COMP26020p1 - Debugging with GDB

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### COMP26020 Programming Languages and Paradigms Part 1: C Programming
***
# Debugging with GDB
## (Please watch the [video](https://online.manchester.ac.uk/webapps/blackboard/execute/content/blankPage?cmd=view&content_id=_11916441_1&course_id=_62150_1), these slides are just a quick recap)

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# Our Program

.leftcol[
```c
#define ARRAY_SIZE  100

/* array[slot] = value */
int update_slot(int *array, int slot,
        int value) {
    array[slot] = value;
    printf("Updated index %d to %d\n",
        slot, value);
}

int process_array(int *array, int size) {
    int ii = 1000000;

for(int i=0; i<size; i++) {
        // If the value is even,
        // change it to 1000000
        if(!(array[i] % 2)) {
            update_slot(array, ii, i);
        }
    }
}
```
]

.rightcol[
```c
int fill_array(int *array, int size) {
    for(int i=0; i<size; i++)
        array[i] = rand()%10;
}

int main(int argc, char **argv) {
    int *array = malloc(ARRAY_SIZE *
        sizeof(int));

fill_array(array, ARRAY_SIZE);
    process_array(array, ARRAY_SIZE);

free(array);
    return 0;
}

//
```
.codelink[<a href="src/buggy-program.c" download>`19-debugging/buggy-program.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/0c2pyONpZgAlr" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]
]

---
# GDB Essentials

- Compile with `-g` flag to include debug symbols
- To launch GDB on a particular binary : `gdb program` in a shell
- `b test.c:12` places a breakpoint on the instruction at line 18 in `test.c`
- `b my_func` places a breakpoint on the first instruction of `my_func`
- `b test.c:12 if x == 42` is a conditional breakpoint, will pop only if x is equal to 42 when the corresponding statement is executed
- `run` starts the program
- In single-step mode:
   - `up` and `down` allows to navigate the function call stack
   - `next` advances the execution, jumping over function calls
   - `step` advances the execution, diving into function calls
   - `continue` exits single step mode

---
# Further Resources

- Official documentation: https://sourceware.org/gdb/current/onlinedocs/gdb/index.html
- GDB reference card: https://users.ece.utexas.edu/~adnan/gdb-refcard.pdf
- GDB's interface can be heavily customised, for example:
  https://github.com/cyrus-and/gdb-dashboard

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