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A simple implementation of a mutable String in C

_(Brought over from [SE](https://codereview.stackexchange.com/q/156339/98306).)_

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The following code implements a simple interface to operate on mutable* `String`s in C. It is composed of two files: one for the structure definition and the available operations, and the other with the actual implementation.

_* = This can be inexact, as only adding characters to the end is implemented._

Any comments on coding style and improvements are greatly appreciated.

## `String.h`

    #ifndef __STRING_H__
    #define __STRING_H__

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

    #define BLOCK_SIZE 256

    typedef struct {
        char *data;
        int length;
        int blocks;
    } String;

    String * string_create();
    void string_dispose(String *str);

    bool string_empty(String *str);
    void string_append_char(String *str, char c);

    char * string_get_all(String *str);
    char string_get(String *str, int pos);

    int string_find(String *str, char c);

    #endif /* __STRING_H__ */

## `String.c`

    #include "String.h"

    /** Create a String */
    String * string_create() {
        String *ans = calloc(1, sizeof *ans);

        if (ans != NULL) {
            ans->data = malloc( BLOCK_SIZE * sizeof *(ans->data) );

            ans->length = 0;
            ans->blocks = (ans->data == NULL) ? 0 : 1;
        }

        return ans;
    }

    /** Free the memory associated with a String */
    void string_dispose(String *str) {
        if (str != NULL) {
            free(str->data);
            free(str);
        }
    }

    /** Is the String empty? */
    bool string_empty(String *str) {
        if (str == NULL) return true;
        if (str->length == 0) return true;
        return false;
    }

    /** Add a character to the end of the String */
    void string_append_char(String *str, char c) {
        if (str != NULL) {
            if (str->length == str->blocks * BLOCK_SIZE) {
                char *new_str = realloc( str->data, BLOCK_SIZE * (str->blocks + 1) * sizeof *(str->data));

                if (new_str != NULL) {
                    str->data = new_str;
                    ++(str->blocks);
                }
            }

            if (str->length < str->blocks * BLOCK_SIZE) {
                str->data[str->length] = c;
                ++(str->length);
            }
        }
    }

    /** Get a C-String with the proper null-terminator */
    char * string_get_all(String *str) {
        char *res = NULL;

        if (str != NULL) {
            res = malloc((str->length + 1) * sizeof *str->data);

            if (res != NULL) {
                memcpy(res, str->data, str->length);
                res[str->length] = '\0';
            }
        }

        return res;
    }

    /** Get a character at a given position in the String */
    char string_get(String *str, int pos) {
        char res = '\0';

        if (str != NULL) {
            if (pos >= 0 && pos < str->length) {
                res = str->data[pos];
            }
        }

        return res;
    }

    /** Get where the first occurrence of a character in the String is */
    int string_find(String *str, char c) {
        int pos = -1;

        if (str != NULL) {
            pos = 0;
            while (str->data[pos] != c) ++pos;

            if (pos == str->length) pos = -1;
        }

        return pos;
    }

## `test_string.c`

This is a test file that uses some of the core methods implemented in a `String`.

    #include <stdio.h>

    #include "String.h"

    int main() {
        String *str = string_create();

        char i;
        for (i = 'a'; i <= 'z'; i++)
            string_append_char(str, i);

        char *cstr = string_get_all(str);

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

        int pos = string_find(str, 'f');

        printf("Character 'f' occurs at position %d.\n", pos);
        printf("Reading from the String, we get \"%c\".\n", string_get(str, pos));

        free(cstr);
        string_dispose(str);

        return 0;
    }