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Merged
merged 4 commits into from
Oct 18, 2024
Merged

Add LLVM libc sample #538

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9dd9b6b
Add LLVM libc sample
smithp35 Oct 15, 2024
aab9590
Address review comments
smithp35 Oct 16, 2024
4898208
Further review comments.
smithp35 Oct 17, 2024
ffa667c
rename errno to avoid clash if errno is a macro.
smithp35 Oct 17, 2024
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4 changes: 4 additions & 0 deletions docs/llvmlibc.md
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Expand Up @@ -54,6 +54,10 @@ following command line options, in addition to `--target`, `-march` or
`__llvm_libc_heap_limit` in addition to whatever other memory layout
you want.

* LLVM libc does not define errno. If you are using a function that
sets errno then you must implement the function `int *__llvm_libc_errno()`
that returns the address of your definition of errno.

For example:

```
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4 changes: 2 additions & 2 deletions llvmlibc-samples/src/llvmlibc/baremetal-semihosting/crt0llvmlibc.c
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Expand Up @@ -34,8 +34,8 @@ extern char __tdata_end[];
extern char __tls_end[];

void _start(void) {
memcpy(__data_start, __data_source, (uintptr_t) __data_size);
memset(__bss_start, '\0', (uintptr_t) __bss_size);
memcpy(__data_start, __data_source, (size_t) __data_size);
memset(__bss_start, 0, (size_t) __bss_size);
_platform_init();
_Exit(main(0, NULL));
}
12 changes: 10 additions & 2 deletions llvmlibc-samples/src/llvmlibc/baremetal-semihosting/hello.c
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Expand Up @@ -19,7 +19,13 @@
#include <string.h>
#include <math.h>

/* Example that uses heap, string and math library */
// Implementation of errno
int *__llvm_libc_errno() {
static int errno;
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@statham-arm statham-arm Oct 17, 2024

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Sorry to do another pedantic one, but I recommend calling that internal variable something other than errno. If a user starts from this code and adds #include <errno.h>, then the name errno will surely become a macro, making this function into nonsense. Any other name is fine.

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@smithp35 smithp35 Oct 17, 2024

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Thanks for spotting. I've used a name that doesn't contain errno.

return &errno;
}

// Example that uses heap, string and math library.

int main(void) {
const char *hello_s = "hello ";
Expand All @@ -32,7 +38,9 @@ int main(void) {
strncpy(out_s, hello_s, hello_s_len + 1);
assert(out_s_len >= strlen(out_s) + world_s_len + 1);
strncat(out_s, world_s, world_s_len + 1);
printf("%s %d\n", out_s, abs(-3));
// 2024-10-17 Embedded printf implementation does not currently
// support printing floating point numbers.
printf("%s %li\n", out_s, lround(400000 * atanf(1.0f)));
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@statham-arm statham-arm Oct 17, 2024

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Because you wrote lround rather than lroundf, the float returned from atanf is being converted into a double, so this code is testing a mix of single and double precision operations.

I think that's actually a good thing – it makes it a better test! I point it out because you probably didn't do it on purpose, but I quite like it, and perhaps it's better not to "fix" it :-)

free(out_s);
return 0;
}
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