Due 3/1

malloc(…): Join strings

Learning goals

You should learn how to:

Use malloc(…) to allocate buffers on the heap.
Write bug-resistant code that uses malloc(…).
Debug common problems that occur when using dynamic memory.

Overview

In this assignment, you will create a few functions for manipulating strings. While these functions are operations that are genuinely useful in some contexts, the real purpose of this assignment is to give you some practice using dynamic memory, starting with something simple (copying a string to the heap) and moving up to something a little fancier (joining several strings with a separator string).

Dynamic memory (aka “heap memory”)

As we discussed in class, memory is organized in segments.

The stack segment is where local variables and arguments are stored. One limitation of the stack is that you have to know in advance how big any array, string, or object will be. If your code will take input from a file, a user, the network, or external code that you didn't write, then you can't predict how big things need to be. Another limitation is that when a function returns, all local variables and arguments are invalidated (i.e., become unavailable).

The text segment is where your compiled code is loaded in memory when you run your program.

The data segment is where most string literals are stored (e.g., for code like char* s = "xyz";). Those are stored in a read-only section of the data segment. (There is also a writable section, where most global and static variables are stored, but we don't talk much about that since you won't use any global or static variables in this class.)

The heap segment lets you specify exactly how many bytes you need. You use the malloc(…) function to allocate (reserve) that space. You allocate a buffer (a bunch of bytes) into which you can store a string, array, or any data you like. Your buffer says allocated until you explicitly deallocate it by calling the free(…) function.

Example: `"abc"`

Let's start with an example. This code just prints abc.

More specifically, it allocates (i.e. reserves space for) a buffer on the heap sufficient for the letters abc and the null terminator, initializes the string with those letters, prints the string, and then deallocates (“frees”) the buffer so that space in memory can be used for something else.

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

char* get_abc_string() {
  // Allocate a buffer on the heap for the new string, and store its address in abc_string.
  char* abc_string = malloc(sizeof(*abc_string) * (3 + 1)); // +1 is for the '\0'

  // malloc(…) takes the number of bytes you want to allocate, allocates (reserves) that
  // space on the heap segment, and returns the address of the first byte.

  // Initialize the characters in the newly allocated buffer.
  abc_string[0] = 'a';
  abc_string[1] = 'b';
  abc_string[2] = 'c';
  abc_string[3] = '\0'; // null terminator (DON'T FORGET!!!)

  return abc_string;
  // Because our string is on the heap (not the stack), it will remain accessible by
  // the caller for as long as it is needed.
}

int main(int argc, char* argv[]) {
  char* abc_string = get_abc_string();

  // Print the string
  log_str(abc_string);  // output: abc_string == "abc"

  // Free the buffer (DON'T FORGET!!!)
  free(abc_string);

  // free(…) takes the address of a buffer that was previously allocated using malloc(…), and
  // deallocates (“frees”) it, making it available to be used for other purposes (i.e., other
  // parts of the system).

  return EXIT_SUCCESS;
}

In the sections below, we will break down exactly how each part of this code works.

Advantages of the heap

Storing data on the heap has two main advantages.

Advantage #1: You can create an array (e.g., a string) in a function and return it to the caller. With the stack, if you create an array in a function (ex: char s[] = "xyz";), that array exists only in the stack frame of the function where it was created. After that function returns, the array no longer exists. If you try to return it (ex: return s;), the caller will end up trying to access an array that no longer exists—and probably contains junk stored by some other part of the system. If you create the same string on the heap and return it, it continues to exist indefinitely—until you explicitly deallocate (aka “free”) it. This allows the get_abc_string(…) function above to return a new string that will continue to exist for as long as the caller needs it.

Advantage #2: You can specify how much memory to allocate as your program is running. In contrast, when declaring an array on the stack, you have to specify how many bytes you need—at the time you write your code. One of the things you will do for this assignment is create a function, called copy_string(…), that copies a string that is passed to the function. At the time you write the function, you have no way of knowing the size of string that will be passed to the function. With the heap, you can make a new string of any size, depending on what is passed to your function.

How to allocate a buffer on the heap using `malloc(…)`

malloc(…) is a function that takes the number of bytes you want to allocate, allocates (reserves) that much space on the heap for you, and returns the address of the newly allocated space (often called a “buffer” or “block”).

For a very simple example, malloc(100) would allocate 100 bytes for you and return the address. Although your C compiler would allow that, no sane programmer would call malloc(…) in that way. Over time, programmers learned new methods of coding defensively to avoid bugs. Following modern best practices, we require that you use a very specific form when calling malloc(…)

To create an array of TYPE elements on the heap (e.g., array of ints or longs), use the following syntax:

TYPE* NAME = malloc(sizeof(*NAME) * # of elements);

The variable NAME will be initialized to the address of the first element in the array stored in your newly allocated buffer on the heap. You must then initialize each element of the array (e.g., NAME[0] = …;, etc.).

Since a string is just an array of char elements, we use malloc(…) to allocate a buffer for a string using this syntax:

char* NAME = malloc(sizeof(*NAME) * # of characters (including '\0'));

Example:

char* abc_string = malloc(sizeof(*abc_string) * (3 + 1)); // +1 is for the null terminator ('')

The variable abc_string will be initialized to the address of the first character of the string in the newly allocated buffer on the heap. You must then initialize each character (e.g., abc_string[0] = 'a';, etc.). Don't forget the null terminator (ex: abc_string[3] = '\0';).

☛ Use descriptive variable names. NAME is just a placeholder.

⚠ Every buffer allocated by malloc(…) must be deallocated (i.e., “freed”) once (and only once) by calling free(…) with the address to the buffer. Keep reading to learn how.

About `sizeof(…)`

We use sizeof(…) when calling malloc(…) to ensure that we allocate the right amount of memory. For example, if were allocating a buffer for an array of 4 int's, we would like to do malloc(4 * number_of_bytes_per_int). However, like most types in C, the size of an int can vary depending on the computer where you compile the code. On our platform, an int occupies 4 bytes, but there is no guarantee that will always be true, and in any case, we don't want to hard-code the size of a type in our code; we might make mistakes. sizeof(…) solves this.

sizeof(…) tells you the number of bytes required to store an object of a particular type (e.g., the number of variables required to store an int).

The C language allows you to use sizeof(…) in two seemingly different ways. (Only one is allowed in this class. Keep reading.)

sizeof(TYPE) gives you the number of bytes required to store for a variable or array element of type TYPE. For example, sizeof(int) gives you the number of bytes per int; on our platform that would be 4.
sizeof(EXPRESSION) gives you the number of bytes required to store a variable or element of the same type as EXPRESSION. For example, sizeof(100) would be the same as sizeof(int) because 100 is an int. Thus, on our platform, sizeof(100) would be 4 (because on our platform, an int occupies 4 bytes).

Modern best practices—and this class requires—that you use only the second form: sizeof(EXPRESSION)).

Why? In the case of calling malloc(…), the old way of using sizeof(…) duplicates information (i.e., the type of each element). As the code evolves, you may find that you need to change the type. If you change it in one place, but not another—a very common type of mistake programmers make—you will end up allocating the wrong amount of memory. For example, suppose you had this code to allocate a buffer for an array of 10 integers.

int* whole_numbers = malloc(sizeof(int) * 10);  // BAD
whole_numbers[0] = …;
…
whole_numbers[99] = …;

On our platform, it would allocate a 40-byte buffer (4 bytes/int * 10 ints). As we fill the array, the second element will be stored 4 bytes after the first element (and so forth).

Then, suppose you decided to change the type to long, but you only update the type of the variable whole_numbers. You end up with this:

// int* whole_numbers = malloc(sizeof(int) * 10);
long* whole_numbers = malloc(sizeof(int) * 10);   // 🐛 BUG!!!  
whole_numbers[0] = …;
…
whole_numbers[99] = …;  // 🔥 out of bounds!!!

This will allocate the same number of bytes as before (i.e., 40 bytes on our platform). Since the type of variable whole_numbers is now long*, and a long occupies 8 bytes on our platform, as we fill the array, the second element will be stored 8 bytes after the first element (and so forth). Thus, the entire second half of the array will be outside 40-byte buffer that we allocated. That can cause major bugs—including serious security vulnerabilities—that can be difficult to debug.

Using the form required in this class, we would instead have this:

// int* whole_numbers = malloc(sizeof(*whole_numbers) * 10);
long* whole_numbers = malloc(sizeof(*whole_numbers) * 10);  //
whole_numbers[0] = …;
…
whole_numbers[99] = …;  // 👌

⚠ Do not use sizeof(TYPE). Always use sizeof(EXPRESSION) only. This applies to all code you write for this class, but the issue most often comes up with respect to malloc(…)

How to free buffers on the heap using `free(…)`

To free any buffer that was allocated using malloc(…), call free(…) as follows:

free(NAME);

Memory leaks

If you forget to deallocate (free(…)) a buffer that you allocated with malloc(…), you will have a memory leak. In this class, you will receive a penalty for any memory leaks. In the real world, memory leaks make programs run slowly and sometimes cause applications or systems to crash.) You can detect memory leaks using Valgrind (described below), but it is much easier to avoid them in the first place.

☛ For every call to malloc(…), you should have a corresponding call to free(…).

Pitfalls

⚠ Do not use a plain integer constant with `malloc(…)`.

Specify the number of bytes to allocate as an expression using sizeof(…).

✘	char* abc_string = malloc(4);
✔	char* abc_string = malloc(sizeof(abc_string) 4);

⚠ Do not use a type with `sizeof(…)`

The C language also allows another form of the sizeof(…) operator: sizeof(TYPE) (← BAD!). Do not use this form. Some people might write the above malloc call as malloc(sizeof(char) * 4) (← BAD!). That form is outdated because it is bug-prone. Don't use it. If you later change the type of the type on the left, you may end up allocating the wrong amount of memory. That can cause serious bugs with security consequences. To avoid those problems, whenever you call malloc(…), pass an expression using sizeof(…) with an expression, not a type.

✘	char* abc_string = malloc(sizeof(int) * 4);
✔	char* abc_string = malloc(sizeof(abc_string) 4);

⚠ Don't forget the asterisk ("*") in the `sizeof(…)` expression.

If you forget the asterisk in your sizeof(…) expression, you may allocate the wrong amount of memory, leading to bufffer overflow errors which can be challenging to find. Remember your asterisk!

✘	char* abc_string = malloc(sizeof(▌abc_string) * 4);
✔	char* abc_string = malloc(sizeof(abc_string) 4);

⚠ Do not typecast the return value of `malloc(…)` in C code.

Some people who learned older C coding practices were taught to always typecast the result of malloc(…). For example, they would do the above call like this: char* abc_string = (char*)malloc(sizeof(*abc_string) * 4). That is not needed in C. malloc(…) returns a void* (generic memory address with no expectations about what type will be stored at that address). You are assigning that to a char*. That requires no typecast. The typecast only creates opportunities for bugs. Typecasts are not allowed in this class, except where you know a clear reason why the typecast is necessary and why it is safe.

✘	char* abc_string = (char)malloc(sizeof(abc_string) * 4);
✔	char* abc_string = malloc(sizeof(abc_string) 4);

⚠ Do not use any typecasts anywhere in any file for HW09.

If you think you need a typecase, you are probably making a mistake.

✘	char* abc_string = (char)malloc(sizeof(abc_string) * 4);
✔	char* abc_string = malloc(sizeof(abc_string) 4);

Typecasts are EVIL!!! They tell the compiler, “I know what I'm doing, so don't warn me if I seem to be making a mistake with types.” Sometimes they are a necessary evil, but you should use them only when they are truly necessary and safe, and you can articulate why.

⚠ Remember to free (i.e., `free(…)`) every buffer (i.e., string) that you allocate `malloc(…)`.

For every call to malloc(…), there should be exactly one call to free(…). You pass to free(…) whatever address was returned by malloc(…).

⚠ Every buffer must be freed exactly once—and no more.

The following would cause an error and/or crash.

char* s = malloc(…);
free(s);
free(s);

This is known as a “double free”. The error will typically be a mess, but will mention something about “double free”.

⚠ Do not attempt to access a buffer after it has been freed.

The following would cause an error and/or crash.

char* s = malloc(…);
free(s);
printf("%s\n", s);  // BAD!!!  Memory has already been freed.

When you check your code with Valgrind, this may result in an Invalid Read message.

⚠ Do not attempt to access memory outside the bounds of the buffer you allocated.

The following would cause an error and/or crash.

char* s = malloc(sizeof(*s) * 2); // just enough for "A" (including the \0)
s[0] = 'A';
s[1] = 'B';
s[2] = '\0';  // BAD!!!
free(s);

When you check your code with Valgrind, this may result in an Invalid Write message.

⚠ Do not forget to write the null terminator `'\0'` after the string.

The following would cause an error and/or crash.

char* s = malloc(sizeof(*s) * 2); // just enough for "A" (including the \0)
s[0] = 'A';
printf("%s\n", s);  // BAD!!!
free(s);

When you check your code with Valgrind, this may result in an Invalid Read or Conditional Jump Depends on Uninitialized Values error.

Valgrind will be covered in class. In short, you check your code by compiling it (normally) and then running…

$ valgrind ./{{ test_c }}

Messages are explained on the back page of your reference sheet.

Array of strings

An array of strings is just an array where every element has type char*. In other words, every element is the address of the first byte in a string.

In this simple example, the strings are on the data segment. The array of addresses to those strings is on the stack because strings was declared with …[].

char* strings[] = { "ab", "cd" };
log_str( strings[0] );
log_str( strings[1] );

That is roughly equivalent to this. The strings are still on the data segment.

char* s1 = "ab";
char* s2 = "ab";
char* strings[] = { s1, s2 };
log_str( strings[0] );
log_str( strings[1] );

This time, the strings and array are on the stack.

char s1[] = "ab"; // on the stack because s1 is declared with …[].
char s2[] = "cd"; // "  "   "     "       s2 "  "        "    "
char* strings[] = { s1, s2 };
log_str( strings[0] );
log_str( strings[1] );

This time, the strings are on the heap. The array is still on the stack.

char* s1 = malloc(sizoef(*s1) * 3);
s1[0] = 'a';
s1[1] = 'b';
s1[2] = '\0';

char* s2 = malloc(sizoef(*s2) * 3);
s2[0] = 'a';
s2[1] = 'b';
s2[2] = '\0';

char* strings[] = { s1, s2 };
log_str( strings[0] );
log_str( strings[1] );

free(s1);
free(s2);

For this final example, the strings and array are both on the heap.

char* s1 = malloc(sizoef(*s1) * 3);
s1[0] = 'a';
s1[1] = 'b';
s1[2] = '\0';

char* s2 = malloc(sizoef(*s2) * 3);
s2[0] = 'a';
s2[1] = 'b';
s2[2] = '\0';

char** strings = malloc(sizeof(*strings) * 2);
strings[0] = s1;
strings[1] = s2;

free(s1);
free(s2);
free(strings);

Getting Started

Get the starter code

you@ecegrid-thin1 ~/264/
$

264get hw09

This will write the following files: 1. hw09/join_strings.h Ok? (y/n)y 1 files were written

you@ecegrid-thin1 ~/264/
$

cd hw09

you@ecegrid-thin1 ~/264/hw09
$

Copy your log_macros.h, miniunit.h, and Makefile from previous assignments.

you@ecegrid-thin1 ~/264/hw09
$

cp ../hw06/log_macros.h ./

you@ecegrid-thin1 ~/264/hw09
$

cp ../hw07/miniunit.h ./

you@ecegrid-thin1 ~/264/hw09
$

cp ../hw08/Makefile ./

you@ecegrid-thin1 ~/264/hw09
$

Update your Makefile

you@ecegrid-thin1 ~/264/hw09
$

vim Makefile

You should only need to modify three lines.

# VARIABLES
ASG_NICKNAME = HW09
BASE_NAME = join_strings
SUBMIT_FILES = $(SRC_C) $(TEST_C) miniunit.h log_macros.h Makefile
…

Your SUBMIT_FILES variable might vary in how it uses the other variables. Just make sure that when all variables have been expanded, SUBMIT_FILES includes all of the files that must be submitted with this assignment.

Create a test file (test_join_strings.c) with one minimal test for `copy_string(…)`

This test will test if copy_string(…) can correctly copy an empty string (i.e., "").

⚠ Do not skip this step. A common mistake on homework assignments like this is to mess up the '\0'. If you have a very simple test that takes care of that—and get it working and tested before doing the rest of this assignment—you will make sure you can solve this issue in isolation before adding any further complexity.

you@ecegrid-thin1 ~/264/hw09
$

vim test_join_strings.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include "join_strings.h"
#include "miniunit.h"

int _test_copy_string_empty() {
    mu_start();
    //────────────────────
    char const* s_orig = "";
    char* s_copy = copy_string(s_orig);
    mu_check_strings_equal(s_copy, s_orig);
    free(s_copy);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    mu_run(_test_copy_string_empty);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

In the above example, s_orig is declared as char const* s_orig. The const ensures that you cannot accidentally write to *s_orig (i.e., you cannot write to the individual characters in s_orig).

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Hint: You will call malloc(…) and implement enough for a buffer to contain just one character (i.e., the '\0'.) Then, write the '\0' to the buffer.

Get this test passing before you add any more tests.

Submit.

Add a second test for `copy_string(…)`.

This will test if copy_string(…) can copy a non-empty string (i.e., "abc").

// …

int _test_copy_string_abc() {
    mu_start();
    //────────────────────
    char const* s_orig = "abc";
    char* s_copy = copy_string(s_orig);
    mu_check_strings_equal(s_copy, s_orig);
    free(s_copy);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_copy_string_abc);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Hint: You can use strlen(…) to get the number of characters in string (not including the '\0').

Hint: When you call malloc(…), be sure to allocate enough memory for all of the characters in string, and the '\0'.

Hint: You can use strcpy(destination string, source string) to copy a string. For example, if s_copy is the new buffer on the heap that you will copy into, then strcpy(s_copy, string) will copy all of the characters in string and the '\0' to the buffer at s_copy.

strlen(…) and strcpy(…) are declared in the standard header file string.h. To you use them, you must include that header file in the same way you would include stdio.h or stdlib.h.

Submit.

Add a minimal test for `wrap_string(…)`.

This will test if wrap_string(…) can correctly copy an empty string, including the specified left and right delimiter characters. (For examples, see the Requirements table below.)

// …

int _test_wrap_string_empty() {
    mu_start();
    //────────────────────
    char* s_copy = wrap_string("", '[', ']');
    mu_check_strings_equal(s_copy, "[]");
    free(s_copy);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_wrap_string_empty);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Hint: For this stage, you should not need strlen(…), strcpy(…), memcpy(…), or any loops.

Submit.

Add a more substantial test for `wrap_string(…)`.

This will test if wrap_string(…) can correctly copy a non-empty string , including the specified left and right delimiter characters.

// …

int _test_wrap_string_abc() {
    mu_start();
    //────────────────────
    char* s_copy = wrap_string("abc", '[', ']');
    mu_check_strings_equal(s_copy, "[abc]");
    free(s_copy);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_wrap_string_abc);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Hint: You can use memcpy(destination, source, num_bytes) to copy a specified number of bytes (num_bytes) from one buffer (source) to another (destination). Unlike strcpy(…), memcpy(…) does not copy the '\0'. It copies a specified number of bytes. memcpy(…) is equivalent to the following code:

void* memcpy(void* destination, void const* source, size_t num_bytes) {
    for(size_t i = 0; i < num_bytes; i++) {
        destination[i] = source[i];  // copy one byte from source to destination
    }
    return destination; // return destination (for convenience)
}

Reminder: void* is a type that means address of anything, and you can assign an anything* to a void*. Since wrap_string(…) will be dealing with a buffer of char, you can imagine the signature of memcpy(…) was char* memcpy(char* destination, char const* source).

Reminder: size_t is an unsigned integer type that is guaranteed to be sufficient to store the number of bytes in any array on the current system. While it may be tempting to use int everywhere, since it is familiar, there could be arrays (including strings) with more than INT_MAX bytes.

Add a minimal test for `join_strings(…)`.

This will test if join_strings(…) can correctly copy an array containing a single string. Since the delimiter only comes between successive strings in the array, the delimiter will be ignored for this test case. (For more examples, see the Requirements table below.)

// …

int _test_join_strings_one() {
    mu_start();
    //────────────────────
    char const* const strings[] = { "abc" };
    char* combined_str = join_strings(strings, 1, "-");
    mu_check_strings_equal(combined_str, "abc");
    free(combined_str);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_join_strings_one);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Note that strings is declared as char const* const strings[]. That means strings is an array of char*. Thus, strings[0] is the address of the 'a' in "abc". The const makes it work with join_strings(…), which has some protections to prevent you from accidentally modifying the array of strings that is passed in, or any character within any of those strings.

join_strings(…) is declared in join_strings.h as follows:

char* join_strings(char const* const* strings, size_t num_strings, char const* separator)

The first const ensures that you cannot accidentally modify **strings (first character in the first string)—or any other character in any of the strings. In other words, if you try to do strings[▒][▒] = '▒';, gcc will give you an error message to prevent you from making that mistake.

The second const ensures that you cannot accidentally modify *strings (the first string)—or any string in the array. In other words, if you had some other string elsewhere in memory (e.g., other_string), you cannot do strings[▒] = other_string;. Gcc would give you an error message to save you.

⚠ Do not use typecasts anywhere in HW09.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Hint: For this stage, join_strings(…) should do nothing more than copy_string(…).

⚠ Do not skip this step and do not try to implement the whole thing at this stage. Just get it working for a single string. If it contains any more code than your copy_string(…), you are not following directions and should expect problems. Get join_strings(…) working for just this simple case first. There are some complexities with the type of the argument that you will want to iron out before you add anything further.

Submit.

Add a more test for `join_strings(…)` with multiple strings but an empty delimiter.

// …

int _test_join_strings_three_empty_delimiter() {
    mu_start();
    //────────────────────
    char const* const strings[] = { "abc", "def", "ghi" };
    char* combined_str = join_strings(strings, 3, "");
    mu_check_strings_equal(combined_str, "abcdefghi");
    free(combined_str);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_join_strings_three_empty_delimiter);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Reminder: Do not use typecasts anywhere in HW09.

Submit.

Add another test for `join_strings(…)` with multiple strings and a non-empty delimiter.

// …

int _test_join_strings_three_nonempty_delimiter() {
    mu_start();
    //────────────────────
    char const* const strings[] = { "abc", "def", "ghi" };
    char* combined_str = join_strings(strings, 3, "-");
    mu_check_strings_equal(combined_str, "abc-def-ghi");
    free(combined_str);
    //────────────────────
    mu_end();
}

int main(int argc, char* argv[]) {
    // …
    mu_run(_test_join_strings_three_nonempty_delimiter);
    return EXIT_SUCCESS;
}
// Okay to copy/adapt code from this example.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Submit.

Continue until finished

You are almost done. You still need to test join_strings(…) for an empty array (i.e., join_strings(NULL, 0, "-")). Also, make sure your tests are adequate.

Requirements

Your submission must contain each of the following files, as specified:

file	contents
join_strings.c	functions	`copy string(char const✶ string)` → return type: char✶ Make a copy of `string` on the heap. Caller is responsible for freeing the heap memory for the copy. Caller is responsible for freeing the heap memory buffer allocated by this function. ⚠ Do not call `free(…)` in this function.
		`wrap string(char const✶ string, char left delimiter, char right delimiter)` → return type: char✶ Return a newly allocated string on the heap containing `left_delimiter` followed by every character in `string` followed by `right_delimiter`. Examples: `wrap_string("flootix", '(', ')')` should create a string on the heap containing `(flootix)`. `wrap_string("moblish", '<', '>')` should create a string on the heap containing `<moblish>`. `wrap_string("moblish", '$', '$')` should create a string on the heap containing $moblish$ . `wrap_string("", '[', ']')` should create a string on the heap containing `[]`. Caller is responsible for freeing the heap memory buffer allocated by this function. ⚠ Do not call `free(…)` in this function.
		`join strings(char const✶ const✶ strings, size t num strings, char const✶ separator)` → return type: char✶ `strings` is an array of strings. `num_strings` is the number of strings in `strings`. `separator` is a string, a copy of which will be copied between each of the strings in `strings` in the output. If `num_strings == 1` (i.e., `strings` contains just one string), then `join_strings(…)` should simply return a copy of `strings[0]`. This is equivalent to `copy_string(strings[0])`. If `num_strings == 0`, then `strings` must be `NULL` and `join_strings(…)` should return a newly allocated empty string. This is equivalent to `copy_string("")`. Examples: This code… `char* strings[] = { "abc", "def"}; char* joined_string = join_strings(strings, 2, "-");` … should set `joined_string` to… `abc-def` Caller is responsible for freeing the heap memory buffer allocated by this function. ⚠ Do not call `free(…)` in this function.
test_join_strings.c	functions	`main(int argc, char✶ argv[])` → return type: int Test the code in your join_strings.c using your miniunit.h. 100% code coverage is required for join_strings.c.
		`test ▒▒▒()` → return type: int Use your `mu_check_strings_equal(…)` from HW07 to check that the code in your join_strings.c is working correctly.
		`test ▒▒▒()` → return type: int
		`test ▒▒▒()` → return type: int
miniunit.h	macros	Same as HW07. Okay to modify, if you wish.
log_macros.h	macros	Same as HW06. Okay to modify, if you wish.
Makefile	macros	Same as HW08. Okay to modify, if you wish.

Do not modify join_strings.h.
⚠ Do not use typecasts anywhere in HW09. The code quality standard says typecasts may not be used, except when they are truly necessary, safe, and you can articulate why. They are not necessary for anything in HW09.

Only the following external header files, functions, and symbols are allowed in join_strings.c.

header	functions/symbols	allowed in…
stdbool.h	`bool`, `true`, `false`	`*`
stdio.h	`*`	`test_join_strings.c`
assert.h	`assert`	`*`
string.h	`strlen`, `strcpy`, `memcpy`	`*`
stdlib.h	`free`, `EXIT_SUCCESS`	`test_join_strings.c`
stdlib.h	`malloc`	`join_strings.c`

All others are prohibited unless approved by the instructor. Feel free to ask if there is something you would like to use.

Submissions must meet the code quality standards and the policies on homework and academic integrity.

Submit

To submit HW09 from within your hw09 directory, type

make submit

That should result in the following command: 264submit HW09 join_strings.c test_join_strings.c miniunit.h log_macros.h Makefile

Pre-tester ●

The pre-tester for HW09 has been released and is ready to use.

Using the pretester

The pretester is a tool for checking your work after you believe you are done, and before we have scored it. It is not a substitute for your own checking, but it may help you avoid big surprises by letting you know if your checking was not adequate. To use the pre-tester, first submit your code. Then, type the following command. (Do this only after you have submitted, and only after you believe your submission is perfect.)

264test hw09

Do not ask TAs or instructors which tests you failed.

Keep in mind:

Pre-testing is intended only for those who believe they are done and believe their submission is perfect.
The pre-tester is not part of the requirements of this or any other assignment.
You are responsible for reading the assignment carefully, and ensuring that your code meets all requirements.
Feedback is limited, to ensure that everyone learns to test their own code.
If your code is failing some tests, review your tests and make sure they are comprehensive enough to catch any bugs (deviations from requirements). Follow the tips given by the pre-tester.
Code quality issues are not reported by the pre-tester; writing clean code is something you must learn to do from the start, not a clean-up step to do at the end.

Logistics:

If we discover that we have not checked some significant part of the assignment requirements, we may add additional tests at any time up to the point when scores are released.
The pre-tester will only be enabled after much of the class has submitted the assignment, and at least a few people have submitted perfect submissions. This is to allow us to test the pre-tester.
The pre-tester checks your most recent submission. You must submit first.
You may be limited to running the pre-tester ≤24 times in a 24-hour period. (This is not implemented yet but will be added.)

Updates

There are no updates to HW09 so far.

Advanced C Programming

Spring 2023 ECE 264 :: Purdue University

malloc(…): Join strings

Learning goals

Overview

Dynamic memory (aka “heap memory”)

Example: `"abc"`

How to allocate a buffer on the heap using `malloc(…)`

How to free buffers on the heap using `free(…)`

Memory leaks

Getting Started

Get the starter code

Copy your log_macros.h, miniunit.h, and Makefile from previous assignments.

Update your Makefile

Create a test file (test_join_strings.c) with one minimal test for `copy_string(…)`

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Add a second test for `copy_string(…)`.

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Add a minimal test for `wrap_string(…)`.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Add a more substantial test for `wrap_string(…)`.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Add a minimal test for `join_strings(…)`.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Add a more test for `join_strings(…)` with multiple strings but an empty delimiter.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Add another test for `join_strings(…)` with multiple strings and a non-empty delimiter.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Continue until finished

Requirements

Submit

Pre-tester ●

Updates

Advanced C Programming

Spring 2023 ECE 264 :: Purdue University

malloc(…): Join strings

Learning goals

Overview

Dynamic memory (aka “heap memory”)

Example: "abc"

How to allocate a buffer on the heap using malloc(…)

How to free buffers on the heap using free(…)

Memory leaks

Getting Started

Get the starter code

Copy your log_macros.h, miniunit.h, and Makefile from previous assignments.

Update your Makefile

Create a test file (test_join_strings.c) with one minimal test for copy_string(…)

Implement just enough of copy_string(…) in join_strings.c to pass that test.

Add a second test for copy_string(…).

Implement just enough of copy_string(…) in join_strings.c to pass that test.

Add a minimal test for wrap_string(…).

Implement just enough of wrap_string(…) in join_strings.c to pass that test.

Add a more substantial test for wrap_string(…).

Implement just enough of wrap_string(…) in join_strings.c to pass that test.

Add a minimal test for join_strings(…).

Implement just enough of join_strings(…) in join_strings.c to pass that test.

Add a more test for join_strings(…) with multiple strings but an empty delimiter.

Implement just enough of join_strings(…) in join_strings.c to pass that test.

Add another test for join_strings(…) with multiple strings and a non-empty delimiter.

Implement just enough of join_strings(…) in join_strings.c to pass that test.

Continue until finished

Requirements

Submit

Pre-tester ●

Updates

Example: `"abc"`

How to allocate a buffer on the heap using `malloc(…)`

How to free buffers on the heap using `free(…)`

Create a test file (test_join_strings.c) with one minimal test for `copy_string(…)`

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Add a second test for `copy_string(…)`.

Implement just enough of `copy_string(…)` in join_strings.c to pass that test.

Add a minimal test for `wrap_string(…)`.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Add a more substantial test for `wrap_string(…)`.

Implement just enough of `wrap_string(…)` in join_strings.c to pass that test.

Add a minimal test for `join_strings(…)`.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Add a more test for `join_strings(…)` with multiple strings but an empty delimiter.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.

Add another test for `join_strings(…)` with multiple strings and a non-empty delimiter.

Implement just enough of `join_strings(…)` in join_strings.c to pass that test.