Question

My code gives me segfault error: which I don’t understand,the debugger says error comes from printing the value from stored_

char *stored_ = NULL;
char testMessage[15];

//strcpy(stored_, testMessage);

for (int a = 0;a < 10; a++)
{
    sprintf(testMessage,"Message::%i\n",a);
    printf("string is:%s;length is %i\n",testMessage,strlen(testMessage));

    stored_ = (char*) realloc (stored_, sizeof(char) * (strlen(testMessage) * (a+1) ));

    strcpy(&stored_[a], testMessage);
} 

for (int b = 0;b < 10; b++)
{
    printf("inside:|%s|\n",stored_[b]);
}

Answer 1

Fist up, sizeof(char) is always 1, you don’t need to multiply by it.

Secondly, when you’re allocating room for a string, you have to use:

malloc (strlen (string) + 1);

In other words, you need room for the null byte at the end.

Thirdly, you appear to be confused between character pointers and character pointer pointers. stored_ is a single block of characters and stored_[1] is only one byte beyond stored_[0], meaning you won’t have enough room to store the string.

stored_[n], n=:   0   1   2   3
                +---+---+---+---+
                |   |   |   |   |...
                +---+---+---+---+
                each of these cells is a single byte.

You’ll either have to manage the single block of characters yourself, leaving enough room for each element (by using sparse indexes), or have a block of character pointers with indexes 0, 1, 2 and so on, but you’ll then have to manage the string allocations separately.

The following code shows how to do this latter one:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main (void) {
    // An array of char pointers (C strings).

    char **stored_ = NULL;
    char testMessage[15];
    int i;

    // Populate them.

    for (i = 0; i < 10; i++) {
        sprintf (testMessage,"Message::%i",i);
        printf ("string is:%s;length is %i\n",testMessage,strlen(testMessage));

        // Reallocate array of char *, allocate room for string, then store it.

        stored_ =  realloc (stored_,sizeof (char*) * (i + 1));
        stored_[i] = malloc (strlen (testMessage) + 1);
        strcpy (stored_[i], testMessage);
    }

That’s the meat of it, the allocation of the array of character pointers separate from the actual arrays of characters forming the C strings.

Then the code below prints them and cleans up.

    // Print them.

    for (i = 0; i < 10; i++) {
        printf("inside:|%s|\n",stored_[i]);
    }

    // Free all memory and return.

    for (i = 0; i < 10; i++) {
        free (stored_[i]);
    }
    free (stored_);

    return 0;
}

The output being, as expected:

string is:Message::0;length is 10
string is:Message::1;length is 10
string is:Message::2;length is 10
string is:Message::3;length is 10
string is:Message::4;length is 10
string is:Message::5;length is 10
string is:Message::6;length is 10
string is:Message::7;length is 10
string is:Message::8;length is 10
string is:Message::9;length is 10
inside:|Message::0|
inside:|Message::1|
inside:|Message::2|
inside:|Message::3|
inside:|Message::4|
inside:|Message::5|
inside:|Message::6|
inside:|Message::7|
inside:|Message::8|
inside:|Message::9|

With this method, each cell is a pointer to an array of characters, separately allocated (which holds the C string):

stored_[n], n=:   0   1   2   3
                +---+---+---+---+
                |   |   |   |   |...
                +---+---+---+---+
                  |   |   |   |     +----------------------+
                  |   |   |   +---> | character array here |
                  |   |   |         +----------------------+
                  |   |   |         +----------------------+
                  |   |   +-------> | character array here |
                  |   |             +----------------------+
                  |   |             +----------------------+
                  |   +-----------> | character array here |
                  |                 +----------------------+
                  |                 +----------------------+
                  +---------------> | character array here |
                                    +----------------------+