diff --git a/sxbp/sxbp.h b/sxbp/sxbp.h
index 89ada9b..11f728e 100644
--- a/sxbp/sxbp.h
+++ b/sxbp/sxbp.h
@@ -389,6 +389,9 @@ sxbp_result_t sxbp_copy_buffer(
  * @returns `SXBP_RESULT_FAIL_MEMORY` or `SXBP_RESULT_FAIL_IO` on failure to copy the file contents
  * @returns `SXBP_RESULT_FAIL_PRECONDITION` if `file_handle` or `buffer` is
  * `NULL`
+ * @returns `SXBP_RESULT_FAIL_IO` if the file's size could not be determined
+ * @returns `SXBP_RESULT_FAIL_UNIMPLEMENTED` if the file size appears to be
+ * greater than 2GiB on Microsoft Windows only
  * @since v0.54.0
  */
 sxbp_result_t sxbp_buffer_from_file(
diff --git a/sxbp/sxbp_internal.h b/sxbp/sxbp_internal.h
index 7266479..2726050 100644
--- a/sxbp/sxbp_internal.h
+++ b/sxbp/sxbp_internal.h
@@ -62,6 +62,14 @@ typedef struct sxbp_bounds_t {
  */
 extern const sxbp_vector_t SXBP_VECTOR_DIRECTIONS[4];
 
+/*
+ * private, portably retrieves the size in bytes of the file at the given handle
+ * and writes this out to file_size
+ * returns SXBP_RESULT_FAIL_IO if an error occurred
+ * returns SXBP_RESULT_OK if successful
+ */
+sxbp_result_t sxbp_get_file_size(FILE* file_handle, size_t* file_size);
+
 /*
  * private, updates the current figure bounds given the location of the end of
  * the most recently-plotted line
diff --git a/sxbp/sxbp_internal_get_file_size.c b/sxbp/sxbp_internal_get_file_size.c
new file mode 100644
index 0000000..6f2ce6c
--- /dev/null
+++ b/sxbp/sxbp_internal_get_file_size.c
@@ -0,0 +1,140 @@
+/*
+ * This source file forms part of sxbp, a library which generates experimental
+ * 2D spiral-like shapes based on input binary data.
+ */
+
+/**
+ * @internal
+ * @file
+ *
+ * @brief This source file provides the implementation for a function which
+ * tries to portably retrieve the size of a file, using OS-specific methods for
+ * POSIX and Windows, falling back to the standard library (which ironically is
+ * not portable due to the standard not requiring the host to meaningfully
+ * support SEEK_END in the ftell() function) in all other cases.
+ *
+ * @author Joshua Saxby <joshua.a.saxby@gmail.com>
+ * @date 2018
+ *
+ * @copyright Copyright (C) Joshua Saxby 2016-2017, 2018
+ *
+ * @copyright
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+#include <assert.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#include "sxbp.h"
+#include "sxbp_internal.h"
+
+
+#ifdef __cplusplus
+#error "This file is ISO C99. It should not be compiled with a C++ Compiler."
+#endif
+
+// TODO: Check these macro constants for correctness on each OS
+
+// POSIX version
+#if defined(__unix__) || defined(__unix) || (defined(__APPLE__) && defined(__MACH__))
+// we're gonna need fstat() for this!
+#include <sys/stat.h>
+
+
+sxbp_result_t sxbp_get_file_size(FILE* file_handle, size_t* file_size) {
+    // preconditional assertions
+    assert(file_handle != NULL);
+    assert(file_size != NULL);
+    // try and get a file descriptor for the file handle
+    int file_descriptor = fileno(file_handle);
+    if (file_descriptor == -1) {
+        // getting a file descriptor failed
+        return SXBP_RESULT_FAIL_IO;
+    }
+    // results from the fstat call are stored here
+    struct stat file_info = { 0 };
+    if (fstat(file_descriptor, &file_info) == 0) {
+        // file size is file_info.st_size
+        *file_size = (size_t)file_info.st_size;
+        // return success
+        return SXBP_RESULT_OK;
+    } else {
+        // return error code
+        return SXBP_RESULT_FAIL_IO;
+    }
+}
+
+// Windows version
+#elif defined(_WIN32)
+// We're gonna need GetFileSizeEx() for this!
+#include <Windows.h>
+
+
+sxbp_result_t sxbp_get_file_size(FILE* file_handle, size_t* file_size) {
+    // preconditional assertions
+    assert(file_handle != NULL);
+    assert(file_size != NULL);
+    // try and convert the file handle to a Windows file descriptor
+    int file_descriptor = _fileno(file_handle);
+    if (file_descriptor < 0) {
+        // Windows API returns a negative code if it couldn't do the operation
+        return SXBP_RESULT_FAIL_IO;
+    }
+    // try and convert the Windows file descriptor to a Windows HANDLE
+    intptr_t windows_file_handle = _get_osfhandle(file_descriptor);
+    if (windows_file_handle == INVALID_HANDLE_VALUE) {
+        // Windows couldn't do it, return an error
+        return SXBP_RESULT_FAIL_IO;
+    }
+    // the file's size from the Windows API call will be stored in this struct
+    LARGE_INTEGER file_size_info;
+    // call the Windows API to check the file size
+    if (!GetFileSizeEx((HANDLE)windows_file_handle, &file_size_info)) {
+        // if it failed, return an error code
+        return SXBP_RESULT_FAIL_IO;
+    }
+    /*
+     * LARGE_INTEGER is actually a struct of two DWORDS (32-bits) with an extra
+     * member which is 64 bits on 64-bit systems.
+     * As it happens, we don't care about files that are larger than 1GiB so if
+     * the higher DWORD is set, we can return an error code instead as we don't
+     * need it (can't use it for input to the algorithm)
+     */
+    if (file_size_info.HighPart != 0) {
+        return SXBP_RESULT_FAIL_UNIMPLEMENTED;
+    } else {
+        *file_size = (size_t)file_size_info.LowPart;
+        return SXBP_RESULT_OK;
+    }
+}
+
+// Generic version
+#else
+sxbp_result_t sxbp_get_file_size(FILE* file_handle, size_t* file_size) {
+    // preconditional assertions
+    assert(file_handle != NULL);
+    assert(file_size != NULL);
+    /*
+     * seek to end
+     * NOTE: This isn't portable due to lack of meaningful support of `SEEK_END`
+     */
+    if (fseek(file_handle, 0, SEEK_END) != 0) {
+        // handle fseek() error
+        return SXBP_RESULT_FAIL_IO;
+    }
+    // get size and check it, before storing it or throwing an error
+    int check_file_size = ftell(file_handle);
+    if (check_file_size < 0) {
+        // handle ftell() error
+        return SXBP_RESULT_FAIL_IO;
+    }
+    *file_size = (size_t)check_file_size;
+    // seek to start again
+    fseek(file_handle, 0, SEEK_SET);
+    // return the calculated file size
+    return SXBP_RESULT_OK;
+}
+#endif
diff --git a/sxbp/utils.c b/sxbp/utils.c
index f2e2a5d..5d74bfd 100644
--- a/sxbp/utils.c
+++ b/sxbp/utils.c
@@ -116,21 +116,6 @@ sxbp_result_t sxbp_copy_buffer(
     }
 }
 
-/*
- * private, works out and returns the size of the file referred to by the given
- * file handle
- */
-static size_t sxbp_get_file_size(FILE* file_handle) {
-    // seek to end
-    // NOTE: This isn't portable due to lack of meaningful support of `SEEK_END`
-    fseek(file_handle, 0, SEEK_END);
-    // get size
-    size_t file_size = (size_t)ftell(file_handle);
-    // seek to start again
-    fseek(file_handle, 0, SEEK_SET);
-    return file_size;
-}
-
 sxbp_result_t sxbp_buffer_from_file(
     FILE* file_handle,
     sxbp_buffer_t* const buffer
@@ -140,8 +125,15 @@ sxbp_result_t sxbp_buffer_from_file(
     SXBP_RETURN_FAIL_IF_NULL(buffer);
     // erase buffer
     sxbp_free_buffer(buffer);
-    // get the file's size
-    buffer->size = sxbp_get_file_size(file_handle);
+    /*
+     * get the file's size
+     * we'll store any errors encountered by this operation here
+     */
+    sxbp_result_t status = SXBP_RESULT_UNKNOWN;
+    if (!sxbp_check(sxbp_get_file_size(file_handle, &buffer->size), &status)) {
+        // handle error
+        return status;
+    }
     // allocate the buffer to this size and handle error if this failed
     if (!sxbp_success(sxbp_init_buffer(buffer))) {
         // allocation failed - this can only be a memory error