a manual copy loop?
Bonus question: if(use_sync) __sync_synchronize() inside the loop.
Since the compiler can't tell the value of use_sync it shouldn't reorder.
Optimization is possible to asm that works something like if(use_sync) { slow barrier loop } else { no-barrier loop }. This is called "loop unswitching": making two loops and branching once to decide which to run, instead of every iteration. GCC has been able to do that optimization (in some cases) since 3.4. So that defeats your attempt to take advantage of how the compiler would compile to trick it into doing more ordering than the source actually requires.
And the HW reordering will be done only if use_sync is true.
Yes, that part is correct.
Also, inlining and constant-propagation of use_sync could easily defeat this, unless use_sync was a volatile global or something. At that point you might as well just make a separate _Atomic unsigned char array_init_done flag / guard variable.
And you can use it for mutual exclusion by having threads try to set it to 1 with int old = guard.exchange(1), with the winner of the race being the one to run init while they spin-wait (or C++20 .wait(1)) for the guard variable to become 2 or -1 or something, which the winner of the race will set after finishing init.
Have a look at the asm GCC makes for non-constant-initialized static local vars; they check a guard variable with an acquire load, only doing locking to have one thread do the run_once init stuff and the others wait for that result. IIRC there's a Q&A about doing that yourself with atomics.

Volatile keyword in C [duplicate]

This question already has answers here:
Why is volatile needed in C?
(18 answers)
Closed 9 years ago.
I am writing program for ARM with Linux environment. its not a low level program, say app level
Can you clarify me what is the difference between,
int iData;
vs
volatile int iData;
Does it have hardware specific impact ?
Basically, volatile tells the compiler "the value here might be changed by something external to this program".
It's useful when you're (for instance) dealing with hardware registers, that often change "on their own", or when passing data to/from interrupts.
The point is that it tells the compiler that each access of the variable in the C code must generate a "real" access to the relevant address, it can't be buffered or held in a register since then you wouldn't "see" changes done by external parties.
For regular application-level code, volatile should never be needed unless (of course) you're interacting with something a lot lower-level.
The volatile keyword specifies that variable can be modified at any moment not by a program.
If we are talking about embedded, then it can be e.g. hardware state register. The value that it contains may be modified by the hardware at any unpredictable moment.
That is why, from the compiler point of view that means that compiler is forbidden to apply optimizations on this variable, as any kind of assumption is wrong and can cause unpredictable result on the program execution.
By making a variable volatile, every time you access the variable, you force the CPU to fetch it from memory rather than from a cache. This is helpful in multithreaded programs where many threads could reuse the value of a variable in a cache. To prevent such reuse ( in multithreaded program) volatile keyword is used. This ensures that any read or write to an volatile variable is stable (not cached)
Generally speaking, the volatile keyword is intended to prevent the compiler from applying any optimizations on the code that assume values of variables cannot change "on their own."
(from Wikipedia)
Now, what does this mean?
If you have a variable that could have its contents changed at any time, usually due to another thread acting on it while you are possibly referencing this variable in a main thread, then you may wish to mark it as volatile. This is because typically a variable's contents can be "depended on" with certainty for the scope and nature of the context in which the variable is used. But if code outside your scope or thread is also affecting the variable, your program needs to know to expect this and query the variable's true contents whenever necessary, more than the normal.
This is a simplification of what is going on, of course, but I doubt you will need to use volatile in most programming tasks.
In the following example, global_data is not explicitly modified. so when the optimization is done, compiler thinks that, it is not going to modified anyway. so it assigns global_data with 0. And uses 0, whereever global_data is used.
But actually global_data updated through some other process/method(say through ptrace ). by using volatile you can force it to read always from memory. so you can get updated result.
#include <stdio.h>
volatile int global_data = 0;
int main()
{
FILE *fp = NULL;
int data = 0;
printf("\n Address of global_data:%x \n", &global_data);
while(1)
{
if(global_data == 0)
{
continue;
}
else if(global_data == 2)
{
;
break;
}
}
return 0;
}
volatile keyword can be used,
when the object is a memory mapped io port.
An 8 bit memory mapped io port at physical address 0x15 can be declared as
char const ptr = (char *) 0x15;
Suppose that we want to change the value at that port at periodic intervals.
*ptr = 0 ;
while(*ptr){
*ptr = 4;//Setting a value
*ptr = 0; // Clearing after setting
}
It may get optimized as
*ptr = 0 ;
while(0){
}
Volatile supress the compiler optimization and compiler assumes that tha value can
be changed at any time even if no explicit code modify it.
Volatile char *const ptr = (volatile char * )0x15;
Used when the object is a modified by ISR.
Sometimes ISR may change tha values used in the mainline codes
static int num;
void interrupt(void){
++num;
}
int main(){
int val;
val = num;
while(val != num)
val = num;
return val;
}
Here the compiler do some optimizations to the while statement.ie the compiler
produce the code it such a way that the value of num will always read form the cpu
registers instead of reading from the memory.The while statement will always be
false.But in actual scenario the valu of num may get changed in the ISR and it will
reflect in the memory.So if the variable is declared as volatile the compiler will know
that the value should always read from the memory
volatile means that variables value could be change any time by any external source. in GCC if we dont use volatile than it optimize the code which is sometimes gives unwanted behavior.
For example if we try to get real time from an external real time clock and if we don't use volatile there then what compiler do is it will always display the value which is stored in cpu register so it will not work the way we want. if we use volatile keyword there then every time it will read from the real time clock so it will serve our purpose....
But as u said you are not dealing with any low level hardware programming then i don't think you need to use volatile anywhere
thanks

How do I correctly assign a const char array to a struct?

I'm having problems with const char array being stored to a struct, then when calling the value from a struct I don't always get the expected value.
Below is the code:
typedef struct
{
char *update_type;
char *transaction;
} TickType;
In a thread I have:
const char tx_types[] = "INV/ADD/MOD/DEL/RPL";
const char upd_types[] = "INS/OVR/MOV/DEL";
tick->transaction = &tx_types[4*upd.xpbu_transaction_type];
tick->update_type = &upd_types[4*upd.xpbu_update_type];
This upd.xpbu_transaction_type and this upd.xpbu_update_type return ints (0-4) and (0-3) respectively. In another thread we have printing to file:
fprintf(out, "%3.3s/%3.3s:\n",tick->transaction, tick->update_type);
fflush(out);
The problem is when checking out the output file I see the following:
+MOD/DEL:
+ / Â +:
+MOD/DEL:
+MOD/ :
/#Ea:
/<90>Ea:
/Ã Ea:
/0Fa:
/ :
So as you can see it is only right sometimes.
I'm sure my mistake is in the struct assignment. Unfortunately I can not offer a better look into the code due to it being proprietary software.
I suspect the problem is where you put the definitions of your arrays.
You should either put them in global address space or declare them static somewhere in order to be able to share the address to the strings between the tasks.
Alternatively as BlackBear suggested allocate memory and copy the substring in there.
First of all, is this tick shared among threads? If so, then you have to protect the assignment to its members in a critical section, otherwise there might be a chance when the variable is being printed, another thread is updating it, causing garbage while printing. I'm not sure about the thread safety of fprintf() and fflush(), you should find it out too because it could be another factor that affect.
You have to malloc both update_type and transaction then using strcpy and copy the important part. You also have to edit a bit your strings: "INV\x00ADD\x00MOD\x00DEL\x00RPL" and "INS\x00OVR\x00MOV\x00DEL"
Your code doesn't check that upd.xpbu_transaction_type or upd.xpbu_update_type is within the proper ranges.
Also you need to use a Mutex or something for thread safety.
Also it is not sufficiently clear which memory region these strings are actually stored in. Different environments have different rules regarding where const strings are going to be located. Make sure it is in global memory that is always accessible by both threads of execution. The easiest way to ensure this is to define the strings as const outside any function. If it must be in a function it has to be declared static const.
Depending upon your environment something as simple as this would suffice:
Thread A:
/* validate upd values, etc*/
switch (upd.xpbu_transaction_type)
{
...
default:
xpbu_tt = 0;
}
...
taskLock();
tick->transaction = &tx_types[4*xpbu_tt];
tick->update_type = &upd_types[4*xpbu_ut];
taskUnlock();
Thread B:
While (1)
{
...
taskLock();
t = tick->transaction;
u = tick->update_type;
taskUnlock();
fprintf(out, "%3.3s/%3.3s:\n",t,u);
fflush(out);
}

Initializing variables in C

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();

initializing char pointers

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.
}

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