Hacker News Reader

Hacker News

Comment by formerly_proven | Hacker News Reader

Parent Full thread

formerly_proven

goto is the idiomatic way to emulate RAII in C. It's everywhere.

View on HN

Please explain a little more with an example. Certainly not in my C code, though I a huge fan of RAII in C++.

Here is an example with RAII and goto: https://wiki.sei.cmu.edu/confluence/display/c/MEM12-C.+Consi...

Btw that is a Cert C recommendation to use goto for RAII!

Take a look at the Linux kernel sometime, goto is everywhere there too https://www.kernel.org/doc/html/v4.10/process/coding-style.h...

Not many of us are kernel developers, or agree whole-heartedly with their practices.

A function that allocates resources in several steps jumps, on error, to the teardown part at the end to deallocate (in reverse order of allocation) only the resources that were allocated. The others are skipped (jumped over). It's very popular in the Linux kernel.

> only the resources that were allocated

How does it know which were allocated and which were not? If you are talking about memory allocation, then you could simply deallocate all the pointers provided they were initially set to NULL. Freeing a NULL pointer is well-defined in C.

The information is in the instruction pointer. You jump to the place that deallocates the last resource that was allocated, followed by the second-to-last etc.

Also, free(NULL) is not well-defined in C, and other resource types aren't so easy to check. delete nullptr in C++ is well-defined, but you rarely need it.

> free(NULL) is not well-defined in C

Oh yes it is: https://stackoverflow.com/questions/1938735/does-freeptr-whe...

I stand corrected. So, these days, null checks before free() are just for compatibility with very old systems, or more likely "tradition". I wonder where I got my wrong information? I have even seen null checks before delete in C++ code recently written by C programmers :o

When it is in the normal execution path, the test avoids a function call, so there's a little bit of performance gain.

It can also be a bit self-documenting, or be a place-holder if there is for example stuff to delete in a loop (elements of an array) before releasing the pointer itself (the array).

I don't know, when you read it, sometimes it feels natural and meaningful, and sometimes you know that the author didn't know that free() handles NULL all right.

Files, sockets, mutexes are all examples of resources that aren't memory. Someone posted an example in this comment thread - [0].

[0] https://wiki.sei.cmu.edu/confluence/display/c/MEM12-C.+Consi...

> Files, sockets, mutexes are all examples of resources that aren't memory

I know - that's why I explicitly mentioned memory.

Reply on news.ycombinator.com