[c] Dynamically create an array of strings with malloc

I am trying to create an array of strings in C using malloc. The number of strings that the array will hold can change at run time, but the length of the strings will always be consistent.

I've attempted this (see below), but am having trouble, any tips in the right direction will be much appreciated!

#define ID_LEN 5
char *orderedIds;
int i;
int variableNumberOfElements = 5; /* Hard coded here */

orderedIds = malloc(variableNumberOfElements * (ID_LEN + 1));

Ultimately I want to be able to use the array to do this:

strcpy(orderedIds[0], string1);
strcpy(orderedIds[1], string2);
/* etc */

This question is related to c arrays malloc

The answer is



#define ID_LEN 5
char **orderedIds;
int i;
int variableNumberOfElements = 5; /* Hard coded here */

orderedIds = (char **)malloc(variableNumberOfElements * (ID_LEN + 1) * sizeof(char));

..


Given that your strings are all fixed-length (presumably at compile-time?), you can do the following:

char (*orderedIds)[ID_LEN+1]
    = malloc(variableNumberOfElements * sizeof(*orderedIds));

// Clear-up
free(orderedIds);

A more cumbersome, but more general, solution, is to assign an array of pointers, and psuedo-initialising them to point at elements of a raw backing array:

char *raw = malloc(variableNumberOfElements * (ID_LEN + 1));
char **orderedIds = malloc(sizeof(*orderedIds) * variableNumberOfElements);

// Set each pointer to the start of its corresponding section of the raw buffer.
for (i = 0; i < variableNumberOfElements; i++)
{
    orderedIds[i] = &raw[i * (ID_LEN+1)];
}

...

// Clear-up pointer array
free(orderedIds);
// Clear-up raw array
free(raw);

char **orderIds;

orderIds = malloc(variableNumberOfElements * sizeof(char*));

for(int i = 0; i < variableNumberOfElements; i++) {
  orderIds[i] = malloc((ID_LEN + 1) * sizeof(char));
  strcpy(orderIds[i], your_string[i]);
}

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