Reading file to char** using fgets and memcpy

Question

I'm trying to read a set of lines from a file to an array. I'm doing this to learn malloc and realloc.

#define MAX_LINE 301

char** read_file_lines(char* filename) {
    char** ptr = NULL;
    int max = 5;
    int i = 0;
    FILE *fp = fopen(filename, "r");

    if(fp != NULL) {
        char line[MAX_LINE];
        while(fgets(line, MAX_LINE, fp) != NULL) {

            /* allocate some extra memory for some more lines */
            if(i == max) {
                int new_max = max * 2;
                int nr_bytes = new_max * sizeof(char) * MAX_LINE;
                char **ptr2 = realloc(ptr, nr_bytes);
                if(ptr2 != NULL) {
                    ptr = ptr2;
                    ptr2 = NULL;
                    max = new_max;
                }
            }

            // ptr[i] = line;
            // strcpy(ptr[i], line);
            memcpy(ptr[i], line, strlen(line));
            i++;
        }
        fclose(fp);
    }
    else {
        printf("Error opening file %s\n", filename);
    }

    return ptr;
}

The code compiles. However, when it is executed, an error occurs (the program crashes).

I did some debugging and determined that the problem is in up in the memcpy () instruction. I had previously tried using strcpy , which also gives a similar problem.

I went to check memcpy ()'s protocolo and it is as followS:

void * memcpy ( void * destination, const void * source, size_t num );

Now, if ptr is char** , isn't ptr[i] equivalent to a char* ?

Thanks for your comments.

Answer 1

It looks like ptr isn't initialized to point to any memory at all. Also, you're not allocating any memory for the individual lines.

To initialize ptr , change the declaration to:

int max = 5;
char** ptr = malloc(max * sizeof(char*));

Try adding this before the call to memcpy:

        ptr[i] = malloc(strlen(line) + 1);

and change the calculation for the realloc call:

            int nr_bytes = new_max * sizeof(char*);

EDIT: To explain in more detail: ptr is a pointer to an array of pointers. You have to allocate memory for ptr (that is, enough memory just to store individual pointers). In addition to this , you also have to allocate each individual array of characters that the individual elements of ptr will point to.

The first change I suggested ensures that ptr always points to enough memory to hold 5 pointers (or more, once it's been realloc'd.)

The second change ensures that each member of ptr always points to valid memory before you try to access it as a pointer.

And the third change is required because ptr points to elements that are pointers to char , not char .

Answer 2

问题是，在第一执行存储器未分配： i是0， max是5， if条件为假并且realloc永远不会执行。

Answer 3

Nah. Arrays are not pointers. Pointers to pointers are not arrays of arrays. If you want a two-dimensional dynamic array, then you have to allocate memory for 1. the array of pointers that point to the individual lines, and 2. for the lines themselves too.