小编AR8*_*R89的帖子

我正在玩这个代码:

#include <stdlib.h>
#include <stdio.h>

void function(int *p){
    free(p);
}

int main(){
    int *i = calloc(3, sizeof(int));
    function(i);
    i[0] = 'a';
    printf("%c\n", i[0]);
    return 0;
}

我希望得到一个错误,但它打印'a',如果'a'被解除分配,为什么打印它？函数(int*p)中发生的事情有什么影响,如果main()？(如果可以的话,解释那个'p'指针发生了什么)

假设我在同一个函数中有这两个:

int *a = malloc(...);
int *b = a;

它们都指向同一块内存,但是当我必须释放它时,我应该在它们两个上调用free,还是只调用其中一个？(再次解释为什么可能)

来自Mike Ash的评论:https: //www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html#comment-3abf26dd771b7bf2f28d04106993c07b

这是代码:

void Tester(int ign, float x, char y) 
{ 
    printf("float: %f char: %d\n", x, y); 
} 

int main(int argc, char **argv) 
{ 
    float x = 42; 
    float y = 42; 
    Tester(0, x, y); 

    void (*TesterAlt)(int, ...) = (void *)Tester; 
    TesterAlt(0, x, y); 

    return 0; 
}

他在主要功能中所做的演员对我来说非常不清楚.

TesterAlt是一个返回void的函数的指针,它与函数Tester的返回类型相同.他分配给这个函数指针,函数Tester,但是他将后一种返回类型转换为void类型的指针(我不确定这一点).

如果我编译代码改变该行:

void (*TesterAlt)(int, ...) = (void)Tester;

我收到编译器错误:

initializing 'void (*)(int, ...)' with an expression of incompatible type 'void'
void (*TesterAlt)(int, ...) = (void) Tester;

为什么他要做这个演员？他的语法是什么意思？

编辑:我对原始问题不是很清楚,我不明白这种语法以及我必须如何阅读它.

(void *)Tester; …

可能重复:
由CACurrentMediaTime()引起的体系结构i386编译错误的未定义符号

我试图使用这个函数:CACurrentMediaTime(),因为我需要当前时间,但是我得到了这个错误

Ld /Users/aruffolo/Library/Developer/Xcode/DerivedData/Star-hbmohpuyghsjeseyswvvpevxkesj/Build/Products/Debug/Star.app/Contents/MacOS/Star normal x86_64
cd "/Users/aruffolo/Documents/Programmazione/Learning Objective-c/Star"
setenv MACOSX_DEPLOYMENT_TARGET 10.8
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk -L/Users/aruffolo/Library/Developer/Xcode/DerivedData/Star-hbmohpuyghsjeseyswvvpevxkesj/Build/Products/Debug -F/Users/aruffolo/Library/Developer/Xcode/DerivedData/Star-hbmohpuyghsjeseyswvvpevxkesj/Build/Products/Debug -filelist /Users/aruffolo/Library/Developer/Xcode/DerivedData/Star-hbmohpuyghsjeseyswvvpevxkesj/Build/Intermediates/Star.build/Debug/Star.build/Objects-normal/x86_64/Star.LinkFileList -mmacosx-version-min=10.8 -fobjc-arc -fobjc-link-runtime -framework SystemConfiguration -framework CoreMedia -framework Cocoa -o /Users/aruffolo/Library/Developer/Xcode/DerivedData/Star-hbmohpuyghsjeseyswvvpevxkesj/Build/Products/Debug/Star.app/Contents/MacOS/Star

Undefined symbols for architecture x86_64:
  "_CACurrentMediaTime", referenced from:
  -[MyView runLoop] in MyView.o
ld: symbol(s) not found for architecture x86_64

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Tetris extends JFrame {

public Tetris() {

    add(new GamePanel());
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setResizable(false);
    setSize(800, 600);
    setVisible(true);
    setLocationRelativeTo(null);
    setTitle("Tetris");
}

public class GamePanel extends JPanel {


    public GamePanel(){
        TetrisBoard tetraBoard= new TetrisBoard();
        GridBagLayout layout= new GridBagLayout();
        this.setLayout(layout);
        GridBagConstraints c = new GridBagConstraints();
        c.gridx = 2;
        c.gridy = 1;
        c.ipadx = 190;
        c.ipady = 390;
        c.insets.left= 360;
        layout.setConstraints(tetraBoard, c);
        this.add(tetraBoard);
        setBackground(Color.WHITE);
    }

   @Override
   public void paint(Graphics g){
       super.paint(g); …

我有一个练习:给定一个AtomicIntegers矩阵(初始化为0),我必须为每一行运行一个线程,并为每一列的一个线程,在我减去1的行上,在我添加1的列上,所以在结束矩阵应保持原样.问题是矩阵发生了变化!下面是代码:该对象采用矩阵,一个布尔值,它将告诉他对列或行进行操作,以及它必须操作的列/行的索引.

import java.util.concurrent.atomic.AtomicInteger;

public class FifthExerciseSafe implements Runnable{
    private Thread thread;
    private boolean onRows;//tells if the object operates on the rows or on the columns
    private AtomicInteger[][] matrix;
    private int index;

public FifthExerciseSafe(AtomicInteger[][] matrix, boolean onRows, int index){
    this.matrix = matrix;
    this.onRows = onRows;
    this.index = index;
}

public void start(){
    thread = new Thread(this);
    thread.start();
}

public boolean isOnRows() {
    return onRows;
}

public void setOnRows(boolean onRows) {
    this.onRows = onRows;
}

public AtomicInteger[][] getMatrix() {
    return matrix;
} …

我试图了解如何使用xmm寄存器比较两个浮点数(32位).为了测试我在C中编写了这段代码(在程序集中调用代码):

#include "stdio.h"

extern int compare();

int main()
{

    printf("Result: %d\n", compare());

    return 0;
}

这是程序集,我想测试是否b <c,在这种情况下确实如此,代码应返回1,但它返回0:

section .data

a:  dd 5.5555
b:  dd 1.1111
c:  dd 5.5555

section .text

global compare

compare:
            ; -------------------------------------------
            ; Entrace sequence
            ; -------------------------------------------
            push    ebp         ; save base pointer
            mov     ebp, esp    ; point to current stack frame 
            push    ebx         ; save general registers
            push    ecx
            push    edx
            push    esi         
            push    edi

            movss xmm0, [b]
            movss xmm1, [c]
            comiss xmm0, xmm1
            jl change …