Fix missing aligned_alloc() on some Android devices

Some Android versions lack `aligned_alloc()` in `<stdlib.h>`. Compiling
on Termux 0.118.0 yields this error:

```
cmd-discard.c:383:11: warning: call to undeclared library function \
'aligned_alloc' with type 'void *(unsigned long, unsigned long)'; ISO \
C99 and later do not support implicit function declarations \
[-Wimplicit-function-declaration]

        buffer = aligned_alloc(lba_size, lba_size);
                 ^
```

To avoid making large changes in tests, define a helper function that
wraps `posix_memalign()` as our own `aligned_alloc()`.

Just another day of working around platform quirks.

Co-authored-by: Alviro Iskandar Setiawan <[email protected]>
Signed-off-by: Alviro Iskandar Setiawan <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

examples/helpers.c

@@ -60,3 +60,13 @@ int setup_listening_socket(int port, int ipv6)
 
 	return fd;
 }
+
+void *aligned_alloc(size_t alignment, size_t size)
+{
+	void *ret;
+
+	if (posix_memalign(&ret, alignment, size))
+		return NULL;
+
+	return ret;
+}

examples/helpers.h

@@ -4,4 +4,11 @@
 
 int setup_listening_socket(int port, int ipv6);
 
+/*
+ * Some Android versions lack aligned_alloc in stdlib.h.
+ * To avoid making large changes in tests, define a helper
+ * function that wraps posix_memalign as our own aligned_alloc.
+ */
+void *aligned_alloc(size_t alignment, size_t size);
+
 #endif

examples/reg-wait.c

@@ -13,6 +13,8 @@
 #include <sys/time.h>
 #include <liburing.h>
 
+#include "helpers.h"
+
 static unsigned long long mtime_since(const struct timeval *s,
 				      const struct timeval *e)
 {

test/helpers.c

@@ -363,3 +363,13 @@ unsigned long long utime_since_now(struct timeval *tv)
 	gettimeofday(&end, NULL);
 	return utime_since(tv, &end);
 }
+
+void *aligned_alloc(size_t alignment, size_t size)
+{
+	void *ret;
+
+	if (posix_memalign(&ret, alignment, size))
+		return NULL;
+
+	return ret;
+}

test/helpers.h

@@ -13,6 +13,7 @@ extern "C" {
 #include "../src/setup.h"
 #include <arpa/inet.h>
 #include <sys/time.h>
+#include <stdlib.h>
 
 enum t_setup_ret {
 	T_SETUP_OK	= 0,
@@ -25,6 +26,13 @@ enum t_test_result {
 	T_EXIT_SKIP   = 77,
 };
 
+/*
+ * Some Android versions lack aligned_alloc in stdlib.h.
+ * To avoid making large changes in tests, define a helper
+ * function that wraps posix_memalign as our own aligned_alloc.
+ */
+void *aligned_alloc(size_t alignment, size_t size);
+
 /*
  * Helper for binding socket to an ephemeral port.
  * The port number to be bound is returned in @addr->sin_port.