diff --git a/src/fuchsia/mod.rs b/src/fuchsia/mod.rs
index bf1a543609406..af334ea455c3b 100644
--- a/src/fuchsia/mod.rs
+++ b/src/fuchsia/mod.rs
@@ -380,7 +380,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_ulong; FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct tm {
@@ -1827,7 +1827,7 @@ pub const SS_DISABLE: ::c_int = 2;
 
 pub const PATH_MAX: ::c_int = 4096;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const EPOLLIN: ::c_int = 0x1;
 pub const EPOLLPRI: ::c_int = 0x2;
diff --git a/src/unix/aix/mod.rs b/src/unix/aix/mod.rs
index 0d7c2ed1d1b65..2ea524954c6f3 100644
--- a/src/unix/aix/mod.rs
+++ b/src/unix/aix/mod.rs
@@ -2140,7 +2140,7 @@ pub const POWER_8: ::c_int = 0x10000;
 pub const POWER_9: ::c_int = 0x20000;
 
 // sys/time.h
-pub const FD_SETSIZE: usize = 65534;
+pub const FD_SETSIZE: ::c_int = 65534;
 pub const TIMEOFDAY: ::c_int = 9;
 pub const CLOCK_REALTIME: ::clockid_t = TIMEOFDAY as clockid_t;
 pub const CLOCK_MONOTONIC: ::clockid_t = 10;
diff --git a/src/unix/bsd/apple/mod.rs b/src/unix/bsd/apple/mod.rs
index 21efb23eb2e32..f7437a9637e9f 100644
--- a/src/unix/bsd/apple/mod.rs
+++ b/src/unix/bsd/apple/mod.rs
@@ -4364,7 +4364,7 @@ pub const OS_SIGNPOST_INTERVAL_END: ::os_signpost_type_t = 0x02;
 pub const MINSIGSTKSZ: ::size_t = 32768;
 pub const SIGSTKSZ: ::size_t = 131072;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const ST_NOSUID: ::c_ulong = 2;
 
diff --git a/src/unix/bsd/freebsdlike/mod.rs b/src/unix/bsd/freebsdlike/mod.rs
index d2cd7794b107e..faeda848d07c9 100644
--- a/src/unix/bsd/freebsdlike/mod.rs
+++ b/src/unix/bsd/freebsdlike/mod.rs
@@ -1204,7 +1204,7 @@ pub const SCHED_FIFO: ::c_int = 1;
 pub const SCHED_OTHER: ::c_int = 2;
 pub const SCHED_RR: ::c_int = 3;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const ST_NOSUID: ::c_ulong = 2;
 
diff --git a/src/unix/bsd/mod.rs b/src/unix/bsd/mod.rs
index 6ce041357ebee..c3563abd4ebe2 100644
--- a/src/unix/bsd/mod.rs
+++ b/src/unix/bsd/mod.rs
@@ -59,10 +59,10 @@ s! {
     pub struct fd_set {
         #[cfg(all(target_pointer_width = "64",
                   any(target_os = "freebsd", target_os = "dragonfly")))]
-        fds_bits: [i64; FD_SETSIZE / 64],
+        fds_bits: [i64; FD_SETSIZE as usize / 64],
         #[cfg(not(all(target_pointer_width = "64",
                       any(target_os = "freebsd", target_os = "dragonfly"))))]
-        fds_bits: [i32; FD_SETSIZE / 32],
+        fds_bits: [i32; FD_SETSIZE as usize / 32],
     }
 
     pub struct tm {
diff --git a/src/unix/bsd/netbsdlike/netbsd/mod.rs b/src/unix/bsd/netbsdlike/netbsd/mod.rs
index 45f42d74165ee..0c9daabceb214 100644
--- a/src/unix/bsd/netbsdlike/netbsd/mod.rs
+++ b/src/unix/bsd/netbsdlike/netbsd/mod.rs
@@ -1860,7 +1860,7 @@ pub const _SC_SCHED_RT_TS: ::c_int = 2001;
 pub const _SC_SCHED_PRI_MIN: ::c_int = 2002;
 pub const _SC_SCHED_PRI_MAX: ::c_int = 2003;
 
-pub const FD_SETSIZE: usize = 0x100;
+pub const FD_SETSIZE: ::c_int = 0x100;
 
 pub const ST_NOSUID: ::c_ulong = 8;
 
diff --git a/src/unix/bsd/netbsdlike/openbsd/mod.rs b/src/unix/bsd/netbsdlike/openbsd/mod.rs
index 17dfa6571568f..a8ebdf53cf98c 100644
--- a/src/unix/bsd/netbsdlike/openbsd/mod.rs
+++ b/src/unix/bsd/netbsdlike/openbsd/mod.rs
@@ -1374,7 +1374,7 @@ pub const _SC_AVPHYS_PAGES: ::c_int = 501;
 pub const _SC_NPROCESSORS_CONF: ::c_int = 502;
 pub const _SC_NPROCESSORS_ONLN: ::c_int = 503;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const SCHED_FIFO: ::c_int = 1;
 pub const SCHED_OTHER: ::c_int = 2;
diff --git a/src/unix/haiku/mod.rs b/src/unix/haiku/mod.rs
index 00e9523c9865a..e742878f824b9 100644
--- a/src/unix/haiku/mod.rs
+++ b/src/unix/haiku/mod.rs
@@ -1100,7 +1100,7 @@ pub const SA_ONESHOT: ::c_int = SA_RESETHAND;
 pub const SS_ONSTACK: ::c_int = 0x1;
 pub const SS_DISABLE: ::c_int = 0x2;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const RTLD_LOCAL: ::c_int = 0x0;
 pub const RTLD_NOW: ::c_int = 0x1;
diff --git a/src/unix/hurd/mod.rs b/src/unix/hurd/mod.rs
index 75a272e4dd7de..9ab0e0bbcd1d8 100644
--- a/src/unix/hurd/mod.rs
+++ b/src/unix/hurd/mod.rs
@@ -1084,7 +1084,7 @@ pub const SHM_UNLOCK: ::c_int = 12;
 pub const STDIN_FILENO: ::c_int = 0;
 pub const STDOUT_FILENO: ::c_int = 1;
 pub const STDERR_FILENO: ::c_int = 2;
-pub const __FD_SETSIZE: usize = 256;
+pub const __FD_SETSIZE: ::c_int = 256;
 pub const R_OK: ::c_int = 4;
 pub const W_OK: ::c_int = 2;
 pub const X_OK: ::c_int = 1;
@@ -1129,7 +1129,7 @@ pub const PDP_ENDIAN: usize = 3412;
 pub const BYTE_ORDER: usize = 1234;
 
 // sys/select.h
-pub const FD_SETSIZE: usize = 256;
+pub const FD_SETSIZE: ::c_int = 256;
 pub const __SIZEOF_PTHREAD_MUTEX_T: usize = 32;
 pub const __SIZEOF_PTHREAD_ATTR_T: usize = 32;
 pub const __SIZEOF_PTHREAD_RWLOCK_T: usize = 28;
@@ -1452,8 +1452,8 @@ pub const _POSIX_MQ_OPEN_MAX: usize = 8;
 pub const _POSIX_MQ_PRIO_MAX: usize = 32;
 pub const _POSIX_NAME_MAX: usize = 14;
 pub const _POSIX_NGROUPS_MAX: usize = 8;
-pub const _POSIX_OPEN_MAX: usize = 20;
-pub const _POSIX_FD_SETSIZE: usize = 20;
+pub const _POSIX_OPEN_MAX: ::c_int = 20;
+pub const _POSIX_FD_SETSIZE: ::c_int = 20;
 pub const _POSIX_PATH_MAX: usize = 256;
 pub const _POSIX_PIPE_BUF: usize = 512;
 pub const _POSIX_RE_DUP_MAX: usize = 255;
diff --git a/src/unix/linux_like/mod.rs b/src/unix/linux_like/mod.rs
index 26fd68de49c7c..5ec1eb3f8b378 100644
--- a/src/unix/linux_like/mod.rs
+++ b/src/unix/linux_like/mod.rs
@@ -89,7 +89,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_ulong; FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct tm {
@@ -1069,7 +1069,7 @@ pub const PATH_MAX: ::c_int = 4096;
 
 pub const UIO_MAXIOV: ::c_int = 1024;
 
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 pub const EPOLLIN: ::c_int = 0x1;
 pub const EPOLLPRI: ::c_int = 0x2;
diff --git a/src/unix/newlib/mod.rs b/src/unix/newlib/mod.rs
index a572cc38bfda9..e8e7e191819dd 100644
--- a/src/unix/newlib/mod.rs
+++ b/src/unix/newlib/mod.rs
@@ -171,7 +171,7 @@ s! {
     }
 
     pub struct fd_set { // Unverified
-        fds_bits: [::c_ulong; FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct passwd { // Unverified
@@ -283,11 +283,11 @@ pub const PTHREAD_MUTEX_ERRORCHECK: ::c_int = 2;
 
 cfg_if! {
     if #[cfg(any(target_os = "horizon", target_os = "espidf"))] {
-        pub const FD_SETSIZE: usize = 64;
+        pub const FD_SETSIZE: ::c_int = 64;
     } else if #[cfg(target_os = "vita")] {
-        pub const FD_SETSIZE: usize = 256;
+        pub const FD_SETSIZE: ::c_int = 256;
     } else {
-        pub const FD_SETSIZE: usize = 1024;
+        pub const FD_SETSIZE: ::c_int = 1024;
     }
 }
 // intentionally not public, only used for fd_set
diff --git a/src/unix/nto/mod.rs b/src/unix/nto/mod.rs
index 91e531ffd7961..79c45e8413960 100644
--- a/src/unix/nto/mod.rs
+++ b/src/unix/nto/mod.rs
@@ -154,7 +154,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_uint; 2 * FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_uint; 2 * FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct tm {
@@ -1383,7 +1383,7 @@ pub const PATH_MAX: ::c_int = 1024;
 
 pub const UIO_MAXIOV: ::c_int = 1024;
 
-pub const FD_SETSIZE: usize = 256;
+pub const FD_SETSIZE: ::c_int = 256;
 
 pub const TCIOFF: ::c_int = 0x0002;
 pub const TCION: ::c_int = 0x0003;
diff --git a/src/unix/redox/mod.rs b/src/unix/redox/mod.rs
index cd219a777288f..a0ead95e98c12 100644
--- a/src/unix/redox/mod.rs
+++ b/src/unix/redox/mod.rs
@@ -116,7 +116,7 @@ s! {
     }
 
     pub struct fd_set {
-        fds_bits: [::c_ulong; ::FD_SETSIZE / ULONG_SIZE],
+        fds_bits: [::c_ulong; ::FD_SETSIZE as usize / ULONG_SIZE],
     }
 
     pub struct in_addr {
@@ -758,7 +758,7 @@ pub const MS_INVALIDATE: ::c_int = 0x0002;
 pub const MS_SYNC: ::c_int = 0x0004;
 
 // sys/select.h
-pub const FD_SETSIZE: usize = 1024;
+pub const FD_SETSIZE: ::c_int = 1024;
 
 // sys/socket.h
 pub const AF_INET: ::c_int = 2;
diff --git a/src/unix/solarish/mod.rs b/src/unix/solarish/mod.rs
index a3fa56a65a67e..ccaa012088b2d 100644
--- a/src/unix/solarish/mod.rs
+++ b/src/unix/solarish/mod.rs
@@ -518,9 +518,9 @@ s_no_extra_traits! {
 
     pub struct fd_set {
         #[cfg(target_pointer_width = "64")]
-        fds_bits: [i64; FD_SETSIZE / 64],
+        fds_bits: [i64; FD_SETSIZE as usize / 64],
         #[cfg(target_pointer_width = "32")]
-        fds_bits: [i32; FD_SETSIZE / 32],
+        fds_bits: [i32; FD_SETSIZE as usize / 32],
     }
 
     pub struct sockaddr_storage {
@@ -1250,9 +1250,9 @@ pub const IPV6_V6ONLY: ::c_int = 0x27;
 
 cfg_if! {
     if #[cfg(target_pointer_width = "64")] {
-        pub const FD_SETSIZE: usize = 65536;
+        pub const FD_SETSIZE: ::c_int = 65536;
     } else {
-        pub const FD_SETSIZE: usize = 1024;
+        pub const FD_SETSIZE: ::c_int = 1024;
     }
 }
 
diff --git a/src/wasi.rs b/src/wasi.rs
index 1a855e0e0fe77..16d859ab5d8f3 100644
--- a/src/wasi.rs
+++ b/src/wasi.rs
@@ -206,7 +206,7 @@ pub const F_SETFD: c_int = 2;
 pub const F_GETFL: c_int = 3;
 pub const F_SETFL: c_int = 4;
 pub const FD_CLOEXEC: c_int = 1;
-pub const FD_SETSIZE: size_t = 1024;
+pub const FD_SETSIZE: c_int = 1024;
 pub const O_APPEND: c_int = 0x0001;
 pub const O_DSYNC: c_int = 0x0002;
 pub const O_NONBLOCK: c_int = 0x0004;