如何在GCC中规避格式截断警告? [英] How to circumvent format-truncation warning in GCC?

问题描述

我收到以下gcc格式截断警告:

I'm getting the following gcc format-truncation warning:

test.c:8:33: warning: ‘/input’ directive output may be truncated writing 6 bytes into a region of size between 1 and 20 [-Wformat-truncation=]
snprintf(dst, sizeof(dst), "%s-more", src);
                             ^~~~~~
test.c:8:3: note: ‘snprintf’ output between 7 and 26 bytes into a destination of size 20
snprintf(dst, sizeof(dst), "%s-more", src);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

关于这样的代码:

char dst[20];
char src[20];
scanf("%s", src);
snprintf(dst, sizeof(dst), "%s-more", src);
printf("%s\n", dst);

我知道它可能会被截断-但这正是我首先使用snprintf的原因.有没有一种方法可以使编译器清楚这是预期的(不使用编译指示或-Wno-format-truncation)?

I'm aware that it might be truncated - but this is exactly the reason why I'm using snprintf in the first place. Is there a way how to make it clear to the compiler that this is intended (without using a pragma or -Wno-format-truncation)?

推荐答案

1. 警告已侵入gcc7.1，请参阅 gcc7.1版本更改.
2. 来自 gcc文档:

1. warning has been intruded in gcc7.1, see gcc7.1 release changes.
2. From gcc docs:

-Wformat-truncation [...]的级别1仅警告对有界函数的调用，这些函数的返回值未使用，并且很可能导致输出截断.

Level 1 of -Wformat-truncation [...] warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.

1. 问题出在错误报告，并以NOTABUG的形式关闭:

1. The issue was a bug report and was closed as NOTABUG:

未处理的输出截断通常是程序中的错误. [...]
在需要截断的情况下，调用方通常会检查该函数的返回值并以某种方式进行处理(例如，通过在其上分支).在这种情况下，不会发出警告.警告显示的源代码行表明这不是这些情况之一.该警告正在执行其预期的操作.

Unhandled output truncation is typically a bug in the program. [...]
In cases when truncation is expected the caller typically checks the return value from the function and handles it somehow (e.g., by branching on it). In those cases the warning is not issued. The source line printed by the warning suggests that this is not one of those cases. The warning is doing what it was designed to do.

1. 但是我们只需要检查snprintf的返回值即可，该返回值会在错误时返回负值.

#include <stdio.h>
#include <stdlib.h>
void f(void) {
    char dst[2], src[2];
    // snprintf(dst, sizeof(dst), "%s!", src);

    int ret = snprintf(dst, sizeof(dst), "%s!", src);
    if (ret < 0) {
         abort();
    }

    // But don't we love confusing one liners?
    for (int ret = snprintf(dst, sizeof(dst), "%s!", src); ret < 0;) exit(ret);
    // Can we do better?
    snprintf(dst, sizeof(dst), "%s!", src) < 0 ? abort() : (void)0;
    // Don't we love obfuscation?
#define snprintf_nowarn(...) (snprintf(__VA_ARGS__) < 0 ? abort() : (void)0)
    snprintf_nowarn(dst, sizeof(dst), "%s!", src);
}

使用gcc7.1 gcc7.2 gcc7.3 gcc8.1在 https://godbolt.org/上进行了测试用-O{0,1,2,3} -Wall -Wextra -pedantic.不发出任何警告. gcc8.1优化/删除对abort()的调用，优化程度大于-O1.

Tested on https://godbolt.org/ with gcc7.1 gcc7.2 gcc7.3 gcc8.1 with -O{0,1,2,3} -Wall -Wextra -pedantic. Gives no warning whatsoever. gcc8.1 optimizes/removes the call to abort() with optimization greater then -O1.

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