从C中的函数返回二维数组

Question

I am trying to return a 2d array from a function but for some reason when I call the function in my main it causes a segmentation fault.

char **upper_case_word(char **word,int size){
        int i =0;
        int i2=0;
        // this gets the number of words
        int length = upper_counter(word,size);
        //this gets the size of the largest word
        int size2 = length_upper(word,size);

        char ** upper =malloc(length*sizeof(char*));

        for(i =0; i <length; i++){
                upper[i]= malloc(size2*sizeof(char*));
        }
        i =0;

        for(i =0; i<=size2; i++){

                for(i2 =0; i2<(sizeof(word[i])/sizeof(word[i][0])); i2++){   
                             if(isupper(word[i][i2])!=0){     
                                upper[i]=word[i];
                        }
                }
        }
        return upper;
}
int main(){
   //this is causing the segmentation fault.
   char ** upper = upper_case_word(words,size);

 return(0);
}

Answer 1

I'm not sure this covers all problems but here a few to start with:

What you allocate is not a 2D array. Instead you allocate an array of pointers and for each of these pointer, you allocate an array. That last array should be an array of char. However, your code is:

upper[i]= malloc(size2*sizeof(char*));
                              ^^^^^
                              Should be char, not char*

But that is not the cause of the seg fault - it's just allocating more memory than needed.

The outer array, ie upper is allocated using length like:

char ** upper =malloc(length*sizeof(char*));
                      ^^^^^^

but later you iterate it using size2 - here:

    for(i =0; i<=size2; i++){

So in case size2 is greater than or equal to length , you access out of bounds. That may cause a seg fault.

Then this part:

sizeof(word[i])/sizeof(word[i][0])

Are you sure that is doing what you want?

It is the same as:

sizeof(char*)/sizeof(char)

or just

sizeof(char*)

I doubt that is what you want.

Then you have this code:

                    if(isupper(word[i][i2])!=0){     
                        upper[i]=word[i];
                    }

Here you change the pointer value held by upper[i] . That makes no sense and it's a memory leak because you overwrite a pointer that points to dynamic allocated memory with another pointer.