strncat..strange behaviour
Take a look at this piece of code:
and here is the output:Code:int main (int argc, char *argv[]) { char *str1 = "Foxbat"; char *str2 = "Fulcrum"; printf("str1 = %s\tstr2 = %s\n", str1, str2); strncat(str1, "ABC", 2); printf("str1 = %s\tstr2 = %s\n", str1, str2); return 0; }
why is str2 being spoiled?? it is not supposed to contain value B, right? Any ideas?...i tried to declare str2 as const...but same result..Code:str1 = Foxbat str2 = Fulcrum str1 = FoxbatAB str2 = B
Yes its true infact in my case it does not print the second line
It says unhandled exception writing at some location . But this is strange.
a) str1 is pointing at a string constant, so you're already relying on luck to be able to modify it at all.
(edit - the more usual case is as roaan notes, and the OS kills your program with a memory fault)
b) string constants are only allocated the space they need. You can't arbitrarily extend that without trashing something else.
You (your specific case have)
The two ^ indicate where str1 and str2 point.Code:Foxbat\0Fulcrum ^ ^
You change str1, so it now looks like this
Both pointers still point at the same place.Code:FoxbatAB\0ulcrum ^ ^
Only str2 is now just the tail-end of some much shorter string!
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
I'm surprised that worked at all without a run-time crash. But you don't have any "extra" letters to play with in your str1 and str2, and so any attempt to add letters to them are doomed to failure.
Originally Posted by Adak
Perhaps it's your compiler, mine core dumps (as expected).
The code is broke - there ain't any other way to slice it.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
Ok, could this be a solution to the problem?
Now it prints as i want..Code:char *str1 = "Foxbat"; char *str2; strcpy(str2, "Fulcrum"); printf("str1 = %s\tstr2 = %s\n", str1, str2); strncat(str1, "ABC", 2); printf("str1 = %s\tstr2 = %s\n", str1, str2);
any other solutions?Code:str1 = Foxbat str2 = Fulcrum str1 = FoxbatAB str2 = Fulcrum
NO! This is not a solution! What compiler do you use? This should not work one bit.
First and foremost, read SourceForge.net: Common mistakes and errors - cpwiki
And secondly, listen to what people say! It's a string literal! It cannot be modified, nor does it have the space for the extra string you're adding!
And thirdly, read all the links in the thread!
Solutions?
Code:char str1[20] = "Foxbat"; char str2[20]; // The rest as above.
If what i understood from the wiki page is that having a const before a string literal will prevent any memory fault and will give compilation error if we try to modify a string literal appended with the keyword const. So to prevent abnormal termination of program this should be fine
[insert]// This will give error at compilation tiem and prevent abnormal termination as what i experienced.Code:int main (int argc, char *argv[]) { const char *str1 = "Foxbat"; const char *str2 = "Fulcrum"; printf("str1 = %s\tstr2 = %s\n", str1, str2); strncat(str1, "ABC", 2); printf("str1 = %s\tstr2 = %s\n", str1, str2); return 0; }
Oh please, it just gets worse and worse!
str2 isn't even pointing at ANY memory (just some random location).
Please stop guessing, and learn about some things before trying again.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
i got the point..
so this is the safest method.. this way i just add two elements to the char array..which already contains 6 elements... So, the point is... its better to know how many elements there if u want to do such kind of operations am i right? i read it.
Code:char str1[20] = "Foxbat"; char str2[20] = "Fulcrum"; printf("str1 = %s\tstr2 = %s\n", str1, str2); strncat(str1, "ABC", 2); printf("str1 = %s\tstr2 = %s\n", str1, str2);
I don't get your question, but either you make sure you have enough space beforehand (figure it out!), or you expand your storage as necessary using dynamic memory (malloc/realloc/free).
And this is not the safest method, it's the ONLY method (unless you want to use malloc/free). Your "methods" don't work at all.
hmm...by the way, i am working now on some image processing system..and there i am going to output strings in the image memory... well in other words before i tried to make things work like that:
char *letter = "";
and then i used a lot of strcpy() on the letter.
and then i used a lot of things like *(letter+ SomOffset) inside for loops..
and i found that strings dont print correctly...
what i did now is:
char *letter = (char *)malloc(14*12*sizeof(char));
and now everything works just fine...
i could also do this char letter[168]; but in my code i have specific element accessing method...cuz actually my array is 2dimensional... cuz when u have this: char letter[14][12] and then u do strcpy(letter, "something"); it gives warning..
so...in other words, it now seems to me that using malloc is one of the good ways to solve such issues... ok
This works with regular arrays, too.
1D arrays:
char foo[n];
foo[n]; // Access element n
2D arrays:
char foo[x][y];
foo[x][y]; // Access element x, y.
If you use malloc, you must also use free.
