How do I dynamically allocate a 2d array of chars?

Question

I want to dynamically allocate a 2d array to store strings.

I originally declared the array like this:

char lines[numlines][maxlinelength];

This however gives me a stack overflow when numlines is very huge.

How can I dynamically allocate it to prevent stack overflow?

• Martin James: arr = malloc(numlines[star]maxlinelength[star]sizeof(char))

Answer 1

Use a pointer to an array:

#define maxlinelength 10
char (*lines)[maxlinelength] = malloc( sizeof( char[maxlinelength] ) * numlines ) ;
lines[0][0] = 'A' ;

This requires that the inner most size, maxlinelength, is constant.

You can avoid this limitation if you use pointers to variable-length arrays in which case the syntax remains the same and maxlinelength doesn't have to be a constant. Standards that supports this feature, are C99 and optionally C11.

( A constant is a variable whose value is known at compile time.)

( And to clarify: sizeof( *lines ) is identical to sizeof( char[maxlinelength] ) )

Answer 2

Try this code:

char **arr;
int i;
arr = malloc(numlines*sizeof(char*));
for (i = 0; i < numlines; i++)
 arr[i] = malloc(maxlinelength*sizeof(char));

This will give you a pointer to pointer, but you can handle it as if it was a 2D array.

Also see this very useful link in order to understand what dynamic allocation for 2D arrays actually does in the memory.

• Martin James: That is not a 2D array. It's an array of pointers to 1D arrays.
• Marievi: @MartinJames but the only way to allocate a 2D array dynamically is to actually allocate a double pointer..
• too honest for this site: @Marievi: That is wrong! You can very well have int (*p)[varcols] = malloc(...);
• Peter: @Olaf - I didn't write this answer. I'm expressing disagreement with you that there is a single right answer (the one you're advocating) and that all others are wrong. With EVERY possible solution, it is possible to find a test case that shows a difference from an actual 2D array.
• Peter: My disagreement stands. You are dismissing anything that doesn't comply with your narrow perspective, so no point in my discussing further.

Answer 3

You may allocate a contiguous block of memory with the following:

const int numlines = 10;
const int maxlinelength = 120;
const int arrlen = sizeof(char) * numlines * maxlinelength;
const char **lines = (const char**)malloc(arrlen);
fprintf(stderr, "Allocated %d bytes\n", arrlen);

Add/remove const and typecasting according to taste.

• Peter: The problem with this is that lines[i] does not point to (the first character of) an array of char, so lines[i][j] doesn't work as if it is accessing a 2D array.