|
Hello ! I just started to play around with C lately(and mainly with pointers). For practice, i created this function that is suppose to create a random string:
char *ReturnRandomString(int Range)
{
char StringsToChooseFrom[26] = {'q','w','e','r','t','y','u','i','o','p','a','s','d','f','g','h','j','k','l','z','x','c','v','b','n','m'};
char MyArray[Range + 1];
char *Ptr_str;
int i;
for(i = 0; i < Range; i++)
{
srand(time(NULL));
int RandomInt = rand() & 26;
MyArray[i] = StringsToChooseFrom[RandomInt -1];
}
MyArray[i + 1] = '\0';
Ptr_str = malloc(sizeof(MyArray));
Ptr_str = MyArray;
return(Ptr_str);
}
I am "calling" this function this way:
char *PtrCharArray = ReturnRandomString(5);
printf("\nString Array returned: %s", PtrCharArray);
The output... is nothing. I don't neither an output on the console... either an error.
It's sure that i am missing something, but i don't know what. Thanks in advance !!!
P.S No hate.
|
|
|
|
|
char MyArray[Range + 1];
Ptr_str = malloc(sizeof(MyArray)); Ptr_str = MyArray; return(Ptr_str);
That won't work:
1. You cannot allocate an array on the stack using a variable value.
2. sizeof is compile time so it cannot calculate the actual size of the array. You need to use strlen.
3. You cannot use an assignment to copy data. You need to use a copy function: memcpy or strcpy MyArray to Ptr_str.
|
|
|
|
|
1. You cannot allocate an array on the stack using a variable value.
C99 and C11 both permit this. Additionally, both gcc and clang will accept this when compiling in C89 mode, too.
If you're using a POSIX system, then rather than doing malloc() and strcpy() then perhaps use strdup() instead. MSVC has _strdup(), which does the same thing.
|
|
|
|
|
Thank you. Actually I did think that was the case, but when I tried to compile it the compiler objected, so I assumed I just mis-remembered.
|
|
|
|
|
There are some more errors besides those mentioned by Richard.
Here you would normally use the % operator so that the value is in the range 0 to 25:
int RandomInt = rand() & 26;
Anding with 26 (0x1A) will clear masked out bits so that the possible values are only 0x00, 0x02, 0x08, 0x0A, 0x12, 0x18, and 0x1A.
Here you should remove the subtraction of one because that accesses the array out of bounds when RandomInt is zero:
MyArray[i] = StringsToChooseFrom[RandomInt -1];
See the rand() description:
Quote: The rand() function returns a pseudo-random integer in the range 0 to RAND_MAX inclusive.
|
|
|
|
|
What Jochem said .. RTM and look at the examples you need to use the MODULO trick
Can you also please put this line up before the i loop
srand(time(NULL));
That just sets the random seed to the current time tick which updates every second again RTM on C standard time function.
I assure you that loop will complete far faster than 1 second on any decent computer so you are wasting time setting the same seed i times.
You might as well just do it once before the loop 
In vino veritas
modified 27-Oct-16 9:49am.
|
|
|
|
|
In addition to the other comments and suggestions:
_TCHAR *ReturnRandomString( int Range )
{
_TCHAR *Ptr_str = (_TCHAR *) malloc(Range + 1);
int i;
for (i = 0; i < Range; i++)
{
int RandomInt = rand() % 26;
Ptr_str[i] = (_TCHAR) (RandomInt + 97);
}
Ptr_str[i] = _T('\0');
return Ptr_str;
}
...
srand(time(NULL));
_TCHAR *PtrCharArray = ReturnRandomString(5);
_tprintf(_T("String Array returned: %s\n"), PtrCharArray);
free(PtrCharArray);
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
|
|
|
|
|
_TCHAR *Ptr_str = (_TCHAR *) malloc(Range + 1);
This line only allocates the correct amount of memory if sizeof(_TCHAR) == 1. The correct code would multiply the length by sizeof(_TCHAR).
Ptr_str[i] = (_TCHAR) (RandomInt + 97);
This line only works on character sets that encode the alphabet consecutively. This is true for ASCII and Unicode, but not for all varieties of EBCDIC, for example.
A portable solution would be as follows:
static _TCHAR const chars[] = _T("abcdefghijklmnopqrstuvwxyz");
TCHAR *Ptr_str = (_TCHAR *) malloc((sizeof(_TCHAR)*(Range + 1));
...
Ptr_str[i] = chars[RandomInt];
Where _TCHAR and _T(x) have appropriate definitions for the environment.
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
|
|
|
|
|
May this will help...
char *ReturnRandomString(int Range)
{
char StringsToChooseFrom[26] = {'q','w','e','r','t','y','u','i','o','p','a','s','d','f','g','h','j','k','l','z','x','c','v','b','n','m'};
char MyArray[100] = {}; char *Ptr_str;
int i;
for(i = 0; i < Range; i++)
{
int RandomInt = rand() % 26;
MyArray[i] = StringsToChooseFrom[RandomInt];
}
MyArray[i + 1] = '\0';
Ptr_str = (char*)malloc(sizeof(MyArray)); strcpy(Ptr_str, MyArray ); return(Ptr_str);
}
int _tmain(int argc, _TCHAR* argv[])
{
srand(time(NULL)); for( int i = 0; i < 10; i++ )
{
char *PtrCharArray = ReturnRandomString(i);
printf("\nString Array returned: %s\n", PtrCharArray);
}
return 0;
}
|
|
|
|
|
Only as long as Range < 100.
Which also brings up the question of what happens if Range < 0. It might be better to use size_t for Range and your loop variable i. You should probably check the return value of malloc() too, just to be safe.
Oh. And you're leaking the results of ReturnRandomString(). There should probably be a free() when you are done with it.
|
|
|
|
|
|
Krishnakumartg wrote:
MyArray[i + 1] = '\0';
Why add 1? When the loop ends, i is already 1 past the desired length.
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
|
|
|
|
|
I have a .Net/WPF application which calls methods on a DLL. One of these methods - Load() - calls CoCreateInstance to load a COM server provided by a third party. If I do all this in the Load() thread, it works fine i.e. Load() calls CoCreateInstance directly.
However if Load() creates a worker thread, and then waits for completion, the CoCreateInstance call hangs. If I remove the wait, so that Load() returns before the worker thread completes, it works. The CoCreateInstance call returns AFTER the Load() function returns. 
Anyone know what is happening? It is as if it is serialising the COM access for some reason ...
|
|
|
|
|
|
Thanks for the comments. I have tested using a simple Win32 C++ console application to drive our DLL, and it works fine when using the worker thread to invoke COM commands to the third party component. So clearly this is something to do with the .Net application.
Okay, I have solved it.
A COM STA uses Windows messages to serialise access to the COM object which might not be thread safe. So when I created a worker thread, the main thread then hung waiting for the worker thread to finish, but it never did, as the main thread hanging blocked the message pump! Instead of the main thread waiting, I now make it periodically run a message pump to ensure that messages are process. That has solved the problem. Yikes. I hate COM!
Thank you for your helpful post.
Added later: I have also looked into removing the worker thread and marshalling the COM interfaces across threads and this works. It has the advantage that I do not have to mess around with a message pump, which is akin to spinning the tyres.
modified 10-Nov-16 10:36am.
|
|
|
|
|
You could try calling CoInitialize[^] in the worker thread before the call to CoCreateInstance.
«_Superman_»
I love work. It gives me something to do between weekends.
Microsoft MVP (Visual C++) (October 2009 - September 2013) Polymorphism in C
|
|
|
|
|
Thanks for the comment. Yes I call CoInitialise, that is not the issue. I wish it was.
modified 9-Nov-16 3:51am.
|
|
|
|
|
Hi, i have to ask a question about some codelines, where i have a mistake, because the array of int always returns 1 for every instance and i spent a lot of hours and still cannot find the fail i have maid.
I simply post the necessary codelines in hope that someone can put away the wood infront of my head.
Inside a modal dialogue, these are the calls
for (int y=0;y<5;y++)
{
Schadensnummer[y]=KlendautKlendaut_Gegner[0].Basistrefferermittlung();
}
The class "KlendautKlendaut_Gegner0" - function Basistrefferermittlung:
int CBasis::Basistrefferermittlung()
{
int Zufallstreffer;
Zufallstreffer=rand()%99+1;
int Trefferchance;
Trefferchance=rand()%99+1;
static int Einschlagsnummer;
if (Trefferchance>80)
{
if (Zufallstreffer>95)
{
if (m_Bodyguard>1)
{
Einschlagsnummer=20;
m_Bodyguard=m_Bodyguard-1;
return (Einschlagsnummer);
}
else if (m_Bodyguard<1&&m_Palast>1)
{
Einschlagsnummer=21;
m_Palast=m_Palast-1;
return (Einschlagsnummer);
}
}
else if (Zufallstreffer>90&&Zufallstreffer<95)
{
if (m_Fabrik>1)
{
Einschlagsnummer=16;
m_Fabrik=m_Fabrik-1;
return (Einschlagsnummer);
}
}
else if (Zufallstreffer>85&&Zufallstreffer<90)
{
if (m_Scanner>1)
{
Einschlagsnummer=17;
m_Scanner=m_Scanner-1;
return (Einschlagsnummer);
}
}
else if (Zufallstreffer>80&&Zufallstreffer<85)
{
if (m_Kraftwerk>1)
{
Einschlagsnummer=18;
m_Kraftwerk=m_Kraftwerk-1;
return (Einschlagsnummer);
}
}
else if (Zufallstreffer>75&&Zufallstreffer<80)
{
if (m_Zentrale>1)
{
Einschlagsnummer=19;
m_Zentrale=m_Zentrale-1;
return (Einschlagsnummer);
}
}
}
if (m_Schutzwall>1)
{
Einschlagsnummer=1;
m_Schutzwall=m_Schutzwall-1;
return (Einschlagsnummer);
}
else if (m_Gefechtsstand>1)
{
Einschlagsnummer=2;
m_Gefechtsstand=m_Gefechtsstand-1;
return (Einschlagsnummer);
}
else if (m_Geschuetzturm>1)
{
Einschlagsnummer=3;
m_Geschuetzturm=m_Geschuetzturm-1;
return (Einschlagsnummer);
}
else if (m_Drohnen>1)
{
Einschlagsnummer=4;
m_Drohnen=m_Drohnen-1;
return (Einschlagsnummer);
}
else if (m_Marines>1)
{
Einschlagsnummer=5;
m_Marines=m_Marines-1;
return (Einschlagsnummer);
}
else if (m_Werkspolizei>1)
{
Einschlagsnummer=6;
m_Werkspolizei=m_Werkspolizei-1;
return (Einschlagsnummer);
}
else if (m_Unterkunft>1)
{
Einschlagsnummer=7;
m_Unterkunft=m_Unterkunft-1;
return (Einschlagsnummer);
}
else if (m_ErhZentrum>1)
{
Einschlagsnummer=8;
m_ErhZentrum=m_ErhZentrum-1;
return (Einschlagsnummer);
}
else if (m_EinkZentrum>1)
{
Einschlagsnummer=9;
m_EinkZentrum=m_EinkZentrum-1;
return (Einschlagsnummer);
}
else if (m_Bunker>1)
{
Einschlagsnummer=10;
m_Bunker=m_Bunker-1;
return (Einschlagsnummer);
}
else if (m_Lager>1)
{
Einschlagsnummer=11;
m_Lager=m_Lager-1;
m_Lagerpunkte=m_Lagerpunkte-150;
return (Einschlagsnummer);
}
else if (m_Forschung>1)
{
Einschlagsnummer=12;
m_Forschung=m_Forschung-1;
return (Einschlagsnummer);
}
else if (m_MedZentrum>1)
{
Einschlagsnummer=13;
m_MedZentrum=m_MedZentrum-1;
return (Einschlagsnummer);
}
else if (m_Schild>1)
{
Einschlagsnummer=14;
m_Schild=m_Schild-1;
return (Einschlagsnummer);
}
else if (m_Firewall>1)
{
Einschlagsnummer=15;
m_Firewall=m_Firewall-1;
return (Einschlagsnummer);
}
else if (m_Fabrik>1)
{
Einschlagsnummer=16;
m_Fabrik=m_Fabrik-1;
return (Einschlagsnummer);
}
else if (m_Scanner>1)
{
Einschlagsnummer=17;
m_Scanner=m_Scanner-1;
return (Einschlagsnummer);
}
else if (m_Kraftwerk>1)
{
Einschlagsnummer=18;
m_Kraftwerk=m_Kraftwerk-1;
return (Einschlagsnummer);
}
else if (m_Zentrale>1)
{
Einschlagsnummer=19;
m_Zentrale=m_Zentrale-1;
return (Einschlagsnummer);
}
else if (m_Bodyguard>1)
{
Einschlagsnummer=20;
m_Bodyguard=m_Bodyguard-1;
return (Einschlagsnummer);
}
else if (m_Palast>1)
{
Einschlagsnummer=21;
m_Palast=m_Palast-1;
return (Einschlagsnummer);
};
}
The array of Schadensnummer[5] always return 1 (Einschlagsnummer==1) out of Basistrefferermittlung. But the class KlendautKlendaut_Gegner[0].m_Schutzwall ist declared only 1 at this point and it is reduced in Basistrefferermittlung. So in the second call of this function it must be 0 and so the Schadensnummer[1] has to be 2. But it stays 1 for every instance of Schadensnummer[].
Does someone has a suggestion?
|
|
|
|
|
Use a debugger and step through the function. If a variable's value is not what you are expecting it is a lot easier to find out why by watching when it changes than by peering through the code looking for clues.
|
|
|
|
|
I did that. Debugger s showing the array is filled with "1" in each of the 5 instances.
|
|
|
|
|
You need to delve deeper in the debugger - look at where the array elements are being set to 1, and you should see why (Use F11 to step instead of F10).
Cheers,
Mick
------------------------------------------------
It doesn't matter how often or hard you fall on your arse, eventually you'll roll over and land on your feet.
|
|
|
|
|
Hi,
i made a step per step view on the code. It runs through "Schadensnummer[y]=RestonaGuldon_Gegner[0].Basistrefferermittlung();"
and returns 1 after it doesn't enter the line it should
if (m_Schutzwall>1)
{
Einschlagsnummer=1;
m_Schutzwall=m_Schutzwall-1;
return (Einschlagsnummer);
}
it should enter there, because the class array object has .m_Schutzwall>1.
But it neither enters and sets Einschlagsnummer to 1 or subtracts the m_Schutzwall by 1.
There must be something wrong with my declaration of the array of the class "RestonaGuldon_Gegner[0]" oder initialization of the function "Basistrefferermittlung()". But i can't see what it is. I'm blindfolded. I just did everything how i am used to do and it worked fine till now.
I am stunned.
|
|
|
|
|
Nowhere in your code can we see the defintion of "m_Schutzwall" it would be useful
Are you sure you don't have two, m_Schutzwall's it's quite common to that and easy to miss.
If you turn the compiler up to W4 maximum warning it will tell you if m_Schutzwall is hidden by a local copy.
It's dead simple to do something silly with scope like this .. try it
int m_Schutzwall = 1;
int j;
for (j = 1; j < 10; j++){
int m_Schutzwall = j + 2;
}
Do a search for in files for "m_Schutzwall" and look at all the uses.
In vino veritas
|
|
|
|
|
Is it possible that the array int Schadensnummer[] establishs a copy of the member variable of the class RestonaGuldon_Gegner[0]?
The class is global, the variable m_Schutzwall is public.
The class is capsulated in an array.
I dont see a solution so far. it couldnt be that i have to give the array of the class in its own function. i think i forgot something, but i still cant see it.
|
|
|
|
|
addition:
i feel like an idiot. Maybe i am one.
There must be everything allright.
It just misses a ">=" in the function instead of a ">", because the variable is equal 1.
So the entrypoint is never beeing reached. I ll change it and then we ll see.
Thank you so far, i hope i just gave you an evening with something to smile about.
Thanks.
|
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
