我正在尝试在共享内存上发布一些随机的东西;出于某种奇怪的原因,读者没有领会发件人写的东西
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <sys/types.h>
#include <cstdio>
class SHM {
volatile char* _ptr;
public:
SHM() {
const auto handle = shm_open("myTest", O_RDWR|O_CREAT, 0666);
const auto size = 4 * 1024 * 1024;
if (-1 == ftruncate(handle, size)) {
throw;
}
_ptr = (volatile char*)mmap(0,size , PROT_READ | PROT_WRITE, MAP_SHARED, handle, 0);
if(_ptr == MAP_FAILED){
throw;
}
int rc = fchmod(handle, 0666);
if (rc == -1) {
throw;
}
}
bool read(uint64_t& magic, uint64_t& time) {
const uint64_t newVal = *(uint64_t*)_ptr;
if (newVal != magic) {
magic = newVal;
printf("value changed!!!\n");
time = *(uint64_t*)(_ptr + sizeof(magic));
return true;
}
//printf("old value: %lu\n", newVal);
return false;
}
void publish(const uint64_t time) {
__sync_fetch_and_add((uint64_t*)_ptr, time);
__sync_synchronize();
*(uint64_t*)(_ptr + sizeof(uint64_t)) = time;
}
};
这是发件人:
#include <ctime>
#include <unistd.h>
#include <cstdlib>
#include <cstdint>
#include "shm.h"
int main() {
SHM shm;
timespec t;
for (auto i = 0; i < 10000; i++) {
if (0 == clock_gettime(CLOCK_REALTIME, &t)) {
const uint64_t v = t.tv_sec * 1000 * 1000 * 1000 + t.tv_nsec;
shm.publish(v);
printf("published %lu\n", v);
usleep(100);
}
}
}
这里是读者:
#include <iostream>
#include "shm.h"
int main() {
SHM shm;
uint64_t magic = 0;
uint64_t t = 0;
while (true) {
if (shm.read(magic, t)) {
printf("%lu, %lu\n", magic, t);
}
}
}
如果重新启动阅读器,则阅读器确实能够读取发送方写入的最后一个值。
但是,如果我先启动阅读器,然后再启动发送器,则阅读器不会拾取发送者写的所有值。
为了使这个问题变得更奇怪,如果我在SHM :: read()中取消对printf语句的注释,那么读者有时就可以使用。
有什么主意吗?
GCC版本:
g++ (GCC) 7.2.1 20170829 (Red Hat 7.2.1-1)
答案 0 :(得分:1)
我发现了几个问题,但是,我不确定它们是否可以解决您的问题。
import cv2
import numpy as np
import math
img = cv2.imread('angle.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
ret, threshold = cv2.threshold(gray,170,255,cv2.THRESH_BINARY)
im, contours, hierarchy = cv2.findContours(threshold,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)
for c in contours:
area = cv2.contourArea(c)
perimeter = cv2.arcLength(c, False)
if area < 10001 and 100 < perimeter < 1000:
# first approach - fitting line and calculate with y=kx+n --> angle=tan^(-1)k
rows,cols = img.shape[:2]
[vx,vy,x,y] = cv2.fitLine(c, cv2.DIST_L2,0,0.01,0.01)
lefty = int((-x*vy/vx) + y)
righty = int(((cols-x)*vy/vx)+y)
cv2.line(img,(cols-1,righty),(0,lefty),(0,255,0),2)
(x1, y1) = (cols-1, righty)
(x2, y2) = (0, lefty)
k = (y2-y1)/(x2-x1)
angle = math.atan(k)*180/math.pi
print(angle)
#second approch - cv2.minAreaRect --> returns center (x,y), (width, height), angle of rotation )
rect = cv2.minAreaRect(c)
box = cv2.boxPoints(rect)
box = np.int0(box)
cv2.drawContours(img,[box],0,(0,0,255),2)
print(rect[2])
cv2.imshow('img2', img)
的name
应该以{{1}}开头,以供便携式使用。shm_open
和/
中,强制类型转换不能丢弃read
。例如:publish
。更好的是,放下volatile
并使用const uint64_t newVal = *(uint64_t volatile*)_ptr;
。尽管涉及到不同的过程,但仍然存在相同的对象被多个执行线程访问的情况,并且这些线程中的至少一个会修改共享对象。
我进行了上述更改。使用volatile
对其进行了修复:
std::atomic
使用std::atomic
,您可以拥有更多控制权。例如:
class SHM {
void* _ptr;
public:
SHM() {
const auto handle = shm_open("/myTest", O_RDWR|O_CREAT, 0666);
const auto size = 4 * 1024 * 1024;
if (-1 == ftruncate(handle, size))
throw;
_ptr = mmap(0,size , PROT_READ | PROT_WRITE, MAP_SHARED, handle, 0);
if(_ptr == MAP_FAILED)
throw;
}
bool read(uint64_t& magic, uint64_t& time) {
auto p = static_cast<std::atomic<uint64_t>*>(_ptr);
const uint64_t newVal = p[0];
if (newVal != magic) {
magic = newVal;
printf("value changed!!!\n");
time = p[1];
return true;
}
return false;
}
void publish(const uint64_t time) {
auto p = static_cast<std::atomic<uint64_t>*>(_ptr);
p[0] += time;
p[1] = time;
}
};
void sender() {
SHM shm;
timespec t;
for (auto i = 0; i < 10000; i++) {
if (0 == clock_gettime(CLOCK_REALTIME, &t)) {
const uint64_t v = t.tv_sec * 1000 * 1000 * 1000 + t.tv_nsec;
shm.publish(v);
printf("published %lu\n", v);
usleep(100);
}
}
}
void reader() {
SHM shm;
uint64_t magic = 0;
uint64_t t = 0;
while (true) {
if (shm.read(magic, t)) {
printf("%lu, %lu\n", magic, t);
}
}
}
int main(int ac, char**) {
if(ac > 1)
reader();
else
sender();
}