C dynamic allocation for a grid when rows are not known

I am trying to allocate a array of char*'s in C. I know the开发者_如何学JAVA number of columns in advance, but not the rows and I want to allocate the rows as and when needed.

I tried to use:

char *(*data)[NUMCOLS]; //declare data as pointer to array NUMCOLS of pointer to char

data = malloc(sizeof(char*));

now, the above line should allocate for data[0] ... correct? then, I must be able to use the row like

data[0][1] = strdup("test");
 .
 ..
data[0][NUMCOLS-1] = strdup("temp");

I am getting seg fault. I am not able to understand what is wrong here. can anyone please help.

You haven't allocated enough memory for the things that you want to store. In this particular case, that would be:

data=malloc(sizeof(char*)*NUMCOLS*NUMROWS);

To resize the array, you would use:

data=realloc(data,(size_t)sizeof(char*)*NUMCOLS*NEW_NUMROWS);

More about it (reallocation) here

I would do this:

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

int main(){
  char ***a = NULL;

  a       = realloc( a, 1 * sizeof(char **) ); // resizing the array to contains one raw
  a[0]    = malloc(     3 * sizeof(char  *) ); // the new raw will contains 3 element
  a[0][0] = strdup("a[0][0]");
  a[0][1] = strdup("a[0][1]");
  a[0][2] = strdup("a[0][2]");


  a       = realloc( a, 2 * sizeof(char **) ); // resizing the array to contains two raw
  a[1]    = malloc(     3 * sizeof(char  *) ); // the new raw will contains 3 element
  a[1][0] = strdup("a[1][0]");
  a[1][1] = strdup("a[1][1]");
  a[1][2] = strdup("a[1][2]");

  for( int rows=0; rows<2; rows++ ){
    for( int cols=0; cols<3; cols++ ){
      printf( "a[%i][%i]: '%s'\n", rows, cols, a[rows][cols] );
    }
  }
}