@TOC

描述

-Wstrict-overflow
-Wstrict-overflow=n
第一句其实挺难理解，说这个undefine的意思是“未定义”，我们经常使用的地方就是“UB（undefined behavior）”，未定义行为。这一句的翻译就是：只有在符号数溢出未定义时才会激活这个选项。怎么才叫符号数溢出未定义？
Human: 怎么理解“未定义符号溢出”？
ChatGPT: 未定义符号溢出是指程序未明确定义如何处理溢出情况，即当某个特定的值超出一定的范围时，程序该如何处理这种情况。当使用-Wstrict-overflow参数时，编译器会检查程序中可能存在的溢出行为，并提供相应的提示。

This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level. An optimization that assumes that signed overflow does not occur is perfectly
safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues,
several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example the compiler simplifies x + 1 > x to 1. This level of ‘-Wstrict-overflow’ is enabled by ‘-Wall’; higher levels are not, and must be explicitly requested.

-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0. This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN, which is less than zero. ‘-Wstrict-overflow’ (with no level) is the same as ‘-Wstrict-overflow=2’.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0.
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2.
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

相关的一个bug

https://bugzilla.redhat.com/show_bug.cgi?id=2047022

In file included from /usr/include/sys/socket.h:33,from /builddir/build/BUILD/socket_wrapper-1.3.3/src/socket_wrapper.c:50:
In function '__cmsg_nxthdr',inlined from 'test_sendmsg_cmsg' at /builddir/build/BUILD/socket_wrapper-1.3.3/tests/test_swrap_unit.c:73:9:
/usr/include/bits/socket.h:322:6: error: assuming pointer wraparound does not occur when comparing P +- C1 with P +- C2 [-Werror=strict-overflow]322 |   if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control|      ^

chatGPT

问答式输出：
Human: how to translate “This option is only active when signed overflow is undefined” to chinese?
AI:
AI: 这个选项只有在未定义符号溢出时才会生效。
Human: 怎么理解这一句话？
AI: 这句话的意思是，当符号溢出未定义时，这个选项才会生效。它意味着在算法在未定义符号溢出情况下，这个选项才能提供正确的结果。
Human: 也就是说程序自己需要定义异常的情况？
AI: 是的，如果希望某项算法在特定的情况下得到正确的结果，就必须要定义异常情况。-Wstrict-overflow 参数可以帮助检测数组在溢出的情况，从而能够阻止程序崩溃
Human: 有没有好的例子，展示符合溢出定义？
AI: 有的，以下是一个简单的例子，来展示如何定义符号溢出：让我们假设一个数组只能够容纳5个元素，当数组中元素大于5个时，就会发生溢出。这时，在程序
Human: 有没有好的例子，展示有符合溢出定义的程序？
AI: 可以，下面是一个示例来说明如何使用-Wstrict-overflow参数对程序进行符号溢出定义：
int main() {
int array[5];
unsigned int arraySize = 10;

// Compile with flag -Wstrict-overflow
//gcc -Wstrict-overflow someFile.c

// This line should trigger a warning.
array[arraySize] = 0;

return 0;
}
Human: 上面的程序示例没有定义符号溢出的处理，能否展示一个有符号溢出处理的程序？
AI: 可以，下面是一个有符号溢出处理的示例：
int main() {
int array[5];
unsigned int arraySize = 10;

// Compile with flag -Wstrict-overflow gcc -Wstrict-overflow someFile.c

// This line should trigger a warning.
if (arraySize >= 5) {
printf(“Error: Array size is too large!\n”);
} else {
array[arraySize] = 0;
}

return 0;
}
Human: Is there anyone using -Wstrict-overflow?
AI: Yes, many developers use the -Wstrict-overflow flag to help catch any potential issues in their code. It’s particularly useful in projects where safety and reliability are top priorities.
Human: Is the -Wstrict-overflow included in -Wall?
AI: Yes, -Wstrict-overflow is included in -Wall, which stands for “all warnings”. The -Wall gcc flag enables all the standard compiler warnings that are available. So if you use -Wall then it will also include -Wstrict-overflow.
Human: 那，可以详细说一下strict-overflow的详细说明吗？
AI: 当使用-Wstrict-overflow参数时，编译器会检查你的代码以检测是否可能存在溢出行为，例如，索引值超出了数组的范围，某个变量的最大值小于最小值，以及其他潜在的问题，如果编译器发现有可能存在溢出，则会停止编译，并抛出异常。

总结

chatGPT的用途很广泛，如果赶不上，后果还是挺严重。对于基础教育模式是一个挑战。以后的竞争里来自于创新，素质教育。