C++ reading file into char* array

Question

I have a trouble reading a .txt file into a char* array.

I have a levels.txt file that looks like this:

level1.txt
level2.txt

I have my array defined inside a class as

 char* levels[10];

And my parsing function looks like this:

// Parse the level list file
int Environment::parseLevels() {
    ifstream data;
    data.open("levels.txt");

    char buf[64];

    for (int i=0; i<sizeof(levels); i++) {
        data.getline(buf, 64);
        levels[0] = strtok(buf, " ");
    }
}

If I do cout << levels[0]; right after

levels[0] = strtok(buf, " ");

then I get a fine output. However, when I try to cout << levels[0]; from somewhere else, nothing gets displayed.

What am I doing wrong?

Thank you in advance!

What do you mean by somewhere else? Outside the function?

Coding Mash
– Coding Mash

2012-12-09 01:59:01 +00:00
Commented Dec 9, 2012 at 1:59 — Coding Mash
– Coding Mash, Commented Dec 9, 2012 at 1:59

Jason · Accepted Answer · 2012-12-09 06:19:47Z

2

The pointer returned by strtok is not going to point to valid memory forever since the buffer that you're tokenizing is declared on the stack. You'll need to actually copy the string being pointed to by the return pointer of strtok, not the pointer itself if you want to use the string outside the function body.

So basically modify your code to the following:

levels[0] = new char[64];
char* temp = strtok(buf, " ");

//check for NULL pointer return from strtok()
if (temp) 
{
    //if the pointer is not NULL, copy the contents of the temporary string
    //returned by strtok into more permanent memory allocated on the heap
    //and being pointed to by levels[0]

    //Use strncpy() to prevent the risk of a buffer overflow
    strncpy(levels[0], temp, 64);
}

Then in the destructor for your Environment object, make sure to have some loop to free the memory being pointed to by each of the members of the levels array that are pointing to memory allocated via new []. You do this by calling delete []

edited Dec 9, 2012 at 6:19

answered Dec 9, 2012 at 2:00

Jason

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2 Comments

Maxim Neaga Over a year ago

Thank you! But how can I do that? It doesn't seem like strtok returns a pointer to me...

Josh Elias Over a year ago

Allocate the memory with new.

Josh Elias · Accepted Answer · 2012-12-09 07:19:57Z

1

Here's your function:

// Parse the level list file
int Environment::parseLevels() {
    ifstream data;
    data.open("levels.txt");

    char buf[64];

    for (int i=0; i<sizeof(levels); i++) {
        data.getline(buf, 64);
        levels[0] = strtok(buf, " ");
    }
}

You are assigning levels[0] = strtok(buf," ");

This is creating an object in stack memory (no new operator being used) and returning a pointer to that memory, a pointer you are assigning to levels[0].

After you leave this function, the memory used by it is taken off the stack, destroying any memory it created, this would be the memory that the pointer returned from strtok pointed to.

This is why the pointer is no longer valid.

Somebody else got the proper way to copy the data though, strcopy().

// For every string allocate its length as a char buffer on the heap levels[0] = new char[64]; // Get a pointer to the data char* temp = strtok(buf, " ");

//check for NULL pointer return from strtok() if (temp) tstrncpy(levels[0], temp, 64); // Copy the data from temp, to heap

edited Dec 9, 2012 at 7:19

answered Dec 9, 2012 at 3:24

Josh Elias

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3 Comments

Maxim Neaga Over a year ago

Thank you, however as far as I know "new" requires the datatype to go after it. Adding "new" before strtok() results in the following errors: error: expected type-specifier before ‘strtok’ error: cannot convert ‘int*’ to ‘char*’ in assignment error: expected ‘;’ before ‘strtok’ I tried allocating memory for buf: char * buf = new char[64]; however I still didn't get any output :(

Josh Elias Over a year ago

Maybe using a std::move()?

Jason Over a year ago

I don't see how move semantics is applicable to this situation. An invalid pointer is an invalid pointer. The OP needs to copy the contents of the memory on the stack into a more permanent location, either in some memory allocated via the Environment object instance, or on the heap by calling new. And you don't place new before the call to strtok, you allocate memory for levels[N] via new, then use strcpy to copy the contents being pointed to by the return pointer of strtok into the memory being pointed to by levels[N].

Maxim Neaga · Accepted Answer · 2012-12-09 04:05:23Z

0

Here is what I ended up doing:

int Environment::parseLevels(char* filename) {
    ifstream myfile("levels.txt", ifstream::in);
    int i = 0;
    while(myfile >> levels[i]) {
        i++;
    }
    return 0;
}

answered Dec 9, 2012 at 4:05

Maxim Neaga

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2 Comments

Jason Over a year ago

That's working and not causing any crashes or memory corruption? If it's not, you're getting lucky.

Jason Over a year ago

Nope ... the pointer contained in levels[i], if you did not specifically allocate it via a call to new (or new [] in the case of an array), will not point to valid memory. Thus the operator >> on the std::ifstream object will be copying into memory it's not suppose to access. That should at some point lead to a crash or other very-hard-to-debug error.

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