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# 資安報告!
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---
## Who are you?
nobody
---
## concurrency rule(並行性規則)
[CON32-C](https://wiki.sei.cmu.edu/confluence/display/c/CON32-C.+Prevent+data+races+when+accessing+bit-fields+from+multiple+threads)
[CON33-C](https://wiki.sei.cmu.edu/confluence/display/c/CON33-C.+Avoid+race+conditions+when+using+library+functions)
---
## CON32-C
Prevent data races when accessing bit-fields from multiple threads.
---
### race conditions
```c
#include <stdio.h>
#include <pthread.h>
#define NUM_THREADS 8
int counter=0;
void *threadFunction(void *threadId) {
long tid = (long)threadId;
for (int i = 0; i < 10000; i++) {
counter++;
}
printf("Thread %ld: Counter value: %lu\n", tid, counter);
pthread_exit(NULL);
}
int main() {
pthread_t threads[NUM_THREADS];
int i;
for (i = 0; i < NUM_THREADS; i++) {
if (0 != pthread_create(&threads[i], NULL, threadFunction, (void *)(long)i)) {
printf("Error creating thread %d\n", i);
return 1;
}
}
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf("Final counter value: %ld\n", counter);
return 0;
}
```
---
### 輸出
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<img class ="right_img" src="https://blogger.googleusercontent.com/img/a/AVvXsEgyjOAxskQj7NJoKyl07SJnfTu6NuuVVWZdMV9TZVb0kigS0Hbi9KNEJHambOOeKMq-qavL2vX0nbo5pNUB7YPK6oGQuNGSoHMWkLtWkRqvEFmk8WEsWYSYU6-SA2UDAQmt6tbUG8225MqQjk75jdpwtfmFZX9nfkgdh-8ESaqAX1x5Bs1EbGoi4iMoVw=w640-h472">
```
Thread 0: Counter value: 10000
Thread 4: Counter value: 20000
Thread 1: Counter value: 30000
Thread 2: Counter value: 40165
Thread 3: Counter value: 48295
Thread 5: Counter value: 58295
Thread 7: Counter value: 68295
Thread 6: Counter value: 75784
Final counter value: 75784
```
---
### bit-fields
```c
struct foo {
int a : 1;
int b : 2;
unsigned int c : 3;
unsigned int d : 4;
};
int main() {
struct foo f;
f.a=1;
f.b=1;
f.c=3;
f.d=3;
printf("a=%d\nb=%d\nc=%d\nd=%d\n", f.a, f.b, f.c, f.d);
// a=-1
// b=1
// c=3
// d=3
return 0;
}
```
---
### zero-width bit field
```c
struct foo {
unsigned int a : 3;
unsigned int b : 2;
unsigned int : 0; /* Force alignment to next boundary */
unsigned int c : 4;
unsigned int d : 3;
};
int main() {
int i = 0xFFFF;
struct foo *f = (struct foo *) &i;
printf("a=%d\nb=%d\nc=%d\nd=%d\n", f->a, f->b, f->c, f->d);
//a=7 b=3 c=12 d=2
return 0;
}
```
```
0000 0000 0000 0000 1111 1111 1111 1111
b baaa dddcccc
|← int 32 bits →|← int 32 bits →|
```
---
### mutex
```c
#include <stdio.h>
#include <pthread.h>
#define NUM_THREADS 8
int counter = 0;
pthread_mutex_t mutex;
void *threadFunction(void *threadId) {
long tid = (long)threadId;
for (int i = 0; i < 100000; i++) {
pthread_mutex_lock(&mutex);
counter++;
pthread_mutex_unlock(&mutex);
}
printf("Thread %ld: Counter value: %d\n", tid, counter);
pthread_exit(NULL);
}
int main() {
pthread_t threads[NUM_THREADS];
int i;
pthread_mutex_init(&mutex, NULL);
for (i = 0; i < NUM_THREADS; i++) {
int result = pthread_create(&threads[i], NULL, threadFunction, (void *)(long)i);
if (result != 0) {
printf("Error creating thread %d\n", i);
return 1;
}
}
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
pthread_mutex_destroy(&mutex);
printf("Final counter value: %d\n", counter);
return 0;
}
```
---
### mutex 輸出
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<img class="right_img1" src ="https://forums.autodesk.com/t5/image/serverpage/image-id/344684iCB014721B542E874?v=v2">
```
Thread 5: Counter value: 284318
Thread 7: Counter value: 618112
Thread 4: Counter value: 716860
Thread 1: Counter value: 740933
Thread 3: Counter value: 753300
Thread 2: Counter value: 799071
Thread 0: Counter value: 800000
Thread 6: Counter value: 799140
Final counter value: 800000
```
---
## CON33-C
Avoid race conditions when using library functions.
---
## not thread safe library functions
```
rand(), srand()
getenv(), getenv_s()
strtok()
strerror()
asctime(), ctime(),
localtime(), gmtime() asctime_s(), ctime_s(), localtime_s(), gmtime_s()
setlocale()
```
---
## rand
```c
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#define NUM_THREADS 8
int counter=0;
void *threadFunction(void *threadId) {
long tid = (long)threadId;
int a= rand();
for (int i = 0; i < 10000; i++) {
counter+=a;
}
printf("Thread %ld: Sum value: %lu\n", tid, counter);
pthread_exit(NULL);
}
int main() {
pthread_t threads[NUM_THREADS];
int i;
for (i = 0; i < NUM_THREADS; i++) {
if (0 != pthread_create(&threads[i], NULL, threadFunction, (void *)(long)i)) {
printf("Error creating thread %d\n", i);
return 1;
}
}
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf("Final Sum value: %ld\n", counter);
return 0;
}
```
---
## rand output
first time
```
Thread 1: Sum value: 4031186800
Thread 2: Sum value: 3664539088
Thread 3: Sum value: 1500377952
Thread 0: Sum value: 2360082320
Thread 4: Sum value: 4160134042
Thread 6: Sum value: 3635315610
Thread 7: Sum value: 4123987514
Thread 5: Sum value: 464556282
Final Sum value: 464556282
```
second time
```
Thread 0: Sum value: 4031186800
Thread 1: Sum value: 3664539088
Thread 2: Sum value: 1500377952
Thread 4: Sum value: 2360082320
Thread 5: Sum value: 3377077152
Thread 3: Sum value: 2332738704
Thread 7: Sum value: 2821410608
Thread 6: Sum value: 3456946672
Final Sum value: 3456946672
```
---
## Done
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