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user:embedded_c_code_doesn_t_have_to_be_ugly [2020/05/11 18:23] – [5. Globals] Igor Yefmov | user:embedded_c_code_doesn_t_have_to_be_ugly [2022/04/04 23:32] (current) – external edit 127.0.0.1 | ||
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As such (barring external dependencies) I always advise on using the latest stable language standard supported by the toolchain that your organization is comfortable with. And yes, that means that engineers must continually improve their grasp of the language and be on top of the latest stable standard to efficiently take advantage of the improvements provided by that standard. | As such (barring external dependencies) I always advise on using the latest stable language standard supported by the toolchain that your organization is comfortable with. And yes, that means that engineers must continually improve their grasp of the language and be on top of the latest stable standard to efficiently take advantage of the improvements provided by that standard. | ||
- | ==== 3. #define (and const) vs. enum ==== | + | ==== 3. #define (and const) vs. enum and magic numbers |
So much has been said about the many advantages of pushing as much work as possible away from the preprocessor and into the compiler that is amazes me to still see tons of ''# | So much has been said about the many advantages of pushing as much work as possible away from the preprocessor and into the compiler that is amazes me to still see tons of ''# | ||
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<code C>enum{ s2r_uvc_stream_buf_size = s2r_ep_bulk_video_pkts_count * s2r_ep_bulk_video_pkt_size };</ | <code C>enum{ s2r_uvc_stream_buf_size = s2r_ep_bulk_video_pkts_count * s2r_ep_bulk_video_pkt_size };</ | ||
+ | |||
+ | === Magic numbers === | ||
+ | You know, that kind: | ||
+ | <code C> | ||
+ | |||
+ | Compare that to this code and tell me - which one makes you understand what that code does? | ||
+ | <code C> | ||
==== 4. Bit manipulations vs. structured data ==== | ==== 4. Bit manipulations vs. structured data ==== | ||
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A relic of K&R style '' | A relic of K&R style '' | ||
- | These same compilers are capable of making quite intelligent choices on how to translate your code into zeroes and ones, so that you should truly be not afraid of expressing your functionality in the language of abstraction rather than writing in '' | + | These same compilers are capable of making quite intelligent choices on how to translate your code into zeroes and ones, so that you should truly be not afraid of expressing your functionality in the language of abstraction, rather than writing in '' |
Stack allocation is extremely cheap, even on the most low-end down-to-earth hardware, so concerning yourself with the cost of that operation by grouping all-the-possible-function-variables together at the top should be reserved only for the most critical code((in which case it is very likely that such code should be written directly in Assembler by a highly-skilled engineer who is also extremely familiar with the target platform)). | Stack allocation is extremely cheap, even on the most low-end down-to-earth hardware, so concerning yourself with the cost of that operation by grouping all-the-possible-function-variables together at the top should be reserved only for the most critical code((in which case it is very likely that such code should be written directly in Assembler by a highly-skilled engineer who is also extremely familiar with the target platform)). | ||
=== Cross-function globals === | === Cross-function globals === | ||
- | Oh, these are absolutely the worst as they have a tendency on sneaking | + | Oh, these are absolutely the worst, as they tend to sneak up on you at the least expected time and place! They get declared as globals and then are only set in one place while being accessed in plenty of other places. Especially dangerous ones cross thread boundaries, making for extra-nasty race condition type bugs. |
Even if one doesn' | Even if one doesn' | ||
==== 6. Code blocks' | ==== 6. Code blocks' | ||
+ | Somewhat related to the general question of "how many LoC in a function is too many?" this one goes back to gut feeling that cramming all the code into a single function will somehow have positive optimization benefits overall. Let me be clear here: **it doesn' | ||
+ | This perception completely ignores all the advances made in CPU manufacturing in the past 60 years or so((various speculative execution techniques: branch prediction, prefetching, | ||
+ | |||
+ | Just write the code in manageable (one-two page length) chunks, packaged as individual functions. And if there really **is** a concern for stack depth you always have the option of inlining those functions((a very often overlooked method to my surprise)). | ||
+ | |||
+ | If you have a long-ass '' | ||
+ | |||
+ | And as a general rule: if you see a block of code that doesn' | ||
+ | |||
+ | Think of functions as text paragraphs: if you read some text that consists of one huge paragraph chances are you'll give up soon enough and look for some other source of information. One that is made for human consumption. | ||
+ | |||
+ | === Stack depth === | ||
+ | Speaking of stack depth: partitioning your code into smaller functions may indeed //reduce// your stack requirement as you won't be needing to allocate every-single-variable-ever-used-in-any-branch in one chunk, but instead only use up as much stack as needed for each individual function. | ||
==== 7. Code comments ==== | ==== 7. Code comments ==== | ||
+ | <code C> | ||
+ | |||
+ | int i = 1; // set i to 1 | ||
+ | |||
+ | i++; // increment i by 1</ | ||
+ | The comments above are utterly useless. Not only they don't anything to what is already expressed in the code((which is bad enough on its own)), there' |