---
tags: jserv-Linux
---
# 2021q1 Homework1 (lab0)
contributed by < `asef18766` >

## env
* ubuntu 20.04 64bit on Vmware

$ uname -a
Linux ubuntu 5.8.0-43-generic #49~20.04.1-Ubuntu SMP Fri Feb 5 09:57:56 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

## basic queue features
- [x] new
- [x] free
- [x] insert_head
- [x] insert_tail
- [x] remove_head
- [x] size
- [x] reverse
- [x] sort
### structure

typedef struct {
    list_ele_t *head; /* Linked list of elements */
    list_ele_t *tail; /* Last element of linked list */
    u_int32_t count;  /* current element count of queue */
} queue_t;

### new/free
* new: nothing special
* free: need to add a check for null pointer input. otherwise, iterate through all the elements in the list and free them
### insert_head / insert_tail
1. check if queue pointer is NULL
2. `malloc` a new element and check if it's NULL
3. copy string and link elements
### remove_head
1. check if queue is not null and with first element existed
2. if `sp` is not null, copy the value of removed element into it
3. remove element and relink
### size
1. check if queue pointer is null
2. return `count`
### reverse
1. check if queue pointer is null
2. make each element points to previous element
3. swap head and tail pointer
4. make tail object points to NULL
### sort
* just a merge sort

## ASan debugging
1. enable ASan by ```make SANITIZER=1```
2. use ```make test``` to run through all the testcases
3. from log we can know that there is a global BOF happend

    /* Add a new parameter */
    void add_param(char *name,
                   int *valp,
                   char *documentation,
                   setter_function setter)
    {
        param_ptr next_param = param_list;
        param_ptr *last_loc = &param_list;
        while (next_param && strcmp(name, next_param->name) > 0) {
            last_loc = &next_param->next;
            next_param = next_param->next;
        }

        param_ptr ele = (param_ptr) malloc_or_fail(sizeof(param_ele), "add_param");
        ele->name = name;
        ele->valp = valp; /* error here */
        ele->documentation = documentation;
        ele->setter = setter;
        ele->next = next_param;
        *last_loc = ele;
    }

4. according to the log, the BOF happend with variable ```param_list```. Use GDB to set a hardware watchpoint by ```watch param_list```

    /* Add a new parameter */
    void add_param(char *name,
                   int *valp,
                   char *documentation,
                   setter_function setter)
    {
        param_ptr next_param = param_list;
        param_ptr *last_loc = &param_list;
        while (next_param && strcmp(name, next_param->name) > 0) {
            last_loc = &next_param->next;
            next_param = next_param->next;
        }

        param_ptr ele = (param_ptr) malloc_or_fail(sizeof(param_ele), "add_param");
        ele->name = name;
        ele->valp = valp;
        ele->documentation = documentation;
        ele->setter = setter;
        ele->next = next_param;
        *last_loc = ele; // write happend here
    }

5. after a few step, we can observe that there are some vulnerable force casts

    add_param("simulation", (int*)&simulation, "Start/Stop simulation mode", NULL);
    add_param("verbose", &verblevel, "Verbosity level", NULL);
    add_param("error", &err_limit, "Number of errors until exit", NULL);
    add_param("echo", (int*)&echo, "Do/don't echo commands", NULL);

6. patch them to the correct type

int simulation = false;
...
static int echo = 0;

## valgrind debugging
* in ```linenoise.c```, the global objects ```history``` was not remove until it triggers ```atexit```
* however, the code wasn't register it
* manual free whenever the program finished

    ok = ok && run_console(infile_name);
    ok = ok && finish_cmd();
    linenoiseAtExit();

## dudect
### Student’s t-distribution
* usage: to check whether samples is close to population
* input: samples, population mean,population varience
* output: confidence interval (is the data close to population)
### improvement
* [cpucycle](https://www.itread01.com/p/149683.html)

#include <stdint.h>
#include <time.h>

static inline int64_t cpucycles(void)
{
    /*
     * use this method to avoid time sampling error
    */
    struct timespec t;
    clock_gettime(CLOCK_MONOTONIC, &t);
    return t.tv_nsec;
}