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[klibc] fix: rt_sprintf() writes nothing when libc vsnprintf is used - #11953

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[klibc] fix: rt_sprintf() writes nothing when libc vsnprintf is used#11953
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@Yunlk Yunlk commented Oct 30, 2026

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拉取/合并请求描述：(PR description)

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为什么提交这份PR (why to submit this PR)

修复 #10027：qemu-virt64-aarch64 上开 RT_USING_CI_ACTION 后 klibc.rt_sprintf 失败。

根因

rt_sprintf() 的调用链把无穷大的 size 交给了 libc：

rt_sprintf()  →  rt_vsprintf()  →  rt_vsnprintf(buf, (size_t)-1, ...)

这在内置实现下无害——它只用 size 决定何时停止写。但启用 RT_KLIBC_USING_LIBC_VSNPRINTF 后，rt_vsnprintf() 直接转发给 libc：

int rt_vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
{
    return vsnprintf(buf, size, fmt, args);   /* size = 0xFFFFFFFFFFFFFFFF */
}

libc 用 buf + size 计算缓冲区末尾，64 位下直接溢出，于是调用失败且一个字节都没写。

实测（bsp/qemu-virt64-aarch64，RT_KLIBC_USING_LIBC_VSNPRINTF=y）：

[K27] rt_snprintf size=100    ret= 2  buf="42"       ← 正常
[K27] rt_sprintf SIZE_MAX     ret=-1  buf=0xCC ...   ← 返回 -1，缓冲区未被写过
[K27] sizeof(size_t)=8, (size_t)-1 = 0xffffffffffffffff

0xCC 正是测试宏 SPRINTF_CHECK 里 rt_memset(buffer, 0xCC, ...) 的填充值，所以 utest 报出来的 str-b 是一串 ����：

[E/utest] at (TC_rt_sprintf.c); func: (TC_space_flag:76); msg: (string not equal)
[E/utest] str-a: ( 42); str-b: (����...����)

你的解决方案是什么 (what is your solution)

启用 libc 委托时改用 libc 的 vsprintf()——它本来就不需要 size 参数，正是 rt_vsprintf() 该对应的接口：

int rt_vsprintf(char *buf, const char *format, va_list arg_ptr)
{
#ifdef RT_KLIBC_USING_LIBC_VSNPRINTF
    return vsprintf(buf, format, arg_ptr);
#else
    return rt_vsnprintf(buf, (size_t) - 1, format, arg_ptr);
#endif
}

内置路径完全不动。

请提供验证的bsp和config (provide the config and bsp)

  • BSP: bsp/qemu-virt64-aarch64
  • 配置：RT_USING_CI_ACTION=y（选中 RT_UTEST_USING_AUTO_RUN）、RT_UTEST_TC_USING_KLIBC=y，其余为 BSP 默认
  • action: 未使用 GitHub Actions；本地 QEMU 11.0.0 + xpack aarch64-none-elf-gcc 15.2.1，master 1c801efa

三组结果：

配置 改前 改后
RT_KLIBC_USING_LIBC_VSNPRINTF=y core.klibc.rt_sprintf FAILED，其余 5 个 PASSED 6 个全部 PASSED
LIBC_VSNPRINTF 未开（内置 vsnprintf） 6 个全部 PASSED 6 个全部 PASSED
LIBC_VSNPRINTF 未开 + VSNPRINTF_STANDARD/LONGLONG 全开 6 个全部 PASSED —

第三组是排查时顺带跑的：RT_KLIBC_USING_VSNPRINTF_STANDARD 的默认值是 default y if ARCH_CPU_64BIT，而这块 BSP 保存的 .config 把它显式关掉了，所以默认配置下并不会看到这个失败——必须显式打开 RT_KLIBC_USING_LIBC_VSNPRINTF 才复现。

一点说明

issue 里没有给出当时的完整 .config，我只能从"64 位 + CI"这两个条件反推。如果原作者当时的配置里 RT_KLIBC_USING_LIBC_VSNPRINTF 是关的，那这里修的可能不是他遇到的那一次，需要他确认。不过无论原场景如何，上面这条路径的缺陷本身是确定的，且可以用一行改动修掉。

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

  • 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
  • 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

  • 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
  • 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
  • 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
  • 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
  • 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
  • 代码是高质量的 Code in this PR is of high quality
  • 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
  • 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

@Yunlk
[klibc] fix: rt_sprintf() writes nothing when libc vsnprintf is used
b7cdb29 
rt_vsprintf() passed (size_t)-1 as the buffer size to rt_vsnprintf(). That is
harmless for the built-in implementation, which only uses the size to stop
writing, but with RT_KLIBC_USING_LIBC_VSNPRINTF the call is forwarded to libc:

    rt_sprintf() -> rt_vsprintf() -> rt_vsnprintf(buf, SIZE_MAX, ...) -> vsnprintf()

libc derives the end of the buffer from buf + size, and 0xFFFFFFFFFFFFFFFF
overflows on 64-bit targets, so the call fails without writing a single byte:

    rt_snprintf(buf, 100, "%d", 42) -> ret=2  buf="42"
    rt_sprintf(buf, "%d", 42)       -> ret=-1 buf=0xCC (untouched)

This is what makes core.klibc.rt_sprintf fail on qemu-virt64-aarch64 when
RT_KLIBC_USING_LIBC_VSNPRINTF is enabled: every SPRINTF_CHECK() compares the
0xCC filled buffer against the expected string.

Delegate to libc's vsprintf() instead, which takes no size argument and is the
exact counterpart of what rt_vsprintf() has to do.

Verified on bsp/qemu-virt64-aarch64 with RT_UTEST_TC_USING_KLIBC + CI_ACTION:

    RT_KLIBC_USING_LIBC_VSNPRINTF=y
      before: core.klibc.rt_sprintf FAILED, 5 other klibc cases passed
      after : all 6 klibc cases PASSED

    RT_KLIBC_USING_LIBC_VSNPRINTF not set (built-in rt_vsnprintf path)
      after : all 6 klibc cases PASSED

Refs RT-Thread#10027
@Yunlk Yunlk mentioned this pull request Oct 30, 2026
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👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

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@github-actions github-actions Bot added the Kernel PR has src relate code label Oct 30, 2026
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📌 Code Review Assignment

🏷️ Tag: kernel

Reviewers: @GorrayLi @ReviewSun @hamburger-os @lianux-mm @wdfk-prog @xu18838022837

Changed Files (Click to expand)
  • src/klibc/kstdio.c

📊 Current Review Status (Last Updated: 2026-10-30 11:57 CST)

📝 Review Instructions

  1. 维护者可以通过单击此处来刷新审查状态: 🔄 刷新状态
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  3. PR合并前需至少一位维护者确认
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