Lab 3: Pointers, Arrays and the Heap

Lab sessions Tue Apr 23 to Thu Apr 25

Lab written by Julie Zelenski, with modifications by Nick Troccoli

Learning Goals

During this lab, you will:

  1. investigate how arrays and pointers work in C
  2. get further practice with gdb and valgrind
  3. experiment with code that dynamically allocates memory on the heap

Find an open computer to share with a partner with whom you haven't worked before. Introduce yourself and tell them your thoughts on the best places on campus to get work done.

Get Started

Clone the lab starter code by using the command below. This command creates a lab3 directory containing the project files.

git clone /afs/ir/class/cs107/repos/lab3/shared lab3

Next, pull up the online lab checkoff and have it open in a browser so you can jot things down as you go. Only one checkoff needs to submitted for both you and your partner.


Tip: the GDB and Valgrind guides on the Resources are great references for the various commands and strategies you can use to debug. In particular, the GDB guide has a section on good debugging strategies, including ways to use GDB to fix any issues you encounter. We'd highly recommend reviewing them if you haven't already!


Exercises

1) GDB: Pointers and Arrays (20 minutes)

First, we'll play around with pointer and array syntax, and how the two are similar and different. To do this, we'll use the provided code.c program, which is a not-terribly-useful program that exhibits various behaviors of arrays and pointers.

  • Build the program, start it under gdb, and set a breakpoint at main. When you hit the breakpoint, use info locals to see the state of the uninitialized stack variables. This command lists the values of all local variables in the current function. Step through the initialization statements and use info locals again. Remember that when gdb reports that execution is at line N, this is before line N has executed.
  • The expressions below all refer to the local variable arr. For each expression, first try to figure out what the result of the expression should be, and then evaluate the expression in gdb to confirm that your understanding is correct.

    (gdb) p *arr
    (gdb) p arr[1]
    (gdb) p &arr[1]
    (gdb) p *(arr + 2)
    (gdb) p &arr[3] - &arr[1]
    
    (gdb) p sizeof(arr)
    (gdb) p arr = arr + 1
    
  • The main function initializes ptr to arr. The name of a stack array and a pointer to that array are almost interchangeable, but not entirely. Try re-evaluating the above expressions with ptr substituted for arr. The first five evaluate identically, but the last two produce different results for ptr than arr. What is reported for the size of an array? the size of a pointer? The last one is the trickiest to understand. Why is it allowable to assign to ptr but not arr?

  • execute p ptr = ptr - 1 to reset ptr to its original value.
  • Use the gdb step command to advance into the call binky(arr, ptr). step is like next, but instead of executing the entire line and moving to the next line, it steps into the execution of the line. Once inside binky, use info args to see the values of the two parameters. Print any expression you can think of on a and b and they will evaluate to the same result. This includes the last two expressions from above: sizeof reports the same size for a and b and assignment is permissible for either. What happens in parameter passing to make this so? Try drawing a picture of the state of memory to shed light on the matter.
  • Set a breakpoint on change_char and continue until this breakpoint is hit by executing the GDB c command (for "continue"). When stopped there in gdb, use info args. The arguments shown are from the function call ("frame") for change_char. The default frame of reference is the currently executing function.
  • Use backtrace to show the sequence of function calls that led to where the code is currently executing. You can select a different frame of reference with the gdb frame command. Frames are numbered starting from 0 for the innermost frame, and the numbers are displayed in the output of the backtrace command. Try the command frame 1 to select the frame outside change_char and and then use info locals to see the state from winky. The gdb command up is shorthand for selecting the frame that is one higher than the current one, and down is shorthand for selecting the frame that is one lower than the current one. Type frame 0 or down to return to the line where the debugger is paused.
  • Step through change_char and examine the state before and after each line. Use info args to show the inner frame and up and info locals to show what's happening in the outer frame. Carefully observe the effect of each assignment statement.
  • Step through the call to change_ptr and make the same observations. Which of the assignment statements had a persistent effect in winky, and which did not? Can you explain why?

If you don't understand or can't explain the results you observe, stop here and discuss them with your labmates and lab leader. Having a solid model of what is going on under the hood is an important step toward understanding the commonalities and subtle differences between arrays and pointers.

2) Code Study: Heap Use (30 minutes)

Next, we'll take a look at code that dynamically allocates memory on the heap using malloc, strdup and realloc. Specifically, we'll look at heap.c, which you can run or trace in gdb, containing two sample functions written for this lab called remove_first_occurrence and join.

The function remove_first_occurrence(text, pattern) returns a string which is a copy of text with the first occurrence of pattern removed. The returned string is heap-allocated and becomes the client's responsibility to free when done.

1   char *remove_first_occurrence(const char *text, 
2       const char *pattern) {
3
4       char *found = strstr(text, pattern);
5
6       if (!found) {
7           return strdup(text);
8       }
9
10      int nbefore = found - text;
11      int nremoved = strlen(pattern);
12      char *result = malloc(strlen(text) + 1 - nremoved);
13      assert(result != NULL);
14
15      strncpy(result, text, nbefore);
16      strcpy(result + nbefore, found + nremoved);
17      return result;
18  }

Talk through the following questions with your partner to understand this code. Ask questions about anything confusing!

  • What is the purpose of the library function strstr? What does it return?
  • Lines 6-8 handle the case where there is no occurrence of pattern in text, i.e. nothing to remove. Why does the function return strdup(text) rather than just returning text?
  • Line 10 subtracts two pointers. What is the result of subtracting two pointers? Here's a hint: if q = p + 3, then q - p => 3. How does the sizeof(pointee) affect the result of a subtraction?
  • assert terminates the program if the passed-in expression is false. What is the check on line 13?
  • After line 15 executes, is the result string null-terminated? Why or why not?
  • Your colleague proposes changing line 16 to instead: strcat(result, found + nremoved) claiming it will operate equivalently. You try it and it even seems to work. Is it equivalent? No, not quite... The code is just "getting lucky". In what way does it (wrongly) depend on the initial contents of newly allocated heap memory? How will the function break when that wrong assumption is not true?
  • Try running the code; if you run ./heap with no arguments, it will run a short test of the function, removing "time" from "centimeter".

The function join(strings, strings_count) returns a string which is the concatenation of all strings_count strings in strings. The returned string is heap-allocated and becomes the client's responsibility to free when done. A core function that makes this implementation possible is the realloc function.

1   char *join(char *strings[], int strings_count) {
2       char *result = strdup(strings[0]);
3
4       for (int i = 1; i < strings_count; i++) {
5           result = realloc(result, strlen(result) + strlen(strings[i]) + 1);
6           assert(result != NULL);
7           strcat(result, strings[i]);
8      }
9      return result;
10  }
  • At the outset of the loop, the total size needed is not known, so it cannot do the allocation upfront. Instead it makes an initial allocation and enlarges it for each additional string. This resize-as-you-go approach is an idiomatic use case for realloc.
  • Is the size argument to realloc the number of additional bytes to add to the memory block, or the total number of bytes including any previously allocated part?
  • The variable result is initialized to strdup(arr[0]). Why does it make a copy instead of just assigning result = arr[0]?
  • The realloc manual page says the return value from realloc(ptr, newsize) may be the same as ptr, but also may be different. In the case where it is different, realloc has copied the contents from the old location to the new. The old location is also freed. Change the code to call realloc without using its return value (i.e. don't re-assign ptr, just drop the result). Re-compile and you should get a warning from the compiler. What error is that warning trying to caution you against? A realloc call that doesn't catch its return value is never the right call.
  • Try running the code; if you run ./heap with one or more arguments, it will join all of the arguments together using the join function.

3) Code study: calloc (20 minutes)

Finally, we'll study the implementation of calloc, an alternative to the malloc memory-allocating function, to better understand how heap memory allocation works. First, read the manual page for the function (man calloc) and then review the implementation for musl_calloc shown below (taken from musl):

1   void *musl_calloc(size_t nelems, size_t elemsz) {
2       if (nelems && elemsz > SIZE_MAX/nelems) {
3           return NULL;
4       }
5       size_t total = nelems * elemsz;
6       void *p = malloc(total);
7       if (p != NULL) {
8           long *wp;
9           size_t nw = (total + sizeof(*wp) - 1)/sizeof(*wp);
10          for (wp = p; nw != 0; nw--,wp++) 
11              *wp = 0;
12      }
13      return p;
14  }

The calloc interface is presented as allocating an array of nelems elements each of size elemsz bytes. It initializes the memory contents to zero, which is the feature that distinguishes calloc from ordinary malloc.

Here are some issues to go through with your partner to understand the operation of this function.

  • The calloc interface as described in terms of arrays is somewhat misleading because there is nothing array-specific to its operation. It simply allocates a region of total size nelems * elemsz bytes. The call calloc(10, 2) has exactly the same effect as calloc(4, 5) or calloc(1, 20) and memory allocated by any of those calls could be used to store a length-20 string or an array of 5 integers, for example.
  • What are lines 2-4 of the function doing? What would be the effect on the function's operation if these were removed?
  • A colleague objects to the use of division on line 2 because division is more expensive than multiplication and it requires an extra check for a zero divisor. They propose changing it to if (nelems * elemsz > SIZE_MAX), claiming it will operate equivalently and more efficiently. Explain to them why this plan won't work.
  • Line 7 declares p to be of type void*, which matches the return type of malloc. A void* pointer is a generic pointer. It stores an address, but makes no claim about the type of the pointee. It is illegal to dereference or do pointer arithmetic on a void*, since both of these operations depend on the type of the pointer, which is not known in this case. However, void * can be assigned to/from without an explicit cast.
  • What is the purpose of line 7? What would be the effect on the function's operation if that test were removed and it always proceeded into the loop?
  • Line 9 contains the expression sizeof(*wp). Given that wp was declared and not assigned, this looks to dereference an uninitialized pointer! Not to fear, it turns out that sizeof is evaluated at compile-time by working out the size for an expression from its declared type. Since wp was declared as a long *, the expression *wp is of type long which on Myth systems has size 8 bytes. The compiler will have transformed it into (total + 8 - 1)/8. Phew! There will be no nasty runtime surprises.
  • Let's further dig into line 9. It should remind you of code we've seen previously (roundup in lab1). What is the value of nw for the call calloc(20, 4), for calloc(2, 7), or for calloc(3, 1)? Aha, this is a rounding operation, as suspected. This calculation is making an assumption about how many bytes malloc will actually reserve for a given requested size. What is that assumption? (This behavior is not a documented public feature and no client that was a true outsider should assume this, but calloc and malloc were authored by the same programmer, and thus calloc is being written by someone with firsthand knowledge of the malloc internals and acting as a privileged insider.)
  • The initialization of the for loop on line 10 assigns wp = p, yet wp and p are not of the same declared type. Why is no typecast required here? Would the function behave any differently if you added the typecast wp = (long *)p?
  • The increment of the for loop on line 10 offers a rare glimpse of C's comma operator. You can read more about the comma operator on Wikipedia; the only use you are likely to see in practice is to pack multiple expressions into a loop init or increment. In this case, the increment will both advance the pointer and decrement the count of iterations remaining.
  • Imagine if line 8 instead declared char *wp with no other changes to the code. How would this change the function's behavior? What if the declaration was int *wp? What if the declaration were void *wp? For each, consider how the pointee type affects the operations wp++ and *wp = 0. What do you think is the motivation for the author's choice of long *?

4) Valgrind: Heap Errors (15 minutes)

Last week's lab introduced you to Valgrind. This tool will be increasingly essential as we advance to writing C code with heavy use of pointers and dynamic memory. In particular, Valgrind is great at detecting memory issues relating to dynamic memory, such as writing past what you've allocated, accessing freed memory, freeing memory twice, etc. The buggy.c program has some planted errors that misuse heap memory to let you see how Valgrind handles and reports these errors.

  • Review the program in buggy.c and consider error #1. When the program is invoked as ./buggy 1 argument, it will copy argument into a heap-allocated space of 8 characters. If the argument is too long, this code will write past the end of the allocated space - a "buffer overflow" (because the write overflows past the end of a buffer). This is similar to errors we've seen in the past about writing past stack memory used to store a string, but now the overflow is happening on the heap. What are the consequences of this error? Let's observe.
  • Run ./buggy 1 leland to see that the program runs correctly when the name fits. Now try longer names ./buggy 1 stanford and ./buggy 1 lelandstanford. Surprisingly, these also seem to "work", apparently getting lucky when the overrun is still relatively small. Pushing further to ./buggy 1 lelandstanfordjunioruniversity, you'll eventually get the crash you expect.
  • Many crashes have nothing more to say that just "segmentation fault", but this particular crash gives a dump of the program state. This error-reporting is coming from inside the free function of the standard library. The dump is not presented in a particularly user-friendly form, and rather than trying to decipher it, just take it as an indication of a memory problem, for which we will turn to Valgrind for further help.
  • Try valgrind ./buggy 1 stanford and review Valgrind's report to see what help it can offer. Whether the write goes too far by 1 byte or 1000 bytes, Valgrind will helpfully report the error at the moment of transgression. Hooray!
  • Errors 2, 3, and 4 explore the consequences of other misuses of heap memory. Consider these cases one at a time. Review the code to see what the error is, and then run it (e.g. ./buggy 2). Is there an observable result of the error? Now run it under Valgrind. Take note of the terminology that the Valgrind report uses and how it relates back to the root cause.

Be forewarned that memory errors can be very elusive. A program might crash immediately when the error is made, but the more insidious errors silently destroy something that only shows up much later, making it hard to connect the observed problem with the original cause. The most frustrating errors are those that "get lucky" and cause no observable trouble at all, but lie in wait to surprise you at the most inopportune time. Make a habit of using Valgrind early and often in your development to detect and eradicate memory errors!

5) Valgrind: Memory Leaks (20 minutes)

While not memory errors, memory leaks are another type of issue that Valgrind can help detect. A memory leak is when you allocate memory on the heap, but do not free it. These rarely (if ever) cause crashes, and for this reason we recommend that you do not worry about freeing memory until your program is completely written. Then, you can go back and deallocate your memory as appropriate, ensuring correctness at each step. Memory leaks are an issue, however, because your program should be responsible for cleaning up any memory it allocates but no longer needs. In particular, for larger programs, if you never free any memory and allocate an extremely large amount, you may run out of memory in the heap!

The leaks.c program has some planted memory leaks to let you see how Valgrind handles and reports these leaks.

  • Review the program in leaks.c and consider leak #1. When the program is invoked as ./leaks 1, it will allocate 8 bytes on the heap, but then immediately return, causing the program to lose the address of this heap memory. A memory leak! The program terminates fine, however. Let's see how Valgrind can help us detect it.
  • Try valgrind ./leaks 1 and review Valgrind's report to see what help it can offer. You'll notice that Valgrind reports a "LEAK SUMMARY" at the bottom, as well as a total heap usage summary, which shows what memory was still in use at exit (that should have been freed). You can also see the number of reported allocations and frees to not line up, indicating a leak. Helpful!
  • For even more memory leak information, make sure to use the --leak-check=full and --show-leak-kinds=all flags when you run valgrind. Run it again like this: valgrind --leak-check=full --show-leak-kinds=all ./leaks 1. This shows even more information, including where the leaked memory was originally allocated! We recommend always running with these flags to get the most information. Make sure to put these flags before the command to run your actual program. Specifically, the following is not equivalent: valgrind ./leaks 1 --leak-check=full --show-leak-kinds=all, as this passes these flags to your program, instead of valgrind!
  • What is the difference between "definitely lost", "indirectly lost", etc.? Check out our Valgrind guide for an overview.
  • Leaks 2 and 3 explore other possible manifestations of memory leaks. Consider these cases one at a time. Review the code to see what the error is, and then run it (e.g. ./leaks 2). Notice how the program always runs fine. Now run it under Valgrind. Take note of the terminology that the Valgrind report uses and how it relates back to the root cause. Note that leaks do not only have to come from memory you allocate using malloc. For example, functions like strdup allocate memory that it is the caller's responsibility to free!
  • the total heap usage includes more than just your explicit allocations. For example, other functions, like strdup, may allocate/free memory internally as well!

6) Debugging (15 minutes)

It's vital to use good debugging strategies when working on your programs. Developing good debugging strategies consists of both understanding the mechanics of how to use debugging tools like GDB and Valgrind, as well as how to apply them effectively as part of the larger process of finding, narrowing in on, and fixing bugs. Now is the time to invest in mastering the powerful tools provided by a debugger like gdb, and using a careful and systematic debugging approach! Here is a core checklist you should use when approaching debugging (taken from our Guide to GDB and debugging). Take a minute to read through the steps:

  1. Observe the bug. If you never see the bug, you'll likely never fix it. Another reason you want comprehensive testing!
  2. Create a reproducible input. Creating a trivial input that reliably induces the failure is a huge help.
  3. Narrow the search space. Studying the entire program or tracing the execution line-by-line is generally not feasible. Some suggestions for how to narrow down your focus :

    • Start where your intuition believes is the likely culprit, such as a function that recently changed or one you find suspicious.
    • Use binary search to dissect. Set a breakpoint at the midpoint and poke around to determine whether the program state is already corrupt (which indicates the problem is in the front half) or looks good (so you need to focus your attention on the back half). Repeat to further narrow down.
    • Run under Valgrind to identify the root cause of any lurking memory errors.
    • Run under GDB to identify the root cause of any crashes
  4. Analyze. With only a small amount of code under scrutiny, execution tracing becomes feasible. Use gdb to see what the facts (values of variables and flow of control) are telling you. Drawing pictures may help.

  5. Devise and run experiments. Make inferences about the root cause and run experiments to validate your hypothesis. Iterate until you identify the root cause.
  6. Modify code to squash the bug. The fix should be validated by your experiments and passing the original failed test case. You should be able explain the series of facts, tests, and deductions which match the observed symptom to the root cause and the corrected code.

A key tenet of this approach is to not change your code haphazardly. This is like a scientist who changes more than one variable at a time. It makes the observed behavior much more difficult to interpret, and tends to introduce new bugs. That said, if you find buggy code, even if it is not obviously related to the bug you are tracking, you still might want to make a detour to fix it, using a reproducible input to trigger that bug and validate its fix. That bug might be related to or obscuring the original bug and it's good to remove any source of potential interference.

With this debugging approach, you will be extremely effective at finding, understanding and fixing any bugs that occur when working on programs.

To provide a bit of practice with this debugging checklist, we have provided a buggy program, pig_latin.c, with a lurking issue. It is based on the code shown in the previous lecture for translating words and phrases to pig latin. However, it seems to crash on certain inputs! In particular, the most recent change made to the code was to add functionality to return NULL in the pig_latin function if the word cannot be translated to Pig Latin (if it starts with a character that is not a lowercase letter). But when running some custom tests, it seems to fail. Let's follow the debugging processa above to fix it!

  1. Observe the bug. This part is already done for you - there is a custom test case in custom_tests that exhibits the crash. Try running it using sanity check to see!
  2. Create a reproducible input. There is already a provided test case that causes a crash, but it's a complex input that may be unwieldy to use for debugging. It's important to try and reproduce a bug with as small an input as possible to better understand it and make tracing easier later. Try creating such an input in custom tests using the information provided so far. Try doing this without looking at the source code! This approach can sometimes be helpful to not get sidetracked by what you think your code does vs. what it actually does.
  3. Narrow the search space. Now our task is to narrow in on what portion of code we think is causing the issue. This is important to help avoid time-consuming processes like stepping through the entire program execution if the bug is lurking in just one place. Luckily, for a crash, GDB can do part of this for us! If you run a crashing program in GDB, it will pause at the source of the crash, and let you poke around the program state. Try doing this now:
    • run the program in GDB using the test case you found earlier
    • when it crashes, you should see GDB alert you of a segfault (a crash), as well as what line caused it - helpful!
    • try using the backtrace command to see the call stack for the crash - this shows exactly where in the execution the crash happened. You can also print out variable values and use other GDB commands to inspect the state of the program at the crash.
    • Great! Now we've narrowed in on the line and function causing the issue.
  4. Analyze. Now our job is to use GDB to poke around in this state to get more information about why the program crashed. Try printing out values to discover why the crash occurred on that line. Then try using GDB to run the program again, but try breakpointing at the start of the function containing the crash and step through it to get a better understanding of what's going on.
  5. Devise and run experiments. Once you have a hypothesis of what is causing the issue, try coming up with another test input to validate your hypothesis, and use GDB to confirm your suspicion.
  6. Modify code to squash the bug. Now brainstorm and implement a fix for the issue. Make sure you understand exactly what changes you are making and why you are making them. Confirm the changes fix the issue by running your tests from earlier. Woohoo! We fixed the bug - nice work!

Hopefully this process gave you a better idea of how to follow an effective debugging strategy to make every minute of debugging count. Here are a few other key takeaways from this process:

  • Good testing helps find bugs. As step 1 indicates, you won't be able to apply your honed debugging skills if you don't unearth potential bugs in the first place! Make sure to test thoroughly to uncover any lurking issues.
  • Make less work for yourself. the small reproducible input is key to making debugging and tracing easier. If you find the bug with a very large input, it can become unwieldy to trace through it! Similarly, narrowing down what code is causing the issue can let you focus on just that code block rather than the entire program.
  • Good style leads to easier debugging. This program was likely much easier to debug because of the decomposition, commenting, and overall organization. Imagine if the code was poorly commented, or all contained within one function. It would be much more time-consuming!
  • A good debugger is systematic. Every computer scientist at every level spends time debugging. The key to advancing your skills is to know how to spend your time as effectively as possible to narrow in and squash those bugs!

[Optional] Challenge Problem

Finished with lab and itching to further exercise your memory skills? Check out our challenge problem!

Check off with TA

At the end of the lab period, submit the checkoff form and ask your lab TA to approve your submission so you are properly credited for your work. It's okay if you don't completely finish all of the exercises during lab; your sincere participation for the full lab period is sufficient for credit. However, we highly encourage you to finish on your own whatever is need to solidify your knowledge. Also take a chance to reflect on what you got what from this lab and whether you feel ready for what comes next! The takeaways from lab3 should be continued development of your gdb and Valgrind skills, as well as your skills reading and writing code with heavy use of pointers. Arrays and pointers are ubiquitous in C and a good understanding of them is essential. Here are some questions to verify your understanding and get you thinking further about these concepts:

  • If ptr is declared as an char *, what is the effect of ptr++? What if ptr is declared as a int *?
  • Although & applied to the name of a stack-allocated array (e.g. &buffer) is a legal expression and has a defined meaning, it isn't really sensible. Explain why such use may indicate an error/misunderstanding on the part of the programmer.
  • The argument to malloc is size_t (unsigned). Consider the erroneous call malloc(-1), which seems to make no sense at all. The call compiles with nary a complaint -- why is it "ok"? What happens when it executes?
  • What is the purpose of the realloc function? What happens if you attempt to realloc a non-malloc-ed pointer, such as a string constant?
  • What is the difference between a memory error and a memory leak?

xkcd pointers comic