Seg fault on a program spliting a char array to a 2D array

I have tried to use gdb to determine the problem. It is about **output control. You should check address of *output instead of where pointer to pointer to. You are allocating places in for loop until length of the string. It may cause defragmentation. Moreover, the 1D char array should be passed by const to be not changeable. Instead, you should use the snippet

// allocation (in the function)
// protoype: char** my_str2vect(char const* str)
int a;
char** output = malloc(str_len * sizeof(char *));
    output[0] = malloc(str_len * str_len * sizeof(char));
    for(a = 1; a < str_len; a++)
        output[a] = output[0] + a * str_len;

// freeing (in main())  
char ** x;
char const* str = "hello \t\t\n class,\nhow are you?";
x = my_str2vect(str);

free((void *)x[0]);
free((void *)x);

En passant, the source aids to get more knowledge about allocation.

As the debugger is telling you the if (**output) is broken. It's trying to dereference the pointer in the first output array location. That's junk at the point of the if. Hence, the seg fault. You want if (output). When I fix this and use strlen in place of your rewrite, it seems to work fine.

It's considerably simpler to make one copy of the input string and use this for all the strings in the returned vector. You can also use strtok to find the words, but that's not thread safe.

Here's a suggestion:

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

char **split(char *s_org) {
  size_t i;
  // Skip initial whitespace, then copy everything else.
  for (i = 0; s_org[i] && isspace(s_org[i]); ++i) /* skip */;
  char *s = strdup(s_org + i);
  size_t n_rtn = 0, size = 0;
  char **rtn = malloc(sizeof *rtn);
  for (i = 0;;) {
    if (!s[i]) {
      rtn[n_rtn] = NULL;
      return realloc(rtn, (n_rtn + 1) * sizeof *rtn);
    }
    if (n_rtn == size) {
      size = 2 * size + 1;
      rtn = realloc(rtn, size * sizeof *rtn);
    }
    rtn[n_rtn++] = s + i;
    while (s[i] && !isspace(s[i])) ++i;
    if (s[i]) {
      s[i++] = '\0';
      while (isspace(s[i])) ++i;
    }
  }
}

int main(void) {
  char **rtn = split("  hello \t\t\n class,\nhow are you?");
  for (char **p = rtn; *p; ++p)
    printf("%s\n", *p);
  // Freeing the first element frees all strings (or does nothing if none)
  free(rtn[0]);
  free(rtn);
  return 0;
}

This omits checks for NULL returns from malloc and realloc. But they're easy to add.

You asked about the "other problems" with your code. I've fixed some here:

• Use size_t to index arrays.
• Grow the output array as needed. It's really not that hard...
• Avoid unnecessary calls to malloc.
• Avoid strlen when simple checks for the terminating NULL are easier.
• Use the idiom FOO *p = malloc(sizeof *p); to allocate a FOO. It's less error prone than sizeof(FOO).

To correct your code change if (**output) to if (output).

I think your implementation is not memory efficient and could be more elegant.
You are allocating too much memory. I tried to explain in the code the upper bound for the size of output char pointers. If you want to have the exact size, you'll have to count words in the string. It's probably better to do it that way, but for the exercise I think we can go the easier way.

As to your code I can only say:

• I did not see the end of string character '\0' anywhere, which is a bad sign
• I did not see any string copy, which is also a bad sign
• I did not see you make use of standard library, which often makes you reinvent the wheel

Please see below an improved implementation (I use the standard C89):

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

char** my_str2vect(char* s) {
    // Takes a string 
    // Allocates a new vector (array of string ended by a NULL), 
    // Splits apart the input string x at each space character 
    // Returns the newly allocated array of strings
    // Any number of ' ','\t', and '\n's can separate words.
    // I.e. "hello \t\t\n class,\nhow are you?" -> {"hello", "class,", "how", "are","you?", NULL}

    int s_size = strlen(s);
    /*
     * size of output is 1 if string contains non delimiters only
     * size of output is 0 if string contains delimiters only
     * size of output is strlen / 2 if string contains ...
     * ...alternation of delimiter and non delimiter, and that is the max size
     * so we allocate that size (upper bound)
     */
    int max_output_size = (s_size / 2) + 1;
    char **output = (char **) malloc(sizeof (char *) * max_output_size);

    //initialize to NULL for convenience
    int k;
    for (k = 0; k < max_output_size; k++)
        output[k] = NULL;

    //work on a copy of s
    char *str = (char *) malloc(s_size + 1);
    strcpy(str, s);

    //pointer for token and delimiters
    char *ptr;
    char delimiter[] = "\n\t ";

    //initialize and create first token
    ptr = strtok(str, delimiter);

    //
    int i = 0;
    while (ptr != NULL) {
        //allocate memory and copy token
        output[i] = malloc(sizeof (char) * strlen(ptr) + 1);
        strcpy(output[i], ptr);
        //get next token
        ptr = strtok(NULL, delimiter);
        //increment
        i++;
    }

    return output;
}

int main(int argc, char *argv[]) {

    char **result = my_str2vect("hello \t\t\n class,\nhow are you?");

    int i;
    for (i = 0; result[i] != NULL; i++)
        printf("%s\n", result[i]);

    return 0;
}