Thread: Getting strange output!

Could someone explain to me what is going on here? Why does this code:

Code:

char[6] startText;

char data[] = "1.1\nWord\nWord\n0-5\n\0";
char* dataPtr = data;
char** objectData2 = &dataPtr;


std::cout << "Data:" << *objectData2 << std::endl;


char null[1];
null[0] = '\0';
                
strncpy(startText, *objectData2, 5);
strcat(startText, null);
                
std::cout << "startText: " << startText << std::endl;

Give me this output:

Data:1.1
Word
Word
0-5


startText: 1.1
W00 00-00 00-00 00-A


What is this rubbish at the end of the startText???

oh... didn't realise I had to do it as I was using all 6 chars in it, I thought all the rubbish in the startText would be overwritten...

but yes, this sorted the problem:

Code:

for (int i = 0; i < 6; i++) {
    startText[i] = '\0';
}

However, it looks a bit awkward - is this a good way to initialise a char array?

Originally Posted by laserlight

You forgot to initialise startText.

EDIT:
Oh, wait a minute, you used strncpy not strncat on startText, so that removes the problem with failing to initialise it. However, you then attempt to strcat an empty string... before appending the null character. Correct would be:

Code:

strncpy(startText, *objectData2, 5);
startText[5] = '\0';

Sorry about that, I was thrown off by the strange use of a named empty string.

oh right.... isn't appending a null character the same as strcat-int an empty string?

oh! gotcha thanks

this is C with some C++ functionality...

What would I do then in this case:

Code:

char* endText;
endText = strstr(*objectData2, " 5000");

I don't know how long endText would be, but I need to append '\0' to the end of it. I was going to strcat a char array with one null character in it to endText, but I guess this is a bad idea again....

Well I can use things like std::cout in my code for now, but will need to remove them later for the final build, as the final product will need to run on a C-only system.

Originally Posted by laserlight

No, you don't. strstr returns a pointer to somewhere within the search that you are searching, hence it is also null terminated.


Why not just use C facilities then, e.g., printf?

I do use printf in other places, just prefer std::cout for temporary logging as I used to use it in my happy C++ days

Originally Posted by mondaytofriday

I don't know how long endText would be, but I need to append '\0' to the end of it.

Well, you have a contradiction here.
Either you know where "endText" should end or it is impossible to append a '\0'.

If you tell us what you want to achieve, perhaps someone will find a good solution.
Describe the goal, not the step.

Bye, Andreas

This post needs to be moved to C++ forum.