📄 make-05.html
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（在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">MS-DOS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">下有些不同，</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">
MS-DOS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">不存在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">SHELL</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">环境变量。这里不对</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">MS-DOS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">下</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">进行介绍，有兴趣地可以自行参考</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">info
make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">关于此部分的描述）</span></b></p>
<h2><a name="_gnu_make_5.3"><span style="font-size: 14pt" lang="en-us"><b>5</b></span><b><span lang="EN-US" style="font-size: 14.0pt">.3<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7.0pt; font-family: Times New Roman">
</span></span></b><span style="font-size: 14.0pt; font-family: 楷体_GB2312">并发执行命令</span></a></h2>
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GNU make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">支持同时执行多条命令。通常情况下，同一时刻只有一个命令在执行，下一个命令只有在当前命令执行完成之后才能够开始执行。不过可以通过</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的命令行选项“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-j</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">--job</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”来告诉</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">在同一时刻可以允许多条命令同时被执行（注意，在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">MS-DOS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中此选项无效，因为它是单任务操作系统）。</span></p>
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如果选项“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-j</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”之后存在一个整数，其含义是告诉</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">在同一时刻可允许同时执行命令的数目。这个数字被称为“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">job
slots</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”。当“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-j</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”选项之后没有出现一个数字时，那么同一时刻执行的命令数目没有要求。使用默认的“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">job
slots</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”，值为</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">1</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">。表示</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">将串行的执行规则的命令（同一时刻只能有一条命令被执行）。</span></p>
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并行执行命令所带来的问题是显而易见地：</span></p>
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1.<span style="font:7.0pt "Times New Roman""> </span></span>
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多个同时执的命令的输出信息将同时被输出到终端。当出现错误时很难根据一大堆凌乱的信息来区分是哪条命令执行错误。</span></p>
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2.<span style="font:7.0pt "Times New Roman""> </span></span>
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在同一时刻可能会存在多个命令执行进程同时读取标准输入，但是对于标准输入设备来说，在同一时刻只能存在一个进程访问它。就是说在某个时间点，</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">只能保证此刻正在执行的进程中的一个进程读取标准输入流，而其它进程的标准输入流将置无效。因此在一时刻多个执行命令的进程中只能有一个进程获得标准输入，而其它需要读取标准输入流的进程由于输入流无效而导致致命错误（通常此进程会得到操作系统的管道破裂信号而被终止）。</span></p>
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这是因为：执行中的命令在什么时候会读取标准输入流（终端输入或重定向的标准输入）是不可预测的。而得到标准输入的顺序总是按照先来先获得的原则。那个命令首先被执行，那么它就可以首先得到标准输入设备。而其它后续需要获取标准输入设备的命令执行进程，由于不能得到标准输入而产生致命错误。在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">规则中如果存在很多命令需要读取标准输入设备，而它们又被允许并行执行时，就会出现这样的错误。</span></p>
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为了解决这个问题。我们可以修改</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">规则的命令使之在执行过程中避免使用标准输入。当然也可以只存在一个命令在执行时会访问标准输入流的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">。</span></p>
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3.<span style="font:7.0pt "Times New Roman""> </span></span>
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会导致</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的递归调用出现问题。</span></p>
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当</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">在执行命令时，如果某一条命令执行失败（被一个信号中止，或非零退出），且该条命令产生的错误不可忽略，那么其它的用于重建同一目标的命令执行也将会被终止。此种情况下，如果</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">没有使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-k</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">--keep-going</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”选项，</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">将停止执行而退出。另外：如果</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">在执行时，由某种原因（包括信号）被中止，此时它的子进程（那些执行规则命令行的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">子进程）正在运行，那么</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">将等到所有这些子进程结束之后才真正退出。</span></p>
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执行</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">时，如果系统运行于重负荷状态下，我们需要控制（减轻）系统在执行</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">时的负荷。可以使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-l</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”选项告诉</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">限制当前运行的任务的数量（</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">所限制的只是它本身所需要占用的系统负载，而不能通过它去控制其它的任务所占用的系统负载）。“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">-l</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">--max-load</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”选项一般后边需要跟一个浮点数。如：</span></p>
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