diff --git a/t/Makefile.am b/t/Makefile.am
index abfe4491e..bd9e1152f 100644
--- a/t/Makefile.am
+++ b/t/Makefile.am
@@ -51,6 +51,7 @@ libexec_PROGRAMS = \
 	std/stdio-static.t \
 	sys/sysio-gotcha.t \
 	sys/sysio-static.t \
+	multi/stdio-static.t \
 	unifyfs_unmount.t
 
 
@@ -153,6 +154,12 @@ std_stdio_static_t_CPPFLAGS = $(test_cppflags)
 std_stdio_static_t_LDADD = $(test_static_ldadd)
 std_stdio_static_t_LDFLAGS = $(test_static_ldflags)
 
+multi_stdio_static_t_SOURCES = multi/open_multi.c
+
+multi_stdio_static_t_CPPFLAGS = $(test_cppflags)
+multi_stdio_static_t_LDADD = $(test_static_ldadd)
+multi_stdio_static_t_LDFLAGS = $(test_static_ldflags)
+
 unifyfs_unmount_t_SOURCES = unifyfs_unmount.c
 unifyfs_unmount_t_CPPFLAGS = $(test_cppflags)
 unifyfs_unmount_t_LDADD = $(test_static_ldadd)
diff --git a/open_multi.c b/t/multi/open_multi.c
similarity index 61%
rename from open_multi.c
rename to t/multi/open_multi.c
index 921eee1f9..e0413df6d 100644
--- a/open_multi.c
+++ b/t/multi/open_multi.c
@@ -1,14 +1,30 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
 
+#include "t/lib/tap.h"
+#include "t/lib/testutil.h"
+
 #include "mpi.h"
 #include "unifyfs.h"
 
+#define all_ok(condition, comm, ...) \
+    do { \
+        int input = (int) condition; \
+        int output; \
+        MPI_Allreduce(&input, &output, 1, MPI_INT, MPI_LAND, comm); \
+        int rank; \
+        MPI_Comm_rank(comm, &rank); \
+        if (rank == 0) { \
+            ok_at_loc(__FILE__, __LINE__, output, __VA_ARGS__, NULL); \
+        } \
+    } while (0)
+
 int mpi_sum(int val)
 {
     int sum;
@@ -37,7 +53,7 @@ int main(int argc, char* argv[])
     MPI_Comm_rank(MPI_COMM_WORLD, &rank);
     MPI_Comm_size(MPI_COMM_WORLD, &ranks);
 
-    int ret = unifyfs_mount(mountpoint, rank, ranks, 0);
+    unifyfs_mount(mountpoint, rank, ranks, 0);
 
     char file[256];
     sprintf(file, "%s/testfile", mountpoint);
@@ -53,7 +69,7 @@ int main(int argc, char* argv[])
     errno = 0;
     int success = 0;
     int eexist  = 0;
-    int fd = open(file, O_WRONLY | O_CREAT | O_EXCL);
+    int fd = open(file, O_WRONLY | O_CREAT | O_EXCL, 0700);
     if (fd >= 0) {
         success = 1;
         close(fd);
@@ -63,15 +79,13 @@ int main(int argc, char* argv[])
 
     /* one rank should win */
     int sum = mpi_sum(success);
-    if (sum != 1) {
-    }
+    all_ok(sum == 1, MPI_COMM_WORLD,
+        "More than one process opened file in exclusive mode");
 
     /* all others should get EEXIST */
     sum = mpi_sum(eexist);
-    if (sum != (ranks - 1)) {
-    }
-
-    MPI_Barrier(MPI_COMM_WORLD);
+    all_ok(sum == (ranks - 1), MPI_COMM_WORLD,
+        "All but one process should get EEXIST when opening file in exclusive mode");
 
     /* all delete,
      * one rank should win, all others should get ENOENT */
@@ -87,31 +101,26 @@ int main(int argc, char* argv[])
 
     /* one winner */
     sum = mpi_sum(success);
-    if (sum != 1) {
-    }
+    all_ok(sum == 1, MPI_COMM_WORLD,
+        "More than one process got success on unlink of the same file");
 
     /* everyone else should get ENOENT */
     sum = mpi_sum(enoent);
-    if (sum != (ranks - 1)) {
-    }
-
-    MPI_Barrier(MPI_COMM_WORLD);
+    all_ok(sum == (ranks - 1), MPI_COMM_WORLD,
+        "All but one process should get ENOENT when unlinking the same file");
 
     /* all create, this time not exclusive */
     errno = 0;
     success = 0;
-    fd = open(file, O_WRONLY | O_CREAT | O_TRUNC);
+    fd = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0700);
     if (fd >= 0) {
         success = 1;
         close(fd);
     }
 
     /* all should succeed */
-    sum = mpi_sum(success);
-    if (sum != ranks) {
-    }
-
-    MPI_Barrier(MPI_COMM_WORLD);
+    all_ok(success, MPI_COMM_WORLD,
+        "All processes should open file with O_CREAT and not O_EXCL");
 
     /* open file for writing */
     errno = 0;
@@ -123,9 +132,8 @@ int main(int argc, char* argv[])
     }
 
     /* all should succeeed */
-    sum = mpi_sum(success);
-    if (sum != ranks) {
-    }
+    all_ok(success, MPI_COMM_WORLD,
+        "All processes should open file with O_CREAT for writing");
 
     MPI_Barrier(MPI_COMM_WORLD);
 
@@ -135,10 +143,15 @@ int main(int argc, char* argv[])
 
     /* have all ranks write to a different section of the file,
      * then open file with truncate on one rank to verify size change */
-    fd = open(file, O_WRONLY | O_CREAT);
+    success = 0;
+    fd = open(file, O_WRONLY | O_CREAT, 0700);
     if (fd >= 0) {
+        success = 1;
         off_t offset = (off_t) (rank * bufsize);
-        pwrite(fd, buf, bufsize, offset);
+        ssize_t nwritten = pwrite(fd, buf, bufsize, offset);
+        if (nwritten != bufsize) {
+            success = 0;
+        }
         fsync(fd);
         close(fd);
     }
@@ -146,19 +159,21 @@ int main(int argc, char* argv[])
     if (rank == 0) {
         /* all ranks should have written some data */
         off_t size = getsize(file);
-        if (size != bufsize * ranks) {
-        }
+        ok(size == bufsize * ranks,
+            "File size %lu does not match expected size %lu",
+            size, bufsize * ranks);
 
         /* create file with truncate */
-        fd = open(file, O_WRONLY | O_CREAT | O_TRUNC);
+        fd = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0700);
         if (fd >= 0) {
             close(fd);
         }
 
         /* now file should be 0 length again */
         size = getsize(file);
-        if (size != 0) {
-        }
+        ok(size == 0,
+            "File size %lu does not match expected size %lu",
+            size, 0);
     }
     MPI_Barrier(MPI_COMM_WORLD);