How to catch or handle segfault from free() or delete()

Question

In c++, I have a server code running continuously 24*7 but i am getting segfault sometimes while freeing the buffer. I tried try catch as well.

        try {
                    free(partialBuf);
                } catch (...) {
                    printf("Caught partial buf free error");
                }

Thanks in advance!

`try`, `catch` in `c`? no? in `c++`? why `free()`? why not `delete`? — Sourav Ghosh, Mar 16 '15 at 10:15
You **shouldn't** try to handle a segfault. It's a bug, find the cause and fix it! It's not something you should casually dismiss as *ok*. That being said, if i'm not mistaken a segfault is a signal `SIGSEGV` - read about signal handling in c, and you might be able to ignore it. But it will just bring more pain later. — jepio, Mar 16 '15 at 10:17
`How to catch or handle segfault`... run through debugger and fix the code. Best and easiest way. — Sourav Ghosh, Mar 16 '15 at 10:17
Can you please explain why its a null pointer exception? @user4559929 — Kritpal Singh, Mar 16 '15 at 10:19
Segmentation fault is mainly caused by attempt to access or manipulate a null pointer. — niyasc, Mar 16 '15 at 10:24
@SouravGhosh i tried delete as well instead of free. what i learned on internet are delete mostly call's deconstructor and free() will directly free the address we provided. — Kritpal Singh, Mar 16 '15 at 10:25
There's no such "null pointer exception" in C. It's a plain crash. — JBL, Mar 16 '15 at 10:27
What's your question exactly? Is it "how do I debug a segfault?" Also, why is this tagged `C`? — David Schwartz, Mar 16 '15 at 10:46

score 0 · Answer 1 · edited May 23 '17 at 11:50

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A segfault is not an exception in the sense of other C++ exceptions, hence you cannot catch it with try/catch. A segfault can have any number of reasons, but in 99.9% of cases it's a memory access bug :-) If the segfault happens during a call to delete or free(), chances are that you are having a double-free issue.

edited May 23 '17 at 11:50

Community

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answered Mar 16 '15 at 10:26

edgar.holleis

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Thanks.But is there any way to know that whether pointer got freed before calling free() or its memory still accessible ?? – Kritpal Singh Mar 16 '15 at 10:30
Yes. Use valgrind or a similar tool. – R.. GitHub STOP HELPING ICE Mar 16 '15 at 10:43

DesirePRG · Answer 2 · 2015-03-16T10:48:57.200

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You could use GDB to debug, and find out whether you are trying to free a pointer which was not allocated previously.

edited Mar 16 '15 at 10:48

answered Mar 16 '15 at 10:43

DesirePRG

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score 0 · Accepted Answer · answered Mar 16 '15 at 11:02

Since you're apparently able to use try/catch, you're writing C++ code. It helps to know which language you're using.

The solution then is to use std::shared_ptr. You may have multiple places in which a pointer goes out of scope. With shared_ptr you no longer call free, and as a bonus shared_ptr will call delete only once (after the last pointer goes out of scope).

However, you should now allocate memory with new instead of malloc.

How to catch or handle segfault from free() or delete()

3 Answers3