strcat problem

[hebes_99]

First of all, somewhat newbie to C++. I know the basics, just not all the "fine detail". My question...

I get an error - "The instruction at "mem location" referenced memory at "mem location". The memory could not be "read".

I have searched the archives, and couldn't find an answer to why I am getting this. Here is a snippet of my code. The problem seems to be with the strcat function. I have debugged it to here.

int buno = 0;
char filepath[64] = "C:\\some path here\\";
char filesuffix[32] ="_flt_test.csv";
char bunotostring[5] = "123456";

// Convert buno number (int) to string
sprintf(bunotostring, "%d", buno);

char *fp = strcat(filepath,strcat(filesuffix,bunotostring));

for some reason, when I try to use strcat, I get this error. Any thoughts, and sorry for the newbie post....

[stickben]

It may be because you are only allocating 5 bytes for a string that takes up 7, in the statement char bunotostring[5] = "123456"; There is also a null character at the end of the string that is not shown, but tells you where the string ends. I would bet that strcat is not seeing the null character at the end of bunotostring and going off into the weeds

[cilu]

First, the code you posted is just C. It doesn't have anything C++ specific (except for the declaration of fp).

Second, when I run your code it dosn't give me any error and it prints:
C:\some path here\_flt_test.csv0
which is the corect result of your code.

The only mistake is here:

char bunotostring[5] = "123456";

You cannot initialize with 7 chars a buffer of only 5 elements.

[hebes_99]

My bad... that line should have been ... ...

char bunotostring[6];

But I still get the error. Again, sorry for posting in the wrong discussion group. I am using VisualC++, and some of my code is in C++. Trying to make the switch!

[cilu]

Wrong again. You forget about the null terminating character. You're array is actually {'1','2','3','4','5','6','\0'}, so it has 7 chars, not 6. So it should be

char bunotostring[7] = "123456";

Of course that goes for dinamically memory allocation. If your string has N chars, you need N+1 chars.

int N = 6;
char *p = new char[N+1];
...
delete [] p;

What compiler are you using? Because VisualC++ tells you stuff like that, and don't need to look in the dark for such mistakes. Anyway pay attention to the null terminating char.

[hebes_99]

I am using the built in Visual C++ compiler. The error I get is a runtime error. The code compiles fine, and my code works to this point. Once it hits the strcat function, it crashes. It seems straight forward, but I know I am missing something easy. For clearity, let me repost the correct code snippet....

int buno;
char filepath[64] = "C:\\some path\\";
char filesuffix[32] ="_flt_test.csv";
char bunotostring[6];

// Convert buno number (int) to string
sprintf(bunotostring, "%d", buno);

char *fp = strcat(filepath,strcat(filesuffix,bunotostring));

Note: buno is a number that is 6 digits in size. I am sure there are much easier ways to do this... just new!

[cilu]

You have to initialize your variables, and since buno is not initialized of course it crashes. When you don't do it it starts with an undefined value, for instance: -858993460. When you run this

sprintf(bunotostring, "%d", buno);

It puts more than 6 chars in that buffer, overriding your null terminating char. So your bunotostring would have another lenght, like 21 in the below example.

0012FF14 2D 38 35 38 -858
0012FF18 39 39 33 34 9934
0012FF1C 36 30 00 74 60.t
0012FF20 5F 74 65 73 _tes
0012FF24 74 2E 63 73 t.cs
0012FF28 76 00 00 00 v...
0012FF2C 00 00 00 00 ....

When you try to concatenate this to filesuffix which has only 32 chars it chrashes.

So just initialize your variables before using them.

[hebes_99]

a value for buno is passed in from another part of the prorgam. I posted it as an int for clarity. In actuality, it is passed in.

float myfunction(int buno)
{

char filepath[64] = "C:\\some path\\";
char filesuffix[32] ="_flt_test.csv";
char bunotostring[6];

// Convert buno number (int) to string
sprintf(bunotostring, "%d", buno);

char *fp = strcat(filepath,strcat(filesuffix,bunotostring));

...
//more code....
}

[VictorN]

a value for buno is passed in from another part of the prorgam. I posted it as an int for clarity. In actuality, it is passed in.

float myfunction(int buno)
{

char filepath[64] = "C:\\some path\\";
char filesuffix[32] ="_flt_test.csv";
char bunotostring[6];

// Convert buno number (int) to string
sprintf(bunotostring, "%d", buno);

char *fp = strcat(filepath,strcat(filesuffix,bunotostring));

...
//more code....
}

Your code stays WRONG!
Why don't you want to correct it as cilu and stickben already suggested?
Keep in mind that the length of int may be up to 11 characters (for negativ decimal number) in Win-32.
In this case you MUST declare your char bunotostring array with the size at least 12:

char bunotostring[12];

// Convert buno number (int) to string
sprintf(bunotostring, "%d", buno);
.............

[hebes_99]

tried that, didn't work. And sorry, I am new to the idiosyncrasies of C++. I have played with strcat enough to see that the error only occurs when filepath is the initial argument. ie:

strcat(filepath, filesuffix)
or
strcat(filepath, bunotostring)
etc..etc..

other combinations work, so I am guessing this is some weird thing with my compiler....

again, sorry for being a pain..

[hebes_99]

Well... got it to work, but don't really know why...

If I changed the size of filepath to char filepath[128], then it compiles. Strange, since strlen of filepath is only 57 (in my code).... oh well.. it works...

Thanks to all for your help!!!

[VictorN]

tried that, didn't work. And sorry, I am new to the idiosyncrasies of C++. I have played with strcat enough to see that the error only occurs when filepath is the initial argument. ie:

strcat(filepath, filesuffix)
or
strcat(filepath, bunotostring)
etc..etc..

other combinations work, so I am guessing this is some weird thing with my compiler....

again, sorry for being a pain..

1. Once again: please, read carefully the ryplies to your question! As cilu already wrote, there is no any c++ specific at your code snippet posted to the forum!
2. No need to guess there could be "some weird thing with my compiler...." if your compiler is not from youself!
3. Please, debug your code and see/control all your variables (their values/string length,...).
4. Note, that max length of the file path in Windows is MAX_PATH (260 characters), it is some more than 64!

[hebes_99]

wow...sorry...

I guess this isn't a forum for new programmers. I am sorry for violating the forum rules.

[CJ1]

wow...sorry...

I guess this isn't a forum for new programmers. I am sorry for violating the forum rules.

This forum has many new programmers, and NO you are NOT violating the forum rules.

1. Once again: please, read carefully the ryplies to your question! As cilu already wrote, there is no any c++ specific at your code snippet

VictorN is wrong is saying there is no specific C++ code as all the code you presented was valid C++ code, perhaps it should have been posted in the C++ (Non Visual C++ Issues) forum.

What the replies to your question were trying to convey was that the line:

sprintf(bunotostring, "%d", buno);

If you want to limit the maximum size of the string, then changed the above code to

sprintf(bunotostring, "%.5d", buno);

The period before the 5 means the maximum size will be 5 (the 6th character is for the null at the end of the string.
then change the strcat to strncat as follows

char *fp = strcat(filepath,strcat(filesuffix,bunotostring, 5), 32);

or you could combine them into one call to sprintf as follows

sprintf(bunotostring, "C:\\some path\\%d_flt_test.csv", buno);

Don't get discouraged, C++ is a difficult language to learn and it is easy to get into trouble with this language.

For new programmers, if possible, it is better to start programming in another language, for programs that will run on Windows PCs, C# would be a better starting language. You could never have the above problem in C#.

[VladimirF]

If you want to limit the maximum size of the string, then changed the above code to

sprintf(bunotostring, "%.5d", buno);

The period before the 5 means the maximum size will be 5 (the 6th character is for the null at the end of the string.

Well, this is just wrong! The "%.5d" does NOT limit the output - go check your manuals. It is responsibility of the programmer to provide enough room in the destination buffer (that is why printf's are not safe).

Also, even if this code compiles in C++ compiler, it still has nothing to do with C++. But I do think that it was OK to post the question here.

I am surprised that nobody pointed out an obvious OP's misunderstanding:

char *fp = strcat(filepath,strcat(filesuffix,bunotostring));

strcat appends (concatenates) second string to the first string, returning pointer to the target (first string). It does NOT allocate any new memory for char* fp! In that example, fp is equal to filepath. And filepath buffer is not capable to accept all other strings at its end.

Here comes C++, that provides string classes that take care of the memory management (like std::string and MFC's CString), so you no longer need to guess if 64 bytes will be enough for the file path (you know, it won't).