Thread: 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.

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
  }
}

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.

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?