Add API to let caller handle memory management

This is inspired by [skeeto]'s recent post on [minimalist C libraries]
and allows the caller to decide how to allocate or free memory needed by
libx52. A future addition could let the caller allocate a big enough
buffer up front, and let libx52 manage it.

[skeeto]: https://github.com/skeeto
[minimalist C libraries]: http://nullprogram.com/blog/2018/06/10/

[skip-ci]

lib/libx52/libx52.h

@@ -187,6 +187,24 @@ typedef enum {
 
 } libx52_error_code;
 
+/**
+ * @brief Memory allocation callback type
+ *
+ * This function has the same signature as realloc(3), but is passed
+ * to allow the calling application to manage memory instead of relying
+ * on the standard library.
+ */
+typedef void * (*libx52_memalloc)(void *ptr, size_t size);
+
+/**
+ * @brief Memory free callback type
+ *
+ * This function has the same signature as free(3), but is passed
+ * to allow the calling application to manage memory instead of relying
+ * on the standard library.
+ */
+typedef void (*libx52_memfree)(void *ptr);
+
 /**
  * @brief Initialize the X52 library
  *
@@ -206,6 +224,27 @@ typedef enum {
  */
 int libx52_init(libx52_device ** dev);
 
+/**
+ * @brief Initialize the X52 library
+ *
+ * This function does the exact same functionality as \ref libx52_init,
+ * except that it requires the user to provide function callbacks for
+ * allocating and releasing memory.
+ *
+ * @param[out]  dev         Pointer to a \ref libx52_device *
+ * @param[in]   memalloc    Pointer to a function that returns an allocated
+ *                          block of memory. Function should have the same
+ *                          signature as realloc(3)
+ * @param[in]   memfree     Pointer to a function that frees an allocated
+ *                          block of memory. Function should have the same
+ *                          signature as free(3)
+ *
+ * @returns \ref libx52_error_code indicating status
+ */
+int libx52_init_mem(libx52_device ** dev,
+                    libx52_memalloc memalloc,
+                    libx52_memfree memfree);
+
 /**
  * @brief Exit the library and free up any resources used
  *

lib/libx52/x52_common.h

@@ -34,6 +34,9 @@ struct libx52_device {
     libusb_context *ctx;
     libusb_device_handle *hdl;
 
+    libx52_memalloc memalloc;
+    libx52_memfree memfree;
+
     uint32_t update_mask;
     uint32_t flags;
 

lib/libx52/x52_core.c

@@ -43,6 +43,14 @@ static int libx52_device_is_x52pro(uint16_t idProduct)
 }
 
 int libx52_init(libx52_device **dev)
+{
+    /* Call libx52_init_mem with realloc and free */
+    return libx52_init_mem(dev, realloc, free);
+}
+
+int libx52_init_mem(libx52_device **dev,
+                    libx52_memalloc memalloc,
+                    libx52_memfree memfree)
 {
     int rc;
     ssize_t count;
@@ -58,12 +66,20 @@ int libx52_init(libx52_device **dev)
         return LIBX52_ERROR_INVALID_PARAM;
     }
 
+    /* Make sure that our alloc and free functions are valid */
+    if (memalloc == NULL || memfree == NULL) {
+        return LIBX52_ERROR_INVALID_PARAM;
+    }
+
     /* Allocate memory for the library's data structures */
-    x52_dev = calloc(1, sizeof(libx52_device));
+    x52_dev = (*memalloc)(NULL, sizeof(libx52_device));
     if (!x52_dev) {
         return LIBX52_ERROR_OUT_OF_MEMORY;
     }
 
+    x52_dev->memalloc = memalloc;
+    x52_dev->memfree = memfree;
+
     rc = libusb_init(&(x52_dev->ctx));
     if (rc) {
         rc = LIBX52_ERROR_INIT_FAILURE;
@@ -109,6 +125,6 @@ void libx52_exit(libx52_device *dev)
 {
     libusb_close(dev->hdl);
     libusb_exit(dev->ctx);
-    free(dev);
+    (*(dev->memfree))(dev);
 }