How can I return a character array from a function in C?

Question

is that even possible? Let\'s say that I want to return an array of two characters

char arr[2];
arr[0] = \'c\';
arr[1] = \'a\';

from a func

Answer 1

You can return a pointer for the array from a function, however you can't return pointers to local arrays, the reference will be lost.

So you have 3 options:

1. Use a global variable:

   char arr[2];
   
   char * my_func(void){
       arr[0] = 'c';
       arr[1] = 'a';
       return arr;
   }
   

2. Use dinamic allocation (the caller will have the responsability to free the pointer after using it, make it clear in your documentation)

   char * my_func(void){
   
       char *arr;    
       arr = malloc(2);
       arr[0] = 'c';
       arr[1] = 'a';
   
       return arr;
   }
   

3. Make the caller allocate the array and use as a reference (my recomendation)

   void my_func(char * arr){
   
       arr[0] = 'c';
       arr[1] = 'a';
   }
   

if you realy need the function to return the array, you can return the same reference as:

char * my_func(char * arr){
    arr[0] = 'c';
    arr[1] = 'a';
    return arr;
}

Answer 2

char* getCharArray()
{
  return "ca";
}

Answer 3

This works perfecly:

int comm_read_data(int file_i2c, unsigned char** buffer)
{
    *buffer = malloc(BUFFER_SIZE);
    if (i2c_read_bytes(file_i2c, *buffer, BUFFER_SIZE) != 0)
    {
        return -1;
    }
    return BUFFER_SIZE;
}

And then call the function:

unsigned char* buffer;
int length = comm_read_data(file_i2c, &buffer);

/* parse the buffer here */

free(buffer);

Answer 4

You can pass the array to the function and let the function modify it, like this

void function(char *array)
 {
    array[0] = 'c';
    array[1] = 'a';
 }

and then

char array[2];

function(array);
printf("%c%c\n", array[0], array[1]);

If you want it as a return value, you should use dynamic memroy allocation,

char *function(void)
 {
    char *array;

    array = malloc(2);
    if (array == NULL)
        return NULL;
    array[0] = 'c';
    array[1] = 'a';

    return array;
 }

then

char *array = function();
printf("%c%c\n", array[0], array[1]);
/* done using `array' so free it because you `malloc'ed it*/
free(array);

Important Note:

You should be aware of the fact that the array as filled above is not a string, so you can't for instance do this

printf("%s\n", array);

because the "%s" expects a matching string to be passed, and in c an array is not a string unless it's last character is '\0', so for a 2 character string you need to allocate space for 3 characters and set the last one to '\0'.

Answer 5

You've got several options:

1) Allocate your array on the heap using malloc(), and return a pointer to it. You'll also need to keep track of the length yourself:

void give_me_some_chars(char **arr, size_t *arr_len)
{
    /* This function knows the array will be of length 2 */
    char *result = malloc(2);

    if (result) {
        result[0] = 'c';
        result[1] = 'a';
    }

    /* Set output parameters */
    *arr = result;
    *arr_len = 2;
}

void test(void)
{
    char *ar;
    size_t ar_len;
    int i;

    give_me_some_chars(&ar, &ar_len);

    if (ar) {
        printf("Array:\n");
        for (i=0; i<ar_len; i++) {
            printf(" [%d] = %c\n", i, ar[i]);
        }
        free(ar);
    }
}

2) Allocate space for the array on the stack of the caller, and let the called function populate it:

#define ARRAY_LEN(x)    (sizeof(x) / sizeof(x[0]))

/* Returns the number of items populated, or -1 if not enough space */
int give_me_some_chars(char *arr, int arr_len)
{
    if (arr_len < 2)
        return -1;

    arr[0] = 'c';
    arr[1] = 'a';

    return 2;
}

void test(void)
{
    char ar[2];
    int num_items;

    num_items = give_me_some_chars(ar, ARRAY_LEN(ar));

    printf("Array:\n");
    for (i=0; i<num_items; i++) {
        printf(" [%d] = %c\n", i, ar[i]);
    }
}

DO NOT TRY TO DO THIS

char* bad_bad_bad_bad(void)
{
    char result[2];      /* This is allocated on the stack of this function
                            and is no longer valid after this function returns */

    result[0] = 'c';
    result[1] = 'a';

    return result;    /* BAD! */
}

void test(void)
{
    char *arr = bad_bad_bad_bad();

    /* arr is an invalid pointer! */
}

Answer 6

Since you have a predetermined size of you array you can in-fact return the array if you wrap it with a struct:

struct wrap
{
    char a[2] ;
} ;

struct wrap Get( void )
{
    struct wrap w = { 0 } ;

    w.a[0] = 'c';
    w.a[1] = 'a';

return w ;
}