blocks|key|1796042|text|当你用内存做可疑的事情时，分段错误是不能保证的。这完全取决于很多因素(编译器如何在内存中布局程序，优化等)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1796043|对于C%2B%2B程序来说可能是非法的东西在一般情况下对于程序来说可能不是非法的。例如，操作系统并不关心您是否跳出数组。它甚至不知道数组是什么。然而，它会关心你是否触摸了不属于你的记忆。|1796044|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

A segmentation fault is never guaranteed when you're doing fishy stuff with memory. It all depends on a lot of factors (how the compiler lays out the program in memory, optimizations etc).

What may be illegal for a C++ program may not be illegal for a program in general. For instance the OS doesn't care if you step outside an array. It doesn't even know what an array is. However it does care if you touch memory that doesn't belong to you.

blocks|key|1796052|text|假设您的字符串由另外两个字符串组成，每个字符串的最大长度为20个字符，但是您将其放入一个25个字符的字段中，这就是您的第一个问题所在。你很有可能会越界。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1796053|变量desc的长度应至少为41个字符(插入的空格为20+%2B20%2B1)。|offset|length|style|CODE|1796054|使用valgrind或gdb找出出现seg故障的原因。|1796055|entityMap^0|0|2|4|0|2|8|B|3|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]|$D|S|E|T|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|K|$]]

Given that your string is made up of two other strings each being a max of 20 characters long, yet you are putting it into a field that is 25 characters, that is where your first issue lies. You are have a good potential to overstep your bounds.

The variable <code>desc</code> should be at least 41 characters long (20 + 20 + 1 [for the space you insert]).

Use <code>valgrind</code> or <code>gdb</code> to figure out why you are getting a seg fault.

blocks|key|1097541|text|char+desc[25];
char+name[20];
char+address[20];
sprintf+(desc,+"%25s+%25s",+name,+address);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1097542|仅通过查看这段代码，我就可以假设name和address都可以是20字符长度。如果是这样，那么这是否意味着desc应该是最小的20%2B20%2B1字符长度？(1字符表示name和address之间的空格，如sprintf中所指定的那样)。|unstyled|offset|length|style|CODE|1097543|这可能是segfault的一个原因。也可能有其他原因。例如，如果name比20字符更长怎么办？|1097544|所以最好使用std::string|1097545|std::string+name;
std::string+address;
std::string+desc+=+name+%2B+"+"+%2B+address;
char+const+*char_desc+=+desc.str();+//if+at+all+you+need+this|1097546|entityMap^0|0|G|4|L|7|W|2|1H|4|1R|7|24|1|29|4|2E|7|2S|7|0|W|4|11|2|0|6|B|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@$G|U|H|V|I|J]|$G|W|H|X|I|J]|$G|Y|H|Z|I|J]|$G|10|H|11|I|J]|$G|12|H|13|I|J]|$G|14|H|15|I|J]|$G|16|H|17|I|J]|$G|18|H|19|I|J]|$G|1A|H|1B|I|J]]|9|@]|A|$]]|$1|K|3|L|5|F|7|1C|8|@$G|1D|H|1E|I|J]|$G|1F|H|1G|I|J]]|9|@]|A|$]]|$1|M|3|N|5|F|7|1H|8|@$G|1I|H|1J|I|J]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1K|8|@]|9|@]|A|$B|C]]|$1|Q|3|-4|5|F|7|1L|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>char desc[25];
char name[20];
char address[20];
sprintf (desc, "%s %s", name, address);
</code></pre>

Just by looking at this code, I can assume that <code>name</code> and <code>address</code> each can be <code>20</code> chars long. If that is so, then does it not imply that <code>desc</code> should be minimum <code>20+20+1</code> chars long? (<code>1</code> char for the space between <code>name</code> and <code>address</code>, as specified in the <code>sprintf</code>).

That can be the one reason of segfault. There could be other reasons as well. For example, what if <code>name</code> is longer than <code>20</code> chars?

So better you use <code>std::string</code>:

<pre><code>std::string name;
std::string address;
std::string desc = name + " " + address;
char const *char_desc = desc.str(); //if at all you need this
</code></pre>

blocks|key|5135791|text|在这种情况下，您不知道格式化字符串中包含哪种垃圾。这种垃圾可能会导致将剩余的参数视为“对齐”数据类型(例如int或double)的参数。将未对齐的区域视为对齐的参数肯定会在某些系统上导致SIGBUS。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5135792|entityMap^0|1H|3|1L|6|2L|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

In this particular case, you don't know what kind of garbage you have in the format string. That garbage could potentially result in treating the remaining arguments as those of an "aligned" data type (e.g. <code>int</code> or <code>double</code>). Treating an unaligned area as an aligned argument definitely causes <code>SIGBUS</code> on some systems.

blocks|key|1097607|text|如果您尝试对未映射到进程的虚拟地址进行数据访问，则会发生分段错误。在大多数操作系统上，内存是以几千字节的页进行映射的；这意味着，如果您注销数组的末尾，通常不会得到错误，因为在内存页中，它后面还有其他有效数据。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1097608|总线错误表示更低级别的错误；如您所说，错误对齐的访问或丢失的物理地址是两个原因。然而，第一种情况在这里没有发生，因为您正在处理字节，它没有对齐限制；我认为第二种情况只有在内存完全耗尽时才会发生，而这种情况可能不会发生。|1097609|然而，我认为如果你试图从一个无效的虚拟地址执行代码，你也可能会得到一个总线错误。这很可能就是这里正在发生的事情--通过删除本地数组的末尾，您将覆盖堆栈帧的重要部分，例如函数的返回地址。这会导致函数返回一个无效的地址，这(我认为)会给出一个总线错误。这就是我最好的猜测，你在这里体验到了什么特殊的未定义行为。|1097610|一般来说，您不能依靠分段错误来捕获缓冲区溢出；据我所知，最好的工具是valgrind，尽管它仍然无法捕获某些类型的溢出。在处理字符串时，避免溢出的最好方法是使用std::string，而不是假装正在编写C。|offset|length|style|CODE|1097611|entityMap|0|LINK|mutability|MUTABLE|url|https://valgrind.org/^0|0|0|0|28|B|Y|8|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|V|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|W|8|@$H|X|I|Y|J|K]]|9|@$H|Z|I|10|1|11]]|A|$]]|$1|L|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|M|$N|$5|O|P|Q|A|$R|S]]]]

A segmentation fault occurs if you try to do a data access a virtual address that is not mapped to your process. On most operating systems, memory is mapped in pages of a few kilobytes; this means that you often won't get a fault if you write off the end of an array, since there is other valid data following it in the memory page.

A bus error indicates a more low-level error; a wrongly-aligned access or a missing physical address are two reasons, as you say. However, the first is not happening here, since you're dealing with bytes, which have no alignment restriction; and I think the second can only happen on data accesses when memory is completely exhausted, which probably isn't happening.

However, I think you might also get a bus error if you try to execute code from an invalid virtual address. This could well be what is happening here - by writing off the end of a local array, you will overwrite important parts of the stack frame, such as the function's return address. This will cause the function to return to an invalid address, which (I think) will give a bus error. That's my best guess at what particular flavour of undefined behaviour you are experiencing here.

In general, you can't rely on segmentation faults to catch buffer overruns; the best tool I know of is <a href="https://valgrind.org" rel="nofollow">valgrind</a>, although that will still fail to catch some kinds of overrun. The best way to avoid overruns when working with strings is to use <code>std::string</code>, rather than pretending that you're writing C.

blocks|key|780223|text|正如您可能意识到的，基本原因是程序中未定义的行为。在这种情况下，它导致由硬件检测到的错误，该错误被OS捕获并映射到信号。确切的映射并不是真正指定的(我见过整数除以零会导致SIGFPE)，但通常情况下:当您访问越界时会发生SIGSEGV，当出现其他访问错误时会发生SIGBUS，而对于非法指令则会出现SIGILL。在这种情况下，最有可能的解释是您的边界错误覆盖了堆栈上的返回地址。如果返回地址没有正确对齐，您可能会得到一个SIGBUS，如果对齐了，您将开始执行任何内容，这可能会导致SIGILL。(但将随机字节作为代码执行的可能性是标准委员会在定义“未定义的行为”时所考虑的。特别是在没有内存保护的机器上，您最终可能会直接跳到操作系统中。)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|780224|entityMap^0|2D|6|32|7|3N|6|45|6|5U|6|6O|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|G|$]]

As you probably realize, the base cause is undefined behavior in your
program. In this case, it leads to an error detected by the hardware,
which is caught by the OS and mapped to a signal. The exact mapping 
isn't really specified (and I've seen integral division by zero result
in a <code>SIGFPE</code>), but generally: <code>SIGSEGV</code> occurs when you access out of
bounds, <code>SIGBUS</code> for other accessing errors, and <code>SIGILL</code> for an illegal
instruction. In this case, the most likely explination is that your
bounds error has overwritten the return address on the stack. If the
return address isn't correctly aligned, you'll probably get a <code>SIGBUS</code>,
and if it is, you'll start executing whatever is there, which could
result in a <code>SIGILL</code>. (But the possibility of executing random bytes as
code is what the standards committee had in mind when they defined
&ldquo;undefined behavior&rdquo;. Especially on machines with no memory
protection, where you could end up jumping directly into the OS.)

I had a bug in my code that went like this. 

<pre><code>char desc[25];
char name[20];
char address[20];
sprintf (desc, "%s %s", name, address);
</code></pre>

Ideally this should give a segfault. However, I saw this give a bus error. 
Wikipedia says something to the order of 'Bus error is when the program tries to access an unaligned memory location or when you try to access a physical (not virtual) memory location that does not exist or is not allowed. '

The second part of the above statement sounds similar to a seg fault. So my question is, when do you get a SIGBUS and when a SIGSEGV? 

EDIT:- 
Quite a few people have mentioned the context. I'm not sure what context would be needed but this was a buffer overflow lying inside a static class function that get's called from a number of other class functions. If there's something more specific that I can give which will help, do ask.

Anyways, someone had commented that I should simply write better code. I guess the point of asking this question was "can an application developer infer anything from a SIGBUS versus a SIGSEGV?" (picked from that blog post below)

Why am I not getting a segmentation fault with this code? (Bus error)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

我的代码中有一个bug，是这样的。char desc[25];char name[20];char address[20];sprintf (desc, "%s %s", name, address);理想情况下，这应该会给出一个段错误。然而，我看到这给出了一个总线错误。维基百科说：“当程序试图访问未对齐的内存位置时，...

问为什么这段代码没有出现分段错误？(总线错误)
EN

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么这段代码没有出现分段错误？(总线错误)EN