HalCanary / debug

Using GDB (or LLDB)

https://sourceware.org/gdb/current/onlinedocs/gdb/

c++ -g -o program program.cpp    

gdb -q program

gdb -q --args program foo bar

Don't forget to compile with -g.

Important Commands:

• quit (q)
• help (h)
• list (l) — list source code.
• print (p) — print variable values
• print /x (p/x) — print in hexadecimal
• run (r) — start program from begining. Try run args < infile > outfile.
• start — [gdb only] equivilent to break main followed by run.
• break (b) — set breakpoint. Try break srcfile:line or break functionname.
• step (s) — continue program until control reaches a different source line.
• next (n) — continue to the next source line in the innermost stack frame.
• continue (c) — resume program execution.
• finish — finish running curent function.
• watch — set watchpoint. Break when the variable's value changes.
• backtrace (bt) — print a stacktrace.
• frame (f) — change your current frame.
• up, down — move to next frame up or down.
• [blank line] — repeat previous command.

example 1

void set_to_zero(int* ptr) {
    *ptr = 0;
}
int main() {
    int* ptr = 0;
    set_to_zero(ptr);
}

example 2

#include <thread>
#include <cstdio>
void sleep(double seconds) {
    int64_t mics = 1000000 * seconds;
    auto dur = std::chrono::microseconds(mics);
    std::this_thread::sleep_for(dur);
}
int main() {
    int i = 0;
    while (1) {
        sleep(1);
        printf("%d\n", i++);
        fflush(stdout);
    }
}

example 3

#include <cassert>
#include <cmath>
#include <cstdio>
void print_sqrt(int x) {
    assert(x >= 0);
    printf("%f\n", sqrt(x));
}
int main() {
    int i = -5;
    print_sqrt(i);
}

example 4

#include <cmath>
#include <cstdio>

static void error_msg(const char* file, int line, const char* msg) {
    fprintf(stderr, "%s:%d: %s\n", file, line, msg);
}

#define ABORT(msg) \
    do { error_msg(__FILE__, __LINE__, msg); *(int*)0xDEADBEEF = 1234; } while (0)
#define ASSERT(cond) \
    do { if (!(cond)) { ABORT("assert failed: '" #cond "'."); } } while (0)

void print_sqrt(int x) {
    ASSERT(x >= 0);
    printf("%f\n", sqrt(x));
}
int main() {
    int i = -5;
    print_sqrt(i);
    ABORT("abort");
}