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Add helper for creating CharSlice from a string literal #36

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Mar 21, 2022
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Add helper for creating CharSlice from a string literal #36

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8d1a8ef
Add helper for creating CharSlice from a string literal
ivoanjo Mar 17, 2022
d20aff4
Improve error message when using ddprof_ffi_CharSlice_from_literal
ivoanjo Mar 18, 2022
9672ea7
Merge branch 'main' into ivoanjo/add-charslice-helper
morrisonlevi Mar 18, 2022
04f75dd
Change name to DDPROF_FFI_CHARSLICE_C
morrisonlevi Mar 18, 2022
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4 changes: 4 additions & 0 deletions ddprof-ffi/cbindgen.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,10 @@ style = "both"
no_includes = true
sys_includes = ["stdbool.h", "stddef.h", "stdint.h"]

after_includes = """

#define ddprof_ffi_CharSlice_from_literal(string) ((ddprof_ffi_CharSlice) {.ptr = "" string, .len = sizeof("" string) - 1})"""
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Great suggestion !
But I find sizeof dangerous:

const char* s = "foo";
...
ddprof_ffi_CharSlice_from_literal(s);

will construct an incorrect slice.
Using strlen would avoid such errors and strlen call would be optimized away by the compiler for string literals.
I am also not a big fan of macros, you could use static inline function:

static inline ddprof_ffi_CharSlice ddprof_ffi_CharSlice_from_literal(const char*s) {
    return (ddprof_ffi_CharSlice){.ptr=s, .len=strlen(s); };
}

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Great suggestion !
But I find sizeof dangerous:

const char* s = "foo";
...
ddprof_ffi_CharSlice_from_literal(s);

will construct an incorrect slice.

Actually, by design, it won't. It will give you a compile error :D :D

That's because of the .ptr = "" string instead of .ptr = string -- you can only char *foo = "do " "this " "if " "every " "part " "is " "a " "literal";.

To be fair the error you get is not a great one, but at least it does barf at the usage.

We could have both ddprof_ffi_CharSlice_from_literal as well as a strlen version ddprof_ffi_CharSlice_from_char (?), although I do somewhat dislike that you may be doing strlen without realizing.

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Actually, you got me thinking that the error message could be improved. I've pushed d20aff4 that adds a nice comment which incidentally gets shown by the compiler when you try to pass in a char*:

foo.c: In function 'endpoint_from':
foo.c:133:27: error: expected '}' before 's'
   byte_slice_from_literal(s);
                           ^
foo.c:15:113: note: in definition of macro 'byte_slice_from_literal'
   /* NOTE: Compilation fails if you pass in a char* instead of a literal */ ((ddprof_ffi_CharSlice) {.ptr = "" string, .len = sizeof(string) - 1})
                                                                                                                ^~~~~~
foo.c:15:91: note: to match this '{'
   /* NOTE: Compilation fails if you pass in a char* instead of a literal */ ((ddprof_ffi_CharSlice) {.ptr = "" string, .len = sizeof(string) - 1})


[export]
prefix = "ddprof_ffi_"

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