diff --git a/lib/std/syncio.nim b/lib/std/syncio.nim
index dadad97918..d406c254f9 100644
--- a/lib/std/syncio.nim
+++ b/lib/std/syncio.nim
@@ -105,7 +105,7 @@ proc c_feof(f: File): cint {.
   importc: "feof", header: "<stdio.h>".}
 
 when not declared(c_fwrite):
-  proc c_fwrite(buf: pointer, size, n: csize_t, f: File): cint {.
+  proc c_fwrite(buf: pointer, size, n: csize_t, f: File): csize_t {.
     importc: "fwrite", header: "<stdio.h>".}
 
 # C routine that is used here:
diff --git a/lib/system/ansi_c.nim b/lib/system/ansi_c.nim
index ae4b905048..a8a4bd7678 100644
--- a/lib/system/ansi_c.nim
+++ b/lib/system/ansi_c.nim
@@ -211,7 +211,7 @@ else:
   proc c_realloc*(p: pointer, newsize: csize_t): pointer {.
     importc: "realloc", header: "<stdlib.h>".}
 
-proc c_fwrite*(buf: pointer, size, n: csize_t, f: CFilePtr): cint {.
+proc c_fwrite*(buf: pointer, size, n: csize_t, f: CFilePtr): csize_t {.
   importc: "fwrite", header: "<stdio.h>".}
 
 proc c_fflush*(f: CFilePtr): cint {.