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up vote 2 down vote favorite

I have a tiny problem with my assignment. The whole program is about tree data structures but I do not have problems with that.

My problem is about some basic stuff: reading strings from user input and then storing them in an array list.

char str[1000];

fgets(str, 1000, stdin);

int x = 0;
int y = 0;
int z = 0;

char **list;
list = (char**)malloc((x+1)*sizeof(char));
list[x] = (char*)malloc((y+1)*sizeof(char));

while(str[z] != '\n')
{
    list[x][y] = str[z];
    z++;

    if(str[z] == ',')
    {

        x++;
        y = 0;

        list = (char**)realloc(list, (x+1) * sizeof(char*));
        list[x] = (char*)malloc((y + 1)*sizeof(char));

        z++;
        if(str[z] == ' ') // Skips space after the comma
        {
            z++;
        }

    }
    else if(str[z] == '\n')
    {
        break;
    }
    else
    {
        y++;
        list[x] = (char*)realloc(list[x], (y+1)*sizeof(char));

    }

}

I pass this list array into another function. As an example, inputs could be something like 

Abcde, Fghijk, Lmnop, Qrstu

and I am trying to split each of these words into the array list.

Abcde
Fghijk
Lmnop
Qrstu

When I try to output the strings I sometimes get weird, excessive characters such as upside down question marks and numbers.

printf("%s ", list[some_number]);

gets me

Fghijk¿

or

Fghijk\200

All of my program works as expected except for this minor problem which I am having trouble solving. Even with the same exact inputs the bugs may or may not appear. I am guessing it has to do with memory allocation?

Thanks for your help!

share|improve this question
2  
When you use malloc there are no guarantees as to what is contained in the memory you have set aside, you should initialize it to NULL, or at the very least null terminate your strings. –  ChrisCM Jun 4 '13 at 15:09
    
Can I see the other function that you're passing the stuff to? Also, how are these args being passed into the program? File, console, etc.? –  nerdenator Jun 4 '13 at 15:10

3 Answers 3

up vote 3 down vote accepted

You need to put '\0' at the end of your new string.

share|improve this answer
up vote 2 down vote

See most of the C library functions such as printf and strlen process strings assuming \0 as the end character of all. Otherwise, they keep on reading the memory out of bounds either making a memory violation or gets some where the value 0 and stops and all the bytes in between in the memory are interpreted to their extended ascii equivalent hence you are getting such a strange behaviour.

So, allocate an extra byte for \0 character and assign it to the last byte.

share|improve this answer
up vote 0 down vote

Either initialize your variables to null, or as tomato said, put a null character at the end of the new string.

C lacks many of the luxuries programmers now take for granted when it comes to memory management. You're on the right path with malloc but that function only allocates memory... it doesn't clear it out. As a result, your variables will have the correct amount of space (critical for reducing memory leaks and overflow errors), but will be filled with garbage. This garbage could be anything, and in your case, it's an upside down question mark. Appropriate, don't you think?

I could be mistaken since I can't run the code myself without more information, but after your 

char **list;
list = (char**)malloc((x+1)*sizeof(char));
list[x] = (char*)malloc((y+1)*sizeof(char));

statements, you'll want to do something like this:

list = NULL;

and the like to clear out the garbage.

Furthermore, you may care to use the strlen() function (contained in string.h) to figure out just how many blocks of memory you need to allocate.

Clearing out the spaces you use for variables is a good practice to get into with C. Good to see you learning it as well.

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