Linux 上 C 中的 stdout 线程安全? [英] stdout thread-safe in C on Linux?

问题描述

在 Linux 上使用 printf 线程安全写入标准输出吗?使用较低级别的 write 命令怎么样?

Is writing to stdout using printf thread-safe on Linux? What about using the lower-level write command?

推荐答案

它不是由 C 标准指定的——它取决于您对 C 标准库的实现.事实上，C 标准甚至根本没有提到线程，因为某些系统(例如嵌入式系统)没有多线程.

It's not specified by the C standard -- it depends on your implementation of the C standard library. In fact, the C standard doesn't even mention threads at all, since certain systems (e.g. embedded systems) don't have multithreading.

在 GNU 实现 (glibc) 中，stdio 中处理 FILE* 对象的大多数高级函数都是线程安全的.那些名称中通常没有 unlocked 的(例如 getc_unlocked(3)).但是，线程安全是在每个函数调用级别:例如，如果您多次调用 printf(3)，这些调用中的每一个都保证以原子方式输出，但其他线程可能会打印调用 printf() 之间的事情.如果要确保一系列 I/O 调用以原子方式输出，可以用一对 flockfile(3)/funlockfile(3) 调用将它们包围起来以锁定 FILE句柄.请注意，这些函数是可重入的，因此您可以安全地在它们之间调用 printf()，即使 printf() 本身进行调用，也不会导致死锁flockfile().

In the GNU implementation (glibc), most of the higher-level functions in stdio that deal with FILE* objects are thread-safe. The ones that aren't usually have unlocked in their names (e.g. getc_unlocked(3)). However, the thread safety is at a per-function call level: if you make multiple calls to printf(3), for example, each of those calls is guaranteed to output atomically, but other threads might print things out between your calls to printf(). If you want to ensure that a sequence of I/O calls gets output atomically, you can surround them with a pair of flockfile(3)/funlockfile(3) calls to lock the FILE handle. Note that these functions are reentrant, so you can safely call printf() in between them, and that won't result in deadlock even thought printf() itself makes a call to flockfile().

像 write(2) 这样的低级 I/O 调用应该是线程安全的，但我不是 100% 确定 - write() 对内核进行系统调用以执行 I/O.这究竟如何发生取决于您使用的内核.它可能是 sysenter 指令，或旧系统上的 int(中断)指令.一旦进入内核，就由内核来确保 I/O 是线程安全的.在我刚刚使用 Darwin 内核 8.11.1 版进行的测试中，write(2) 似乎是线程安全的.

The low-level I/O calls such as write(2) should be thread-safe, but I'm not 100% sure of that - write() makes a system call into the kernel to perform I/O. How exactly this happens depends on what kernel you're using. It might be the sysenter instruction, or the int (interrupt) instruction on older systems. Once inside the kernel, it's up to the kernel to make sure that the I/O is thread-safe. In a test I just did with the Darwin Kernel Version 8.11.1, write(2) appears to be thread-safe.

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