C: How to store 2d Char array in a struct?

I want to have a 2d char array, and when I don't use the struct I can loop through the array and print out the 开发者_如何学Cstrings. However if I assign the 2d char array to a struct member, I cannot access the array, why?

typedef struct {
    int num;
    char **names;
} test;


test t;
t.num = 2;
char *names[t.num];
char *tmp;
tmp = "test";

names[0] = "something";
strcpy(tmp,names[0]);
strcat(tmp,"appendedtext");
names[1] = tmp;
names[2] = "something else";


t.names = names;

---

You really should be dynamically allocating your arrays here. There are a lot of problems with the things you are trying to do here.

• Your array is initialized to point to memory on the stack.
• You are storing pointers to string literals and attempting to modify them.
• You are accessing memory beyond the bounds of your array.
• And everything in between.

It just so happens I have some utility functions to dynamically allocate 2-dimensional arrays using a single allocation. Feel free to use them in your code.

static size_t getsize(size_t rows, size_t cols, size_t size)
{
    size_t ptrsize = rows*sizeof(void *);
    if (ptrsize%size != 0)
        ptrsize += size - ptrsize%size;
    return ptrsize + rows*cols*size;
}

static void init2d(void *mem, size_t rows, size_t cols, size_t size)
{
    int i;
    char **ptr = mem;
    char *base = (char *)(ptr + rows);
    size_t rowsize = cols*size;
    size_t ptrsize = rows*sizeof(char *);
    if (ptrsize%size != 0)
        base += size - ptrsize%size;
    for (i = 0; i < rows; i++)
        ptr[i] = base + i*rowsize;
}

void *malloc2d(size_t rows, size_t cols, size_t size)
{
    size_t total_size = getsize(rows, cols, size);
    void *mem = malloc(total_size);
    init2d(mem, rows, cols, size);
    return mem;
}

void *calloc2d(size_t rows, size_t cols, size_t size)
{
    size_t total_size = getsize(rows, cols, size);
    void *mem = calloc(total_size, 1U);
    init2d(mem, rows, cols, size);
    return mem;
}

Then your code would look something like this:

#define MAXWIDTH 100
int num = 3;
test t;
t.num = num;

/* dynamically allocate the memory for t.name */
t.names = calloc2d(t.num, MAXWIDTH, sizeof(char));

/* do your thing here */
const char *tmp = "test";
strcpy(t.names[0], tmp);
strcat(t.names[0], "appendtext"); /* just be careful not to go past MAXWIDTH */

strcpy(t.names[1], tmp);

strcpy(t.names[2], "something else");

/* free the memory that was allocated when done */
free(t.names);    
t.names = NULL;

---

shouldn't you alloc memory for your arrays before trying to access them ?

EDIT:

names[2] = "something else" gets you out of index.. you declared only a 2 string array.

Since you said that the memory is declared automatically as a constant, then you should have noticed:

char *tmp;
tmp = "test";

strcpy(tmp, "something"); //something is longer than test