Having trouble reading strings from stdin

Question

I need to create program that takes input from stdin in this format:

abcde //number of characters in word = number of words => square shape
fghij
klmno
pqrst
uvwxy
        // \n separates first half from second
word1word //any amount of characters, any amount of words
word
word2
sdf
        // \n to end input

My code works, but only about 50% of the time. I have couple of example inputs, that I use for testing, but for some of them my readwords function fails.

Here is my function, that reads words. Since I have no idea how many words or how long they are going to be, I use dynamic arrays and getchar() function.

void readWords(char **p,int *n,int w) /* before calling: n = 50; w = 20; p = 50x20 char array */
{
int i = 0,j = 0,x;
char tmp,prevtmp;

while (1)
{
    prevtmp = tmp;
    tmp = getchar();
    if ((prevtmp == '\n'  && tmp == '\n') || feof(stdin))
        break; /* no more words to read */

    if (tmp == '\n') /* end of word */
    {
        p[i][j] = '\0'; /* add \0 to create string format */

        i++;
        j = 0;
        if (i == *n) /* if there is more words than there is space for them, double the size */
            if (realloc(p,*n*2) != NULL)
                *n*=2;

        continue;
    }

    p[i][j] = tmp;
    j++;
    if (j == w) /* if width of word is larger than allocated space, double it */
    {
        for (x = 0; x < *n;x++);
            if(realloc (p[x],w*2) != NULL);

        w=w*2;
    }

}
*n = i;
}

This is example of input for which this works (note:this function only reads second half after line with only \n):

dsjellivhsanxrr
riemjudhgdffcfz
<skipping>
atnaltapsllcelo
ryedunuhyxhedfy

atlanta
saltlakecity

<skipping 15 words>

hartford
jeffersoncity

And this is input that my function doesn't read properly:

<skipping>
...oywdz.ykasm.pkfwb.zazqy...
....ynu...ftk...zlb...akn....

missouri
delaware

<skipping>

minnesota
southdakota

What my function reads from this input:

e
yoming
xas
florida
lvania
ana
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ

There is no difference between those two inputs (except different words and different amount and length of words), the first half gets read properly no matter what, but only the second half bugs out. How do I fix this?

P.S. sorry for long post, in case you want to see full input without skipped bytes, here is pastebin: http://pastebin.com/hBGn2tej

hmjd · Accepted Answer · 2012-11-22 11:49:54Z

realloc() returns the address of the newly allocated memory, it does not update the argument passed into it. So this (and the other use of realloc()) is incorrect:

if (realloc(p,*n*2) != NULL)

and will results in the code accessing memory incorrectly, causing undefined behaviour. Store the result of realloc() to a temporary variable and check for non-NULL before updating p. The argument to realloc() also indicates the number of bytes, not the number of elements so the size argument calculation is incorrect as p is an array of char* so it should be realloc(p, sizeof(char*) * (*n * 2));. However, the change to p would not be visible to the caller. Also note that the only legal arguments to realloc() are pointers obtained from a previous call to malloc(), realloc() or calloc(). The comment p = 50x20 char array in the code suggests this is not the case.

Here is a small example that allocates an array of char* which should be helpful:

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

void f(char*** p)
{
    /* Allocate space for two 'char*' elements.
       Add a NULL pointer element as sentinel value
       so caller knows where to find end of list. */
    *p = malloc(sizeof(**p) * 3);

    /* Allocate space for the two strings
       and populate. */
    (*p)[0] = malloc(10);
    (*p)[1] = malloc(10);

    strcpy((*p)[0], "hello");
    strcpy((*p)[1], "world");
    (*p)[2] = NULL;

    /* Add a third string. */
    char** tmp = realloc(*p, sizeof(**p) * 4);
    if (tmp)
    {
        *p = tmp;
        (*p)[2] = malloc(10);
        strcpy((*p)[2], "again");
        (*p)[3] = NULL;
    }
}

int main()
{
    char** word_list = 0;
    f(&word_list);

    if (word_list)
    {
        for (int i = 0; word_list[i]; i++)
        {
            printf("%s\n", word_list[i]);
            free(word_list[i]);
        }
    }
    free(word_list);

    return 0;
}

Additionally:

prevtmp has an unknown value upon its first use.
getchar() actually returns an int and not a char.

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