popen返回NULL

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当程序写得越来越大，进程占用的内存也就越来越多，调用popen时会返回空的FILE指针，网上说原因是system或popen这样的系统函数，其内部实现是调用fork函数创建子进程，创建过程中会复制父进程堆、栈等资源，这样就容易造成创建失败，返回NULL。

以下是我写的用vfork替换fork调用的Vpopen类。

Vpopen.h

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#pragma once
 
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
 
class Vpopen
{
        FILE *fp_;
        pid_t pid_;
        int pipeFd_[2];
public:
        Vpopen();
        ~Vpopen();
        FILE *open(const char *cmd, const char *flags);
        void close();
};

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win32渐变颜色矩形函数

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#define FILL_STYLE_VERTICAL	0
#define FILL_STYLE_HORIZONTAL	1
 
void fillGradientRect(HDC hdc, RECT *rc, COLORREF fromRGB, COLORREF toRGB, int fillStyle)
{
	BITMAPINFO bmpInfo;
	memset(&bmpInfo, 0, sizeof(bmpInfo));
	bmpInfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
	bmpInfo.bmiHeader.biWidth = rc->right - rc->left;
	bmpInfo.bmiHeader.biHeight = rc->bottom - rc->top;
	bmpInfo.bmiHeader.biPlanes = 1;
	bmpInfo.bmiHeader.biBitCount = 32;
	bmpInfo.bmiHeader.biCompression = BI_RGB;
 
	BYTE *rgbBuffer = (BYTE *)malloc(bmpInfo.bmiHeader.biWidth * bmpInfo.bmiHeader.biHeight * 4);
 
	int width = bmpInfo.bmiHeader.biWidth;
	int height = bmpInfo.bmiHeader.biHeight;
 
	int rStart = fromRGB & 0xFF;
	int rEnd = toRGB & 0xFF;
	int gStart = (fromRGB & 0xFF00) >> 8;
	int gEnd = (toRGB & 0xFF00) >> 8;
	int bStart = (fromRGB & 0xFF0000) >> 16;
	int bEnd = (toRGB & 0xFF0000) >> 16;
	if (FILL_STYLE_VERTICAL == fillStyle)
	{
		double rStep = (rEnd - rStart) * 1.0 / height;
		double gStep = (gEnd - gStart) * 1.0 / height;
		double bStep = (bEnd - bStart) * 1.0 / height;
		for (int y = 0; y < height; y++)
		{
			BYTE r = (BYTE)(rStart + rStep * y);
			BYTE g = (BYTE)(gStart + gStep * y);
			BYTE b = (BYTE)(bStart + bStep * y);
			for (int x = 0; x < width; x++)
			{
				BYTE *p = rgbBuffer + ((height - y) * width - (width - x)) * 4;
				*p++ = b;
				*p++ = g;
				*p++ = r;
			}
		}
	}
	else
	{
		double rStep = (rEnd - rStart) * 1.0 / width;
		double gStep = (gEnd - gStart) * 1.0 / width;
		double bStep = (bEnd - bStart) * 1.0 / width;
		for (int x = 0; x < width; x++)
		{
			BYTE r = (BYTE)(rStart + rStep * x);
			BYTE g = (BYTE)(gStart + gStep * x);
			BYTE b = (BYTE)(bStart + bStep * x);
			for (int y = 0; y < height; y++)
			{
				BYTE *p = rgbBuffer + ((height - y) * width - (width - x)) * 4;
				*p++ = b;
				*p++ = g;
				*p++ = r;
			}
		}
	}
 
	StretchDIBits(hdc, rc->left, rc->top, width, height, 0, 0, width, height, rgbBuffer, &bmpInfo, DIB_RGB_COLORS, SRCCOPY);
 
	free(rgbBuffer);
}
 
void CMFCApplicationDlg::OnPaint()
{
	CPaintDC dc(this);
	CRect rc(50, 50, 200, 200);
	fillGradientRect(dc.m_hDC, rc, RGB(255, 0, 255), RGB(0, 255, 0), FILL_STYLE_HORIZONTAL);
 
	CDialogEx::OnPaint();
}

win32发消息触发窗口拖动

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	::ReleaseCapture();
	::SendMessage(m_hWnd, WM_NCLBUTTONDOWN, HTCAPTION, NULL);
	::SendMessage(m_hWnd, WM_LBUTTONUP, NULL, NULL);

win32限制改变窗口大小

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响应WM_SIZING消息，消息函数中做如下处理

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void CDemoDlg::OnSizing(UINT fwSide, LPRECT pRect)
{
	if (pRect->right - pRect->left < minWidth_)
	{
		if (fwSide == WMSZ_LEFT
			|| fwSide == WMSZ_TOPLEFT
			|| fwSide == WMSZ_BOTTOMLEFT)
		{
			pRect->left = pRect->right - minWidth_;
		}
		else
			pRect->right = pRect->left + minWidth_;
	}
	if (pRect->bottom - pRect->top < memHdcHeight_)
	{
		if (fwSide == WMSZ_TOP
			|| fwSide == WMSZ_TOPLEFT
			|| fwSide == WMSZ_TOPRIGHT)
		{
			pRect->top = pRect->bottom - minHeight_;
		}
		else
			pRect->bottom = pRect->top + minHeight_;
	}
 
	CDialogEx::OnSizing(fwSide, pRect);
}

如何让安卓APP第二次插上USB设备后不弹系统权限提示窗

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方法是把Android App做成随USB设备插入时自启动就可以了。
具体做法是在App的AndroidManifest.xml里，把要自启动activity的地方加两处USB_DEVICE_ATTACHED就可以了。

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        <activity
            android:name="com.serenegiant.usbcameratest2.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
                <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"/>            </intent-filter>
 
            <meta-data android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"                android:resource="@xml/device_filter"/>        </activity>

refer to: 梦幻贝壳, https://blog.csdn.net/menghuanbeike/article/details/78752715

Javascript技巧

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	<style style="display:block" contentEditable="true">
		body {
			background:rgb(255, 255, 255);
		}
	</style>
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const movies = {
	list: [
		{ title: 'Heat' },
		{ title: 'Interstellar' },
		{ title: 'Savior' }
	],
	[Symbol.iterator]() {
		let index = 0;
		return {
			next: () => {
				if (index < this.list.length) {
					const value = this.list[index++].title;
					return { value, done: false };
				}
				return { done: true };
			}
		};
	}
};
 
const [, secondMovieTitle = 'NonTitle'] = movies;
console.log(secondMovieTitle);
big = {
    foo: 'value Foo',
    bar: 'value Bar'
};
var { foo = 'Unk' } = big;
console.log(foo);
var { ['foo']: name = 'Unknown' } = big;
console.log(name);
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obj1 = {
	a() {
		console.log(this);
	},
	b: 'hi',
	c: () => {
		console.log(this);
	}
};
obj1.a();
obj1.c();
A1 = function() {
	console.log(this);
};
a1 = new A1;
A1();

Fall-back from the nopython compilation path to the object mode compilation

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出现此提示的原因是用@jit标注的函数内存在非数值运算语句。

比如下例中，time()函数和print()函数都不能出现在foo()函数中。正确写法为：

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import numba as nb
import time
 
@nb.jit
def foo(x, y):
	s = 0
	for i in range(x, y):
		s += i
	return s
 
tt = time.time()
print(foo(1, 100000000))
print('Time used: {} secs'.format(time.time() - tt))

本地的Documentation for Android SDK浏览还是太慢？

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在Android Studio中，通过SDK Manager将Documentation for Android SDK下载到本地后，在Android SDK目录下找到docs\index.html，在浏览器中打开，发现加载还是好慢，原因是本地网页又去访问互联网了。

想到的办法就是阻止浏览器访问外网，可参照

成为上帝玩转Chrome之指定阻止访问的网站

在此Chrome插件中加入以下阻止前缀字符串

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http://www.google.com/jsapi

就可在Chrome中极速查看安卓SDK文档了。