Dynamic array of pointers

Hi. I'm learning about pointers and as an exercise I'm trying to create a program that will store an unknown number of pointers to strings that will be input by the user in an array of pointers.

Code:

---

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

int main(void)
{
    char input[100];
    char **strings;

    do {
        printf("Type something: ");
        fgets(input, sizeof(input), stdin);

        /* I don't know what to do now */
    } while (strcmp(input, "exit") != 0);

    /* free the memory that was allocated */

    return 0;
}

---

If I was using something like

Code:

---

char *strings[5000];

---

then I'd know what to do but using **strings is confusing me. Anyone can give me a hint? Thanks.

**strings just means that strings is a pointer which can hold the memory address of some other pointer.

Start with
char **strings = NULL;

Then keep two counters
int numStrings = 0, maxStrings = 0;

Then you do this

Code:

---

if ( numStrings == maxStrings ) {
  void *temp = realloc ( strings, (maxStrings+10) * sizeof *strings );
  if ( temp != NULL ) {
    strings = temp;
    maxStrings += 10;
  } else {
    // handle error
  }
}

---

Code:

---

if ( numStrings == maxStrings ) {
  /* after this, I'll be able to store 10 strings in variable strings, right? */
  void *temp = realloc ( strings, (maxStrings+10) * sizeof *strings );
  if ( temp != NULL ) {
    strings = temp;
    maxStrings += 10;
  } else {
    // handle error
  }
}

---

Yes.

And you increment numStrings after each one you add.

Is this correct?

Code:

---

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

int main(void)
{
    char *p = NULL;
    char linha[100];
    char **strings = NULL;
    int c = 0;
    int nstrings = 0;
    int maxstrings = 0;
    void *temp = NULL;

    for (;;) {
        printf("Type something or exit to exit: ");
        fgets(linha, sizeof(linha), stdin);

        if ((p = strchr(linha, '\n')) != NULL)
            *p = '\0';
        else
            while ((c = getchar()) != '\n' && c != EOF);

        if (strcmp(linha, "exit") == 0)
            break;

        if (nstrings == maxstrings)
            temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
        if (temp) {
            strings = (char **)temp;
            maxstrings += 10;
        }
        p = malloc(strlen(linha) + 1);
        if (p)
            strcpy(p, linha);
        else
            break;
        strings[nstrings++] = p;
    }

    --nstrings;

    for (; nstrings >= 0; nstrings--) {
        printf("%s\n", strings[nstrings]);
        free(strings[nstrings]);
    }

    free(strings);

    return 0;
}

---

Thanks.

You should check the return result of fgets(), and break if it's NULL
You should also break if your realloc fails (like you break if your malloc fails)
The cast (char**) is redundant.

Apart from those easy to fix issues, it's looking good :)

Code:

---

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

int main(void)
{
    char *p = NULL;
    char linha[100];
    char **strings = NULL;
    int c = 0;
    int nstrings = 0;
    int maxstrings = 0;
    void *temp = NULL;

    for (;;) {
        printf("Type something or exit to exit: ");
        if (fgets(linha, sizeof(linha), stdin) == NULL)
            break;

        if ((p = strchr(linha, '\n')) != NULL)
            *p = '\0';
        else
            while ((c = getchar()) != '\n' && c != EOF);

        if (strcmp(linha, "exit") == 0)
            break;

        if (nstrings == maxstrings)
            temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
       
        if (temp) {
            strings = temp;
            maxstrings += 10;
        }
        else
            break;

        p = malloc(strlen(linha) + 1);
        if (p)
            strcpy(p, linha);
        else
            break;
        strings[nstrings++] = p;
    }

    --nstrings;

    for (; nstrings >= 0; nstrings--) {
        printf("%s\n", strings[nstrings]);
        free(strings[nstrings]);
    }

    free(strings);

    return 0;
}

---

Just for testing purposes, I entered all the letters of the alphabet, one letter per line. Here's my output:

Code:

---

Type something or exit to exit: a
Type something or exit to exit: b
Type something or exit to exit: c
Type something or exit to exit: d
Type something or exit to exit: e
Type something or exit to exit: f
Type something or exit to exit: g
Type something or exit to exit: h
Type something or exit to exit: i
Type something or exit to exit: j
Type something or exit to exit: k
Type something or exit to exit: l
Type something or exit to exit: m
Type something or exit to exit: n
Type something or exit to exit: o
Type something or exit to exit: p
Type something or exit to exit: q
Type something or exit to exit: r
Type something or exit to exit: s
Type something or exit to exit: t
Type something or exit to exit: u
Type something or exit to exit: v
Type something or exit to exit: w
Type something or exit to exit: x
Type something or exit to exit: y
Type something or exit to exit: z
Type something or exit to exit: exit
z
y
x
w
v
u
t
s
r
q
p
o
n
m
l
k
j
i
h
g
f
e
d
c

---

A little window pops-up asking if I want to send the error to Microsoft. I can't see the error but I think it has something to do with memory freeing. Can someone help me find the problem, please?

Start making braces compulsory in your code

Code:

---

    if (nstrings == maxstrings) {
      temp = realloc(strings, (maxstrings + 10) * sizeof(*strings));
      if (temp) {
        strings = temp;
        maxstrings += 10;
      } else {
        break;
      }
    }

---