Thread: Arrays, and what not

Hello All,

I am new to this language, so please forgive my ignorance. I am wondering how I can resize a character array after a size has been initially set. I am looking to do something like this:

Code:

#define SIZE 10

static char myarray[SIZE];

void resize_my_array(unsigned int new_size) {
       myarray[new_size];
}

How can this be accomplished in c? Thanks in advance for any guidance
or help!

You can't.

What you can do is fake an array using malloc() and then use realloc() to resize it. This will require you to check the results of the malloc() and realloc() functions (they can fail), and to free() the memory when you're done.

An alternative to achieve this is to make a linked list, it's a bit more involved though but should be more effective if you want to manipulate it more later and do things like insertion and removal of elements and so on.

In a linked list each element is not necessarily placed in consecutive order in memory and contains the data and a link to the next element (or both to the next and the previous).

Thank you everyone for your prompt responses. I am trying the following code below:

Code:

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

#define THESIZE 10

void resize_myarray( unsigned int new_size );
char *myarray;

main (int ac, char **av[]) {
	myarray = malloc(THESIZE * sizeof(char));

	myarray = "0123456789";

	printf("%d\n", sizeof(myarray));

	resize_myarray(THESIZE * 2 * sizeof(char));

	myarray = "";
	myarray = "01234567890123456789";

	printf("%d\n", sizeof(myarray));

	exit(0);
}

void resize_myarray ( unsigned int new_size ) {
	myarray = realloc( myarray, new_size );
}

and am getting the following output:
----------------------------------------
-- $ ./a.exe
-- 4
-- Aborted (core dumped)
----------------------------------------


I am having some trouble understanding...
a) why the size of the array is 4 with the first call to print the size. I figured it would be 10.
b) why I am unable to reset the value of myarray to a 20 character string after resetting the size of the array w/realloc

Again, your help is much appreciated!

Thanks. The only part I suppose I am misunderstanding (from b) is why overwriting the old value creates a leak. In any event, I am now trying the following:

Code:

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


#define THESIZE 10

void resize_myarray( unsigned int new_size );
char *myarray;

main (int ac, char **av[]) {
	myarray = malloc(THESIZE * sizeof(char));

	strncpy(myarray, "0123456789", THESIZE * sizeof(char));

	printf("%d\n", strlen(myarray));

	free(myarray);
	resize_myarray(THESIZE * 2 * sizeof(char));

	strncpy(myarray, "01234567890123456789", (THESIZE * 2 * sizeof(char)));

	printf("%d\n", strlen(myarray));

	exit(0);
}

void resize_myarray ( unsigned int new_size ) {
	myarray = realloc( NULL, new_size );
}

and am confused by my output...
----------------------------------------
-- $ ./a.exe
-- 10
-- 22
----------------------------------------

I understand the 10 string of output, but am confused by 22. I was expecting 20.

First, you should be aware that in C, strings are always terminated by a null byte (that is, a byte with the value 0). Thus for "0123456789" you need 11 bytes; 10 for the digits, one for the null byte.

Next, strncpy() does not always create strings, despite its name. You should always null-terminate the “strings” it creates. That is, strncpy() should be used as follows:

Code:

char target[10];
strncpy(target, "some long string", (sizeof target) - 1);
target[(sizeof target) - 1] = 0;

There are other ways (set target[9] to zero ahead of time, etc), but the basic idea is the same: strncpy() is not a safe string function. It requires extra help.

Also: don't free() before you call realloc(). realloc() needs a pointer to allocated memory (or NULL) to resize. Passing a freed pointer is not good. Call free() only after you're completely done with the memory.

Thanks! I am now getting the expected output...
----------------------------------------
-- $ ./a.exe
-- string: 0123456789; length: 10
-- string: 01234567890123456789; length: 20
----------------------------------------

from the code:

Code:

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


#define THESIZE 10

void resize_myarray( unsigned int new_size );
char *myarray;

main (int ac, char **av[]) {
	myarray = malloc((THESIZE * sizeof(char)) + 1);

	strncpy(myarray, "0123456789", ((THESIZE * sizeof(char)) + 1));
	myarray[THESIZE * sizeof(char)] = 0;

	printf("string: %s; length: %d\n", myarray, strlen(myarray));

	resize_myarray(THESIZE * 2 * sizeof(char));

	strncpy(myarray, "01234567890123456789", ((THESIZE * 2 * sizeof(char)) + 1));
	myarray[THESIZE * 2 * sizeof(char)] = 0;

	printf("string: %s; length: %d\n", myarray, strlen(myarray));

	//free(myarray);
	exit(0);
}

void resize_myarray ( unsigned int new_size ) {
	myarray = realloc( NULL, new_size );
}

However, when I remove the comment in front of free, the program hangs. Any idea why? I figured that I am supposed to call free() to so that the memory attached to the myarray pointer is once again made available.

hi, sorry for my english. i added some comment on your code..

Code:

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


#define THESIZE 10

void resize_myarray (char *arr, unsigned int new_size );
//char *myarray;

main (int ac, char *av[]) {  //NOT char**av[]
	
	char *myarray; //NO global variables NO
	myarray = malloc(THESIZE * sizeof(char));

	//you can store max THESIZE-1 char because there is a \n at the end if the string.
	strncpy(myarray, "012345678", THESIZE * sizeof(char));

	printf("%d\n", (int)strlen(myarray)); //need cast to int

	free(myarray);
	
	resize_myarray(myarray, THESIZE * 2 * sizeof(char));

	//same problem...you can store THESIZE * 2 - 1 char
	strncpy(myarray, "0123456789012345678", (THESIZE * 2 * sizeof(char)));

	printf("%d\n", (int)strlen(myarray)); //need cast to int

	exit(0);
}

void resize_myarray (char *arr, unsigned int new_size ) {
	arr = realloc( NULL, new_size );
}