C: 2-Dimensional String Array Segmentation Fault

Question

Trying to make a small program that separates words within a big string and stores each word (of the big string) in a string (i.e pointer) in an array of strings (i.e pointers); forming a 2-dimensional string array. The word separator is simply a whitespace (32 in ASCII); the big string is:

"Showbizzes Oxygenized Equalizing Liquidized Jaywalking"

Note:

the words are all 10 characters in length
the total length of the string is 54 characters (spaces included)
the total size of the buffer is 55 bytes ('\0' included)

One more thing, the last pointer in the array of pointers must hold a 0 (i.e 1 character: '\0') (this is completely arbitrary).

Here is the program, nothing special, but ...

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

int main(void) {

    // The string that we need to break down into individual words
    char str[] = "Showbizzes Oxygenized Equalizing Liquidized Jaywalking";

    // Allocate memory for 6 char pointers (i.e 6 strings) (5 of which will contain words)
    // the last one will just hold 0 ('\0')
    char **array; array = malloc(sizeof(char *) * 6);

    // i: index for where we are in 'str'
    // r: index for rows of array
    // c: index for columns of array
    int i, r, c;

    // Allocate 10 + 1 bytes for each pointer in the array of pointers (i.e array of strings)
    // +1 for the '\0' character
    for (i = 0; i < 6; i++)
        array[i] = malloc(sizeof(char)*11);

    // Until we reach the end of the big string (i.e until str[i] == '\0');
    for (i = 0, c = 0, r = 0; str[i]; i++) {

        // Word seperator is a whitespace: ' ' (32 in ASCII)
        if (str[i] == ' ') { 

            array[c][r] = '\0';     // cut/end the current word
            r++;                    // go to next row (i.e pointer)
            c = 0;                  // reset index of column/letter in word
        }

        // Copy character from 'str', increment index of column/letter in word
        else { array[c][r] = str[i]; c++; }

    }   

    // cut/end the last word (which is the current word)
    array[c][r] = '\0'; 

    // go to next row (i.e pointer)
    r++; 

    // point it to 0 ('\0')
    array[r] = 0; 



// Print the array of strings in a grid - - - - - - - - - - - - - - 

    printf("       ---------------------------------------\n"); 
    for (r = 0; r < 6; r++) {

        printf("Word %i --> ", r);
        for (c = 0; array[c][r]; c++)
            printf("| %c ", array[c][r]);

        printf("|");printf("\n");
        printf("       ---------------------------------------");
        printf("\n");
    }

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    return 0;
}

.. there's something wrong and I don't understand how to fix it.

For some reason it copies into the first string (i.e pointer), in the array of strings (i.e pointers), the first 6 characters of the big string, then on the 7th it gives a segmentation fault. I've allocated 6 pointers each with 11 bytes.. at least thats what I think the code is doing, so really I have NO clue why this is happening...

Hope someone can help.

array[r] = 0; // point it to 0 ('\0') doesn't seems correct. — David Ranieri
– David Ranieri, Commented Aug 22, 2018 at 12:42
I just arbitrarily want the last pointer in the array of pointers to be 0 (i.e '\0'); in other words the last string in the array of strings to hold 1 character: '\0'. — AymenTM
– AymenTM, Commented Aug 22, 2018 at 12:43
So you mean // point it to 0 (NULL) instead of // point it to 0 ('\0'), isn't it? — David Ranieri
– David Ranieri, Commented Aug 22, 2018 at 12:44
No not NULL, as in the last pointer has memory allocated to it (the container/string exists), it's just that the only character of the string is '\0' (0 in ASCII). — AymenTM
– AymenTM, Commented Aug 22, 2018 at 12:45

David Ranieri · Accepted Answer · 2018-08-22 13:00:48Z

2

Replace all ocurrences of array[c][r] with array[r][c]

The first dimension is the row.

Next time you can check this using a debugger:

Program received signal SIGSEGV, Segmentation fault.
0x00000000004007ea in main () at demo.c:37
37  array[c][r] = str[i];

answered Aug 22, 2018 at 13:00

David Ranieri

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2 Comments

politinsa Over a year ago

Compile with flag -g in gcc, and then gdb yourprog and type run

AymenTM Over a year ago

Thank you @KeineLust. It works now. So rows first, then columns... and for all this time I thought it was the opposite :S

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