avr: provide errno storage

docs/manual/embedded.md

@@ -1530,6 +1530,7 @@ what it calls:
   string in flash. A floating-point conversion prints `?`, as avr-libc's
   does without `-lprintf_flt`; EmbCC has no `printf_flt`;
 - the `<avr/eeprom.h>` functions;
+- `errno`, with one value for the program (AVR has no threads of its own);
 - `setjmp` and `longjmp`, with a 24-byte `jmp_buf`: r2-r17, Y, the stack
   pointer, SREG and the return address.
 

lib/avr/errno.c

@@ -0,0 +1,6 @@
+/* AVR has no threads of its own, so errno is one value for the program. */
+#include <errno.h>
+
+static int g_errno;
+
+int *__errno_location(void) { return &g_errno; }

tests/golden/avr-libc-run.sh

@@ -30,6 +30,7 @@ done
 ${CC:-cc} -w -I$D $D/host.c -o "$out/host" || fail "the host program"
 "$out/host" > "$out/host.txt" || fail "the host program does not run"
 { cat <<'EOF'
+errno 34
 byte 22 fe word 1234 beef
 dword 89abcdef qword 102030405060708 float 5
 ptr world

tests/golden/avr-libc-run/prog.c

@@ -2,6 +2,7 @@
  * and the _P functions, and the printf family, on QEMU's ATmega328P. */
 #include <avr/io.h>
 #include <avr/pgmspace.h>
+#include <errno.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -33,6 +34,9 @@ int main(void)
     UCSR0B = _BV(TXEN0);
     UCSR0C = _BV(UCSZ01) | _BV(UCSZ00);
 
+    errno = ERANGE;
+    SHOW("errno %d", errno);
+
     /* pgm_read_* */
     SHOW("byte %x %x word %x %x", pgm_read_byte(&bytes[1]), pgm_read_byte(&bytes[3]),
          pgm_read_word(&words[0]), pgm_read_word(&words[1]));