dcc helps novice C programmers by catching common errors and providing easy-to-understand explnations.
For example:
dcc add extra runtime checking for errors and prints information likely to be helpful to novice programmers, including printing values of variables and expressions. Run-time checking includes array indices, for example:
$ gcc count_zero.c
$ ./a.out
9
$ dcc count_zero.c
$ ./a.out
count_zero.c.c:7:7: runtime error - index 10 out of bounds for type 'int [10]'
dcc explanation: You are using an illegal array index: 10
Valid indices for an array of size 10 are 0..9
Make sure the size of your array is correct.
Make sure your array indices are correct.
Execution stopped in main() in count_zero.c at line 7:
int main(void) {
int numbers[10] = {0};
int count = 0;
for (int i = 1; i <= 10; i++) {
--> if (numbers[i] > 0) {
count++;
}
}
Values when execution stopped:
count = 0
i = 10
numbers = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
numbers[i] = <uninitialized value>
Run-time checking also includes pointer dereferences, for example:
$ gcc linked_list.c
$ a.out
Segmentation fault (core dumped)
$ dcc linked_list.c
$ a.out
linked_list.c:12:15: runtime error - accessing a field via a NULL pointer
dcc explanation: You are using a pointer which is NULL
A common error is using p->field when p == NULL.
Execution stopped in main() in linked_list.c at line 12:
int main(void) {
struct list_node s = {0};
struct list_node *a = &s;
while (a != NULL) {
--> a->next->data = 42;
a = a->next;
}
}
Values when execution stopped:
s = {next = NULL, data = 0}
a->next = NULL
dcc also embeds code to detect use of uninitialized variables, for example:
$ gcc uninitialised_variable.c
$ a.out
0
$ dcc uninitialised_variable.c
$ a.out
Runtime error: uninitialized variable accessed.
Execution stopped in main() in uninitialised_variable.c at required:
int main(void) {
int numbers[10];
for (int i = 1; i < 10; i++) {
numbers[i] = i;
}
--> printf("%d\n", numbers[0]);
}
Values when execution stopped:
numbers = {<uninitialized value>,1,2,3,4,5,6,7,8,9}
numbers[0] = <uninitialized value>
dcc compiles C programs using clang and adds explanations suitable for novice programmers to compiler messages novice programmers are likely to encounter and not understand. For example:
$ dcc a.c
a.c:3:15: warning: address of stack memory associated with local variable 'counter' returned [-Wreturn-stack-address]
return &counter;
dcc explanation: you are trying to return a pointer to the local variable 'counter'.
You can not do this because counter will not exist after the function returns.
See more information here: https://comp1511unsw.github.io/dcc/stack_use_after_return.html
Uninitialized variables are detected by running valgrind simultaneously as a separate process.
The synchronisation of the 2 processes is only effective for the standard C library (signal.h and threads.h excepted). which should include almost all typical programs written by novice programmers. If synchronisation is lost the 2nd process should terminate silently.
If libraries other the standard C library are used, uninitialized variables does not occur.
dcc can also embed code to check for memory leaks:
$ dcc --leak-check leak.c
$ ./a.out
Error: free not called for memory allocated with malloc in function main in leak.c at line 3.
After reporting a runtime error an executable produced by dcc can optionally run an external program.
After reporting a runtime error a dcc executable checks if an executable
named dcc-runtime-helper exists in $PATH and if so runs it.
An alternate name for the executable file can be supplied in the environment variable DCC_RUNTIME_HELPER
The helper executable is run with a different working directory to the orignal executable. It is run in a temporary directory created by the dcc executable which contains the source to the original executable and dcc infrastructure files.
These environment variable are supplied to the helper script. They may be empty.
DCC_PWD- the original directory where the executable was runHELPER_FILENAME- source filename where error occurredHELPER_LINE_NUMBER- source line number where error occurredHELPER_COLUMN- source column where error occurredHELPER_SOURCE- source lines surrounding errorHELPER_CALL_STACK- function call stackHELPER_VARIABLES- current values of variables near the error locationHELPER_JSON- above variables encoded as JSON
After reporting a compiler message dcc can optionally run an external program.
After reporting a compiler message dcc checks if an executable
named dcc-compile-helper exists in $PATH and if so runs it.
An alternate name for the executable file can be supplied in the environment variable DCC_COMPILE_HELPER
These environment variable are supplied to the helper script. They may be empty.
LOGGER_ARGV- compiler command-line argumentsLOGGER_RETURNCODE- compiler exit statusLOGGER_JSON- above variables encoded as JSON
After completing a compilation message dcc can optionally log the details.
After reporting a compiler message dcc checks if an executable
named dcc-compile-logger exists in $PATH and if so runs it.
An alternate name for the executable file can be supplied in the environment variable DCC_COMPILE_LOGGER
These environment variable are supplied to the helper script. They may be empty.
HELPER_COMPILER_MESSAGE- compiler messageHELPER_MESSAGE_TYPE- message type (e.g warning)HELPER_FILENAME- source filename where error occurredHELPER_LINE_NUMBER- source line number where error occurredHELPER_COLUMN- source column where error occurredHELPER_EXPLANATION- dcc text explaining errorHELPER_JSON- above variables encoded as JSON
dcc can check a program's output is correct. If a program outputs an incorrect line, the program is stopped. A description of why the output is incorrect is printed. The current execution location is shown with the current values of variables & expressions.
The environment variable DCC_EXPECTED_STDOUT should be set to the expected output.
If DCC_IGNORE_CASE is true, case is ignored when checking expected output. Default false.
DCC_IGNORE_WHITE_SPACE is true, white space is ignored when checking expected output. Default false.
DCC_IGNORE_TRAILING_WHITE_SPACE is true, trailing white space is ignored when checking expected output. Default true.
DCC_IGNORE_EMPTY_LINES is true, empty lines are ignored when checking expected output. Default false.
DCC_COMPARE_ONLY_CHARACTERS is set to a non-empty string, the characters not in the string are ignored when checking expected output. New-lines can not be ignored.
DCC_IGNORE_CHARACTERS is set to a non-empty string, the characters in the string are ignored when checking expected output. New-lines can not be ignored.
DCC_IGNORE_CHARACTERS and DCC_IGNORE_WHITE_SPACE take precedence over DCC_COMPARE_ONLY_CHARACTERS
Environment variables are considered true if their value is a non-empty string starting with a character other than '0', 'f' or 'F'. They are considered false otherwise.
$ dcc --use-after-return bad_function.c
$ ./a.out
bad_function.c:22 runtime error - stack use after return
dcc explanation: You have used a pointer to a local variable that no longer exists.
When a function returns its local variables are destroyed.
For more information see: https://comp1511unsw.github.io/dcc//stack_use_after_return.html
Execution stopped here in main() in bad_function at line 22:
int *a = f(42);
--> printf("%d\n", a[0]);
}
valgrind also usually detect this type of error, e.g.:
$ dcc --use_after_return bad_function.c
$ ./a.out
Runtime error: access to function variables after function has returned
You have used a pointer to a local variable that no longer exists.
When a function returns its local variables are destroyed.
For more information see: https://comp1511unsw.github.io/dcc//stack_use_after_return.html'
Execution stopped here in main() in tests/run_time/bad_function.c at line 22:
int main(void) {
--> printf("%d\n", *f(50));
}
-
Deb-based Systems including Debian, Ubuntu, Mint and Windows Subsystem for Linux
curl -L https://github.com/COMP1511UNSW/dcc/releases/download/2.33/dcc_2.33_all.deb -o /tmp/dcc_2.33_all.deb sudo apt install /tmp/dcc_2.33_all.deb
or
sudo apt install clang gcc gdb valgrind python3 curl sudo curl -L https://github.com/COMP1511UNSW/dcc/releases/latest/download/dcc -o /usr/local/bin/dcc sudo chmod o+rx /usr/local/bin/dcc
# on Windows Subsystem for Linux (only) this might be necessary to run programs sudo bash -c "echo 0 > /proc/sys/kernel/yama/ptrace_scope;echo 1 >/proc/sys/vm/overcommit_memory"
The Ubuntu & Mint UndefinedSanitizer builds appear not to allow
__ubsan_on_reportto be intercepted which degrades some error reporting -
ARCH Linux
sudo pacman -S clang gcc gdb valgrind python3 curl sudo curl -L https://github.com/COMP1511UNSW/dcc/releases/latest/download/dcc -o /usr/local/bin/dcc sudo chmod o+rx /usr/local/bin/dcc
-
RPM-based Systems including CentOS, Fedora
sudo yum install clang gcc gdb valgrind python3 curl sudo curl -L https://github.com/COMP1511UNSW/dcc/releases/latest/download/dcc -o /usr/local/bin/dcc sudo chmod o+rx /usr/local/bin/dcc
On OpenSUSE:
sudo zypper install clang gcc gdb valgrind python3 curl sudo curl -L https://github.com/COMP1511UNSW/dcc/releases/latest/download/dcc -o /usr/local/bin/dcc sudo chmod o+rx /usr/local/bin/dcc
-
MacOS Install python3 - see https://docs.python-guide.org/starting/install3/osx/ Install gdb - see https://sourceware.org/gdb/wiki/PermissionsDarwin In your terminal, run:
bash <(curl -s https://raw.githubusercontent.com/COMP1511UNSW/dcc/master/install_scripts/macos_install.sh)Note: It is usually not a good idea to blindly run remote bash scripts in your terminal, you can inspect the file by opening the URL and reading to see what it does yourself.
sudo curl -L https://github.com/COMP1511UNSW/dcc/releases/latest/download/dcc -o /usr/local/bin/dcc sudo chmod o+rx /usr/local/bin/dcc
valgrind and MemorySanitizer are not currently supported on macOS which prevent checking for unitialized variables
There is experimental support for C++ programs if dcc is invoked as d++ or dcc++.
Install by creating a symbolic link, e.g.:
sudo ln -sf dcc /usr/local/bin/d++-
dcc by default enables clang's AddressSanitizer (
-fsanitize=address) and UndefinedBehaviorSanitizer (-fsanitize=undefined) extensions. -
dcc embeds in the binary produced a xz-compressed tar file (see [compile.py]) containing the C source files for the program and some Python code which is executed if a runtime error occurs.
-
Sanitizer errors are intercepted by a shim for the function
__asan_on_errorin [dcc_util.c]. -
A set of signals produced by runtime errors are trapped by
_signal_handlerin [dcc_util.c]. -
Both functions call
_explain_errorin [dcc_util.c] which creates a temporary directory, extracts into it the program source and Python from the embedded tar file, and executes the Python code, which:-
runs the Python ([start_gdb.py]) to print an error message that a novice programmer will understand, then
-
starts gdb, and uses it to print current values of variables used in source lines near where the error occurred.
-
Linux initializes stack pages to zero. As a consequence novice programmers writing small programs with few function calls are likely to find zero in uninitialized local variables. This often results in apparently correct behaviour from a invalid program with uninitialized local variables.
dcc embeds code in the binary which initializes the first few megabytes of the stack to 0xbe (see clear-stack in [dcc_util.c].
For valgrind dcc uses its malloc-fill and --free-fill options to achieve the same result see [dcc_util.c]. AddressSanitizer & MemorySanitizer use a malloc which does this by default.
When printing variable values, dcc prints ints, doubles & pointers consisting of 0xbe bytes as "".
Indirection using pointers consisting of 0xbe bytes will produced an unaligned access error from UndefinedBehaviourSanitizer, unless the pointer is to char. dcc intercepts these and explanations suitable for novice programmers (see explain_ubsan_error in [drive_gdb.py])
$ dcc dereference_uninitialized.c
$ ./a.out
tests/run_time/dereference_uninitialized_with_arrow.c:9:14: runtime error - accessing a field via an uninitialized pointer
dcc explanation: You are using a pointer which has not been initialized
A common error is using p->field without first assigning a value to p.
Execution stopped here in main() in dereference_uninitialized.c at line 9:
int main(void) {
struct list_node *a = malloc(sizeof *a);
--> a->next->data = 42;
}
Values when execution stopped:
a->next = <uninitialized value>
git clone https://github.com/COMP1511UNSW/dcc
cd dcc
make
cp -p ./dcc /usr/local/bin/dccflowchart
dcc["dcc program.c -o program<br> (python)"] --> user_code[program.c]
user_code --> gcc[gcc<br>for extra error-detection only]
gcc --> |compile-time error| dcc_explanation[dcc python]
dcc_explanation ---> |outputs| explanation[error message with explanation added<br> suitable for novice]
dcc --> wrapper_code[dcc wrapper C code]
dcc --> embedded_Python[embeded Python<br>for runtime error-handling]
user_code --> clang1["clang with options for valgrind<br>(no sanitizers)"]
wrapper_code --> clang1
clang1 --> |compile-time error| dcc_explanation
clang1 --> temporary_executable[temporary executable]
wrapper_code --> clang2[clang with options for AddressSanitizer]
user_code --> clang2
embedded_Python --> |tar file embedded by<br>encoding as array initializer| clang2
temporary_executable --> |binary embedded by<br>encoding as array initializer| clang2
clang2 --> program
Assumes the default option of AddressSanitizer + valgrind run in parallel.
flowchart
user1[user runs binary from dcc] --> main
main[execute dcc wrapper code in binary] --> |stack pages initialized to 0xbe| Sanitizer1["execute user's code in binary<br>(compiled with AddressSanitizer)"]
main --> |extract embedded binary<br>to temporary file & fork| Sanitizer2["valgrind executes user's code in temporary file<br>(not compiled with sanitizers)"]
main --> |fork| Watcher[valgrind watcher]
Sanitizer1 --> |runtime error| embedded_C[intercepted by dcc code in binary]
Sanitizer1 <--> | synchronize at system calls | Sanitizer2
embedded_C --> embedded_Python
Watcher --> |error details| embedded_Python
Sanitizer2 --> |runtime error| Watcher
gdb <--> |stack backtrace & variable values| embedded_Python[embedded Python]
embedded_Python --> |outputs| user2[novice friendly error message<br>location in source code<br>variable values<br>extra explanation]
Assumes the default option of AddressSanitizer + valgrind run in parallel.
If you have used DCC in your teaching or research, please cite the paper:
@inproceedings{10.1145/3545945.3569768,
author = {Taylor, Andrew and Renzella, Jake and Vassar, Alexandra},
title = {Foundations First: Improving C's Viability in Introductory Programming Courses with the Debugging C Compiler},
year = {2023},
isbn = {9781450394314},
publisher = {Association for Computing Machinery},
address = {New York, NY, USA},
url = {https://doi.org/10.1145/3545945.3569768},
doi = {10.1145/3545945.3569768},
pages = {346–352},
numpages = {7},
keywords = {educational compiler, c in cs1, cs1 programming languages},
location = {Toronto ON, Canada},
series = {SIGCSE 2023}
}
clang, python3, gdb, valgrind
Andrew Taylor (andrewt@unsw.edu.au)
Code for ANSI colors in colors.py is by Giorgos Verigakis
GPLv3