在装配中使用 printf 会在管道时导致空输出，但在终端上工作 [英] Using printf in assembly leads to empty output when piping, but works on the terminal

问题描述

我尝试从我的汇编代码中使用 printf，这是一个最小的例子，它应该只将 hello 打印到标准输出:

I try to use printf from my assembler code, this is a minimal example which should just print hello to stdout:

.section  .rodata
hello:
    .ascii  "hello
"
.section .text
    .globl _start        
_start:
    movq $hello, %rdi     # first parameter
    xorl %eax, %eax       # 0 - number of used vector registers
    call printf        
#exit   
    movq $60, %rax
    movq $0, %rdi
    syscall

我用

gcc -nostdlib try_printf.s -o try_printf -lc

当我运行它时，它似乎工作:字符串hello被打印出来并且退出状态是0:

and when I run it, it seems to work: the string hello is printed out and the exit status is 0:

XXX$ ./try_printf
hello
XXX$ echo $?
0
XXX$

但是当我尝试捕获文本时，很明显，有些地方工作不正常:

But when I try to capture the text, it is obvious, that something is not working properly:

XXX$ output=$(./try_printf) 
XXX$ echo $output

XXX$ 

变量 output 的值应该是 hello，但为空.

The variable output should have the value hello, but is empty.

我使用 printf 有什么问题?

What is wrong with my usage of printf?

推荐答案

在使用 printf 等 stdio 函数后，使用 call exit 而不是原始的 _exit 系统调用. 这会在进行 exit_group 系统调用之前刷新 stdio 缓冲区(write 系统调用).

Use call exit instead of a raw _exit syscall after using stdio functions like printf. This flushes stdio buffers (write system call) before making an exit_group system call).

(或者如果你的程序定义了一个main而不是_start，从main返回相当于调用exit>. 你不能从 _start 中 ret.) 调用 fflush(NULL) 也应该有效.

(Or if your program defines a main instead of _start, returning from main is equivalent to calling exit. You can't ret from _start.) Calling fflush(NULL) should also work.

正如迈克尔解释的那样，动态链接 C 库是可以的.这也是自下而上编程" 书(见第 8 章).

As Michael explained, it is OK to link the C-library dynamically. This is also how it is introduced in the "Programming bottom up" book (see chapter 8).

然而，为了结束程序而不是绕过它，从 C 库中调用 exit 很重要，这是我错误地调用 exit-syscall.正如迈克尔所暗示的，退出做了很多清理就像冲洗溪流.

However it is important to call exit from the C-library in order to end the program and not to bypass it, which was what I wrongly did by calling exit-syscall. As hinted by Michael, exit does a lot of clean up like flushing streams.

这就是发生的事情:正如here，C 库按如下方式缓冲标准流:

That is what happened: As explained here, the C-library buffers the the standard streams as follows:

1. 没有缓冲标准错误.
2. 如果标准输出/输入是终端，则它是行缓冲的.
3. 如果标准输出/输入不是终端，则它是完全缓冲的，因此在原始退出系统调用之前需要刷新.

在第一次为流调用 printf 时决定哪种情况适用.

Which case applies is decided when printf is called for the first time for a stream.

所以如果在终端直接调用printf_try，可以看到程序的输出，因为hello末尾有 (在行缓冲模式下触发flush)并且是终端，也是2.情况.

So if printf_try is called directly in the terminal, the output of the program can be seen because hello has at the end (which triggers the flush in the line-buffered mode) and it is a terminal, also the 2. case.

通过 $(./printf_try) 调用 printf_try 意味着 stdout 不再是终端(实际上我不知道它是临时文件还是临时文件)内存文件)，因此 3. 情况有效 - 需要显式刷新，即调用 C-exit.

Calling printf_try via $(./printf_try) means that the stdout is no longer a terminal (actually I don't know whether is is a temp file or a memory file) and thus the 3. case is in effect - there is need for an explicit flush i.e. call to C-exit.

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