diff --git a/otherlibs/unix/fchmod.c b/otherlibs/unix/fchmod.c
index a6e8ee903..ba5e55721 100644
--- a/otherlibs/unix/fchmod.c
+++ b/otherlibs/unix/fchmod.c
@@ -21,7 +21,11 @@
 
 CAMLprim value unix_fchmod(value fd, value perm)
 {
-  if (fchmod(Int_val(fd), Int_val(perm)) == -1) uerror("fchmod", Nothing);
+  int result;
+  caml_enter_blocking_section();
+  result = fchmod(Int_val(fd), Int_val(perm));
+  caml_leave_blocking_section();
+  if (result == -1) uerror("fchmod", Nothing);
   return Val_unit;
 }
 
diff --git a/otherlibs/unix/fchown.c b/otherlibs/unix/fchown.c
index 574d3c42b..71e827d76 100644
--- a/otherlibs/unix/fchown.c
+++ b/otherlibs/unix/fchown.c
@@ -19,8 +19,11 @@
 
 CAMLprim value unix_fchown(value fd, value uid, value gid)
 {
-  if (fchown(Int_val(fd), Int_val(uid), Int_val(gid)) == -1)
-    uerror("fchown", Nothing);
+  int result;
+  caml_enter_blocking_section();
+  result = fchown(Int_val(fd), Int_val(uid), Int_val(gid));
+  caml_leave_blocking_section();
+  if (result == -1) uerror("fchown", Nothing);
   return Val_unit;
 }
 
diff --git a/otherlibs/unix/ftruncate.c b/otherlibs/unix/ftruncate.c
index f539a6450..5b401fde0 100644
--- a/otherlibs/unix/ftruncate.c
+++ b/otherlibs/unix/ftruncate.c
@@ -24,15 +24,22 @@
 
 CAMLprim value unix_ftruncate(value fd, value len)
 {
-  if (ftruncate(Int_val(fd), Long_val(len)) == -1)
-    uerror("ftruncate", Nothing);
+  int result;
+  caml_enter_blocking_section();
+  result = ftruncate(Int_val(fd), Long_val(len));
+  caml_leave_blocking_section();
+  if (result == -1) uerror("ftruncate", Nothing);
   return Val_unit;
 }
 
 CAMLprim value unix_ftruncate_64(value fd, value len)
 {
-  if (ftruncate(Int_val(fd), File_offset_val(len)) == -1)
-    uerror("ftruncate", Nothing);
+  int result;
+  file_offset ofs = File_offset_val(len);
+  caml_enter_blocking_section();
+  result = ftruncate(Int_val(fd), ofs);
+  caml_leave_blocking_section();
+  if (result == -1) uerror("ftruncate", Nothing);
   return Val_unit;
 }