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0 pointskazinator4y ago0 comments

It is positively not illegal in any standard verision of C since ANSI C 89.

It's an essential feature used in all sorts of everyday code.

C99 added printf conversion specifiers that are hidden behind macros, and idomatic usage of them relies on string catenation.

  uint32_t x = 0;

  printf("x = " PRIx32 "\n", x);

where PRIx32 might expand to "%lx" (if uint32_t is the same as unsigned long in that compiler).

All sorts of C macrology relies on string catenation. Kernel print messages:

  printk(KERN_EMERG "%s: temperature sensor indicates fire!", dev->name);
                   ^ must not have comma here

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6 comments · 2 top-level

tgv4y ago· 3 in thread

I know that. I meant that “abc” + “def” is most likely illegal (although “abc” + ‘d’ is not).

wott4y ago

> I meant that “abc” + “def” is most likely illegal

That would be adding 2 pointers, and that's indeed illegal.

However, you can subtract them: “abc” - “def” . Now, the result is not a pointer any more, it's a ptrdiff_t (an integer type), so most compilers will warn if you try to assign that to a char *.

kazinatorOP4y ago

You started talking about "adding strings" in a thread about adjacent literals, without mentioning any + operator..

String catenation ("adding") by adjacency (no visible operator) is a thing; "add" doesn't imply that we are talking about a + operator:

  $ awk 'BEGIN { x = "abc-" 2 + 2 "-def"; print x}'
  abc-4-def

tgv4y ago

Because the parent compared Python's behavior to that of C. The difference of course is that adding strings doesn't make sense in C, so there's no danger of misinterpreting "abc" "def" in C, as there is in Python.

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lanstin4y ago· 1 in thread

Interesting. Arguably tho this shows how C is aging. I find that PRIx32 a bit ugly.

Although I just had a (logging) use case in go where I missed cpp macros - wanted the log statement to get something from the file and just had to pass it in as another parameter.

kazinatorOP4y ago

I have also never used PRI-anything. It's a crime against readability.

If I have a uint32_t which needs printing I cast it to (unsigned long) and use %lu or %lx. This requires more typing in the argument list, but keeps the format string tidy. It's important for the format string to be tidy, because that's the reason of its existence: to clearly and concisely convey the shape of what is being printed.

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