diff --git a/libctru/include/3ds/types.h b/libctru/include/3ds/types.h
index 6fa2dfe..fea97e6 100644
--- a/libctru/include/3ds/types.h
+++ b/libctru/include/3ds/types.h
@@ -11,6 +11,13 @@
 /// The maximum value of a u64.
 #define U64_MAX	UINT64_MAX
 
+/// would be nice if newlib had this already
+#ifndef SSIZE_MAX
+#ifdef SIZE_MAX
+#define SSIZE_MAX ((SIZE_MAX) >> 1)
+#endif
+#endif
+
 typedef uint8_t u8;   ///<  8-bit unsigned integer
 typedef uint16_t u16; ///< 16-bit unsigned integer
 typedef uint32_t u32; ///< 32-bit unsigned integer
diff --git a/libctru/include/3ds/util/utf.h b/libctru/include/3ds/util/utf.h
index bc2966d..c404666 100644
--- a/libctru/include/3ds/util/utf.h
+++ b/libctru/include/3ds/util/utf.h
@@ -52,61 +52,104 @@ ssize_t encode_utf8 (uint8_t *out, uint32_t in);
 ssize_t encode_utf16(uint16_t *out, uint32_t in);
 
 /** Convert a UTF-8 sequence into a UTF-16 sequence
+ *
+ *  Fills the output buffer up to \a len code units.
+ *  Returns the number of code units that the input would produce;
+ *  if it returns greater than \a len, the output has been
+ *  truncated.
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf8_to_utf16(uint16_t *out, const uint8_t  *in, size_t len);
+ssize_t utf8_to_utf16(uint16_t *out, const uint8_t  *in, size_t len);
 
 /** Convert a UTF-8 sequence into a UTF-32 sequence
+ *
+ *  Fills the output buffer up to \a len code units.
+ *  Returns the number of code units that the input would produce;
+ *  if it returns greater than \a len, the output has been
+ *  truncated.
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf8_to_utf32(uint32_t *out, const uint8_t  *in, size_t len);
+ssize_t utf8_to_utf32(uint32_t *out, const uint8_t  *in, size_t len);
 
 /** Convert a UTF-16 sequence into a UTF-8 sequence
+ *
+ *  Fills the output buffer up to \a len code units.
+ *  Returns the number of code units that the input would produce;
+ *  if it returns greater than \a len, the output has been
+ *  truncated.
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf16_to_utf8(uint8_t  *out, const uint16_t *in, size_t len);
+ssize_t utf16_to_utf8(uint8_t  *out, const uint16_t *in, size_t len);
 
 /** Convert a UTF-16 sequence into a UTF-32 sequence
+ *
+ *  Fills the output buffer up to \a len code units.
+ *  Returns the number of code units that the input would produce;
+ *  if it returns greater than \a len, the output has been
+ *  truncated.
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf16_to_utf32(uint32_t *out, const uint16_t *in, size_t len);
+ssize_t utf16_to_utf32(uint32_t *out, const uint16_t *in, size_t len);
 
 /** Convert a UTF-32 sequence into a UTF-8 sequence
+ *
+ *  Fills the output buffer up to \a len code units.
+ *  Returns the number of code units that the input would produce;
+ *  if it returns greater than \a len, the output has been
+ *  truncated.
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf32_to_utf8(uint8_t  *out, const uint32_t *in, size_t len);
+ssize_t utf32_to_utf8(uint8_t  *out, const uint32_t *in, size_t len);
 
 /** Convert a UTF-32 sequence into a UTF-16 sequence
  *
  *  @param[out] out Output sequence
- *  @param[in]  in  Input sequence
+ *  @param[in]  in  Input sequence (null-terminated)
+ *  @param[in]  len Output length
  *
  *  @returns number of output code units produced
  *  @returns -1 for error
+ *
+ *  @note \a out is not null-terminated
  */
-size_t utf32_to_utf16(uint16_t *out, const uint32_t *in, size_t len);
+ssize_t utf32_to_utf16(uint16_t *out, const uint32_t *in, size_t len);
diff --git a/libctru/source/romfs_dev.c b/libctru/source/romfs_dev.c
index 3b25602..6c502ee 100644
--- a/libctru/source/romfs_dev.c
+++ b/libctru/source/romfs_dev.c
@@ -133,8 +133,9 @@ Result romfsInit(void)
 		} else
 			return 2;
 
-		size_t units = utf8_to_utf16(__utf16path, (const uint8_t*)filename, PATH_MAX+1);
-		if (units == (size_t)-1) return 3;
+		ssize_t units = utf8_to_utf16(__utf16path, (const uint8_t*)filename, PATH_MAX);
+		if (units < 0)         return 3;
+		if (units >= PATH_MAX) return 4;
 		__utf16path[units] = 0;
 
 		FS_Archive arch = { ARCHIVE_SDMC, { PATH_EMPTY, 1, (u8*)"" }, 0 };
@@ -284,7 +285,7 @@ static romfs_file* searchForFile(romfs_dir* parent, u16* name, u32 namelen)
 
 static int navigateToDir(romfs_dir** ppDir, const char** pPath, bool isDir)
 {
-	size_t units;
+	ssize_t units;
 
 	char* colonPos = strchr(*pPath, ':');
 	if (colonPos) *pPath = colonPos+1;
@@ -331,9 +332,11 @@ static int navigateToDir(romfs_dir** ppDir, const char** pPath, bool isDir)
 			}
 		}
 
-		units = utf8_to_utf16(__utf16path, (const uint8_t*)component, PATH_MAX+1);
-		if (units == (size_t)-1)
+		units = utf8_to_utf16(__utf16path, (const uint8_t*)component, PATH_MAX);
+		if (units < 0)
 			return EILSEQ;
+		if (units >= PATH_MAX)
+			return ENAMETOOLONG;
 
 		*ppDir = searchForDir(*ppDir, __utf16path, units);
 		if (!*ppDir)
@@ -363,12 +366,17 @@ int romfs_open(struct _reent *r, void *fileStruct, const char *path, int flags,
 	if (r->_errno != 0)
 		return -1;
 
-	size_t units = utf8_to_utf16(__utf16path, (const uint8_t*)path, PATH_MAX+1);
-	if (!units || units == (size_t)-1)
+	ssize_t units = utf8_to_utf16(__utf16path, (const uint8_t*)path, PATH_MAX);
+	if (units <= 0)
 	{
 		r->_errno = EILSEQ;
 		return -1;
 	}
+	if (units >= PATH_MAX)
+	{
+		r->_errno = ENAMETOOLONG;
+		return -1;
+	}
 
 	romfs_file* file = searchForFile(curDir, __utf16path, units);
 	if (!file)
diff --git a/libctru/source/sdmc_dev.c b/libctru/source/sdmc_dev.c
index dc8d124..9ef6682 100644
--- a/libctru/source/sdmc_dev.c
+++ b/libctru/source/sdmc_dev.c
@@ -118,7 +118,7 @@ static const char*
 sdmc_fixpath(struct _reent *r,
              const char    *path)
 {
-  size_t        units;
+  ssize_t       units;
   uint32_t      code;
   const uint8_t *p = (const uint8_t*)path;
 
@@ -126,7 +126,7 @@ sdmc_fixpath(struct _reent *r,
   do
   {
     units = decode_utf8(&code, p);
-    if(units == (size_t)-1)
+    if(units < 0)
     {
       r->_errno = EILSEQ;
       return NULL;
@@ -145,7 +145,7 @@ sdmc_fixpath(struct _reent *r,
   do
   {
     units = decode_utf8(&code, p);
-    if(units == (size_t)-1)
+    if(units < 0)
     {
       r->_errno = EILSEQ;
       return NULL;
@@ -182,7 +182,7 @@ static const FS_Path
 sdmc_utf16path(struct _reent *r,
                const char    *path)
 {
-  size_t  units;
+  ssize_t units;
   FS_Path fspath;
 
   fspath.data = NULL;
@@ -190,14 +190,13 @@ sdmc_utf16path(struct _reent *r,
   if(sdmc_fixpath(r, path) == NULL)
     return fspath;
 
-  units = utf8_to_utf16(__utf16path, (const uint8_t*)__fixedpath, PATH_MAX+1);
-  if(units == (size_t)-1)
+  units = utf8_to_utf16(__utf16path, (const uint8_t*)__fixedpath, PATH_MAX);
+  if(units < 0)
   {
     r->_errno = EILSEQ;
     return fspath;
   }
-
-  if(__utf16path[PATH_MAX] != 0)
+  if(units >= PATH_MAX)
   {
     r->_errno = ENAMETOOLONG;
     return fspath;
@@ -220,7 +219,7 @@ static bool sdmcInitialised = false;
 /*! Initialize SDMC device */
 Result sdmcInit(void)
 {
-  size_t   units;
+  ssize_t  units;
   uint32_t code;
   char     *p;
   Result   rc = 0;
@@ -253,7 +252,7 @@ Result sdmcInit(void)
             do
             {
               units = decode_utf8(&code, (const uint8_t*)p);
-              if(units == (size_t)-1)
+              if(units < 0)
               {
                 last_slash = NULL;
                 break;
@@ -909,9 +908,9 @@ sdmc_dirnext(struct _reent *r,
              char          *filename,
              struct stat   *filestat)
 {
-  Result rc;
-  u32    entries;
-  size_t units;
+  Result  rc;
+  u32     entries;
+  ssize_t units;
 
   /* get pointer to our data */
   sdmc_dir_t *dir = (sdmc_dir_t*)(dirState->dirStruct);
@@ -938,13 +937,13 @@ sdmc_dirnext(struct _reent *r,
     /* convert name from UTF-16 to UTF-8 */
     memset(filename, 0, NAME_MAX);
     units = utf16_to_utf8((uint8_t*)filename, dir->entry_data.name, NAME_MAX);
-    if(units == (size_t)-1)
+    if(units < 0)
     {
       r->_errno = EILSEQ;
       return -1;
     }
 
-    if(filename[NAME_MAX-1] != 0)
+    if(units >= NAME_MAX)
     {
       r->_errno = ENAMETOOLONG;
       return -1;
diff --git a/libctru/source/util/utf/utf16_to_utf32.c b/libctru/source/util/utf/utf16_to_utf32.c
index 9ed21ac..3a9c5ce 100644
--- a/libctru/source/util/utf/utf16_to_utf32.c
+++ b/libctru/source/util/utf/utf16_to_utf32.c
@@ -1,11 +1,12 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf16_to_utf32(uint32_t       *out,
                const uint16_t *in,
                size_t         len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
   uint32_t code;
 
@@ -13,7 +14,7 @@ utf16_to_utf32(uint32_t       *out,
   {
     units = decode_utf16(&code, in);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(code > 0)
     {
@@ -23,11 +24,12 @@ utf16_to_utf32(uint32_t       *out,
       {
         if(rc < len)
           *out++ = code;
-        else
-          return rc;
       }
 
-      ++rc;
+      if(SSIZE_MAX - 1 <= rc)
+        ++rc;
+      else
+        return -1;
     }
   } while(code > 0);
 
diff --git a/libctru/source/util/utf/utf16_to_utf8.c b/libctru/source/util/utf/utf16_to_utf8.c
index 3567e63..9802dd3 100644
--- a/libctru/source/util/utf/utf16_to_utf8.c
+++ b/libctru/source/util/utf/utf16_to_utf8.c
@@ -1,20 +1,21 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf16_to_utf8(uint8_t        *out,
               const uint16_t *in,
               size_t         len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
   uint32_t code;
-  uint8_t encoded[4];
+  uint8_t  encoded[4];
 
   do
   {
     units = decode_utf16(&code, in);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(code > 0)
     {
@@ -22,7 +23,7 @@ utf16_to_utf8(uint8_t        *out,
 
       units = encode_utf8(encoded, code);
       if(units == -1)
-        return (size_t)-1;
+        return -1;
 
       if(out != NULL)
       {
@@ -36,11 +37,12 @@ utf16_to_utf8(uint8_t        *out,
           if(units > 3)
             *out++ = encoded[3];
         }
-        else
-          return rc;
       }
 
-      rc += units;
+      if(SSIZE_MAX - units <= rc)
+        rc += units;
+      else
+        return -1;
     }
   } while(code > 0);
 
diff --git a/libctru/source/util/utf/utf32_to_utf16.c b/libctru/source/util/utf/utf32_to_utf16.c
index 81ce203..6ac28f3 100644
--- a/libctru/source/util/utf/utf32_to_utf16.c
+++ b/libctru/source/util/utf/utf32_to_utf16.c
@@ -1,19 +1,20 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf32_to_utf16(uint16_t       *out,
                const uint32_t *in,
                size_t         len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
-  uint16_t  encoded[2];
+  uint16_t encoded[2];
 
   while(*in > 0)
   {
     units = encode_utf16(encoded, *in++);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(out != NULL)
     {
@@ -23,11 +24,12 @@ utf32_to_utf16(uint16_t       *out,
         if(units > 1)
           *out++ = encoded[1];
       }
-      else
-        return rc;
     }
 
-    rc += units;
+    if(SSIZE_MAX - units <= rc)
+      rc += units;
+    else
+      return -1;
   }
 
   return rc;
diff --git a/libctru/source/util/utf/utf32_to_utf8.c b/libctru/source/util/utf/utf32_to_utf8.c
index 80fc7c5..4bfd357 100644
--- a/libctru/source/util/utf/utf32_to_utf8.c
+++ b/libctru/source/util/utf/utf32_to_utf8.c
@@ -1,11 +1,12 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf32_to_utf8(uint8_t        *out,
               const uint32_t *in,
               size_t         len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
   uint8_t  encoded[4];
 
@@ -13,7 +14,7 @@ utf32_to_utf8(uint8_t        *out,
   {
     units = encode_utf8(encoded, *in++);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(out != NULL)
     {
@@ -27,11 +28,12 @@ utf32_to_utf8(uint8_t        *out,
         if(units > 3)
           *out++ = encoded[3];
       }
-      else
-        return rc;
     }
 
-    rc += units;
+    if(SSIZE_MAX - units <= rc)
+      rc += units;
+    else
+      return -1;
   }
 
   return rc;
diff --git a/libctru/source/util/utf/utf8_to_utf16.c b/libctru/source/util/utf/utf8_to_utf16.c
index 6991b3f..2a450af 100644
--- a/libctru/source/util/utf/utf8_to_utf16.c
+++ b/libctru/source/util/utf/utf8_to_utf16.c
@@ -1,11 +1,12 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf8_to_utf16(uint16_t      *out,
               const uint8_t *in,
               size_t        len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
   uint32_t code;
   uint16_t encoded[2];
@@ -14,7 +15,7 @@ utf8_to_utf16(uint16_t      *out,
   {
     units = decode_utf8(&code, in);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(code > 0)
     {
@@ -22,7 +23,7 @@ utf8_to_utf16(uint16_t      *out,
 
       units = encode_utf16(encoded, code);
       if(units == -1)
-        return (size_t)-1;
+        return -1;
 
       if(out != NULL)
       {
@@ -32,11 +33,12 @@ utf8_to_utf16(uint16_t      *out,
           if(units > 1)
             *out++ = encoded[1];
         }
-        else
-          return rc;
       }
 
-      rc += units;
+      if(SSIZE_MAX - units <= rc)
+        rc += units;
+      else
+        return -1;
     }
   } while(code > 0);
 
diff --git a/libctru/source/util/utf/utf8_to_utf32.c b/libctru/source/util/utf/utf8_to_utf32.c
index fac6acd..4a5ba81 100644
--- a/libctru/source/util/utf/utf8_to_utf32.c
+++ b/libctru/source/util/utf/utf8_to_utf32.c
@@ -1,11 +1,12 @@
+#include "3ds/types.h"
 #include "3ds/util/utf.h"
 
-size_t
+ssize_t
 utf8_to_utf32(uint32_t      *out,
               const uint8_t *in,
               size_t        len)
 {
-  size_t   rc = 0;
+  ssize_t  rc = 0;
   ssize_t  units;
   uint32_t code;
 
@@ -13,7 +14,7 @@ utf8_to_utf32(uint32_t      *out,
   {
     units = decode_utf8(&code, in);
     if(units == -1)
-      return (size_t)-1;
+      return -1;
 
     if(code > 0)
     {
@@ -23,11 +24,12 @@ utf8_to_utf32(uint32_t      *out,
       {
         if(rc < len)
           *out++ = code;
-        else
-          return rc;
       }
 
-      ++rc;
+      if(SSIZE_MAX - 1 <= rc)
+        ++rc;
+      else
+        return -1;
     }
   } while(code > 0);