diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
index c2fbaae..5bf5f28 100644
--- a/qemu-io-cmds.c
+++ b/qemu-io-cmds.c
@@ -65,13 +65,13 @@
     return 1;
 }
 
-static void command(BlockBackend *blk, const cmdinfo_t *ct, int argc,
-                    char **argv)
+static int command(BlockBackend *blk, const cmdinfo_t *ct, int argc,
+                   char **argv)
 {
     char *cmd = argv[0];
 
     if (!init_check_command(blk, ct)) {
-        return;
+        return -EINVAL;
     }
 
     if (argc - 1 < ct->argmin || (ct->argmax != -1 && argc - 1 > ct->argmax)) {
@@ -88,7 +88,7 @@
                     "bad argument count %d to %s, expected between %d and %d arguments\n",
                     argc-1, cmd, ct->argmin, ct->argmax);
         }
-        return;
+        return -EINVAL;
     }
 
     /* Request additional permissions if necessary for this command. The caller
@@ -108,13 +108,13 @@
             ret = blk_set_perm(blk, new_perm, orig_shared_perm, &local_err);
             if (ret < 0) {
                 error_report_err(local_err);
-                return;
+                return ret;
             }
         }
     }
 
     optind = 0;
-    ct->cfunc(blk, argc, argv);
+    return ct->cfunc(blk, argc, argv);
 }
 
 static const cmdinfo_t *find_command(const char *cmd)
@@ -633,7 +633,7 @@
 "\n");
 }
 
-static void read_f(BlockBackend *blk, int argc, char **argv);
+static int read_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t read_cmd = {
     .name       = "read",
@@ -646,12 +646,12 @@
     .help       = read_help,
 };
 
-static void read_f(BlockBackend *blk, int argc, char **argv)
+static int read_f(BlockBackend *blk, int argc, char **argv)
 {
     struct timeval t1, t2;
     bool Cflag = false, qflag = false, vflag = false;
     bool Pflag = false, sflag = false, lflag = false, bflag = false;
-    int c, cnt;
+    int c, cnt, ret;
     char *buf;
     int64_t offset;
     int64_t count;
@@ -673,7 +673,7 @@
             pattern_count = cvtnum(optarg);
             if (pattern_count < 0) {
                 print_cvtnum_err(pattern_count, optarg);
-                return;
+                return pattern_count;
             }
             break;
         case 'p':
@@ -683,7 +683,7 @@
             Pflag = true;
             pattern = parse_pattern(optarg);
             if (pattern < 0) {
-                return;
+                return -EINVAL;
             }
             break;
         case 'q':
@@ -694,7 +694,7 @@
             pattern_offset = cvtnum(optarg);
             if (pattern_offset < 0) {
                 print_cvtnum_err(pattern_offset, optarg);
-                return;
+                return pattern_offset;
             }
             break;
         case 'v':
@@ -702,35 +702,35 @@
             break;
         default:
             qemuio_command_usage(&read_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind != argc - 2) {
         qemuio_command_usage(&read_cmd);
-        return;
+        return -EINVAL;
     }
 
     offset = cvtnum(argv[optind]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[optind]);
-        return;
+        return offset;
     }
 
     optind++;
     count = cvtnum(argv[optind]);
     if (count < 0) {
         print_cvtnum_err(count, argv[optind]);
-        return;
+        return count;
     } else if (count > BDRV_REQUEST_MAX_BYTES) {
         printf("length cannot exceed %" PRIu64 ", given %s\n",
                (uint64_t)BDRV_REQUEST_MAX_BYTES, argv[optind]);
-        return;
+        return -EINVAL;
     }
 
     if (!Pflag && (lflag || sflag)) {
         qemuio_command_usage(&read_cmd);
-        return;
+        return -EINVAL;
     }
 
     if (!lflag) {
@@ -739,19 +739,19 @@
 
     if ((pattern_count < 0) || (pattern_count + pattern_offset > count))  {
         printf("pattern verification range exceeds end of read data\n");
-        return;
+        return -EINVAL;
     }
 
     if (bflag) {
         if (!QEMU_IS_ALIGNED(offset, BDRV_SECTOR_SIZE)) {
             printf("%" PRId64 " is not a sector-aligned value for 'offset'\n",
                    offset);
-            return;
+            return -EINVAL;
         }
         if (!QEMU_IS_ALIGNED(count, BDRV_SECTOR_SIZE)) {
             printf("%"PRId64" is not a sector-aligned value for 'count'\n",
                    count);
-            return;
+            return -EINVAL;
         }
     }
 
@@ -759,16 +759,19 @@
 
     gettimeofday(&t1, NULL);
     if (bflag) {
-        cnt = do_load_vmstate(blk, buf, offset, count, &total);
+        ret = do_load_vmstate(blk, buf, offset, count, &total);
     } else {
-        cnt = do_pread(blk, buf, offset, count, &total);
+        ret = do_pread(blk, buf, offset, count, &total);
     }
     gettimeofday(&t2, NULL);
 
-    if (cnt < 0) {
-        printf("read failed: %s\n", strerror(-cnt));
+    if (ret < 0) {
+        printf("read failed: %s\n", strerror(-ret));
         goto out;
     }
+    cnt = ret;
+
+    ret = 0;
 
     if (Pflag) {
         void *cmp_buf = g_malloc(pattern_count);
@@ -777,6 +780,7 @@
             printf("Pattern verification failed at offset %"
                    PRId64 ", %"PRId64" bytes\n",
                    offset + pattern_offset, pattern_count);
+            ret = -EINVAL;
         }
         g_free(cmp_buf);
     }
@@ -795,6 +799,7 @@
 
 out:
     qemu_io_free(buf);
+    return ret;
 }
 
 static void readv_help(void)
@@ -816,7 +821,7 @@
 "\n");
 }
 
-static void readv_f(BlockBackend *blk, int argc, char **argv);
+static int readv_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t readv_cmd = {
     .name       = "readv",
@@ -828,11 +833,11 @@
     .help       = readv_help,
 };
 
-static void readv_f(BlockBackend *blk, int argc, char **argv)
+static int readv_f(BlockBackend *blk, int argc, char **argv)
 {
     struct timeval t1, t2;
     bool Cflag = false, qflag = false, vflag = false;
-    int c, cnt;
+    int c, cnt, ret;
     char *buf;
     int64_t offset;
     /* Some compilers get confused and warn if this is not initialized.  */
@@ -851,7 +856,7 @@
             Pflag = true;
             pattern = parse_pattern(optarg);
             if (pattern < 0) {
-                return;
+                return -EINVAL;
             }
             break;
         case 'q':
@@ -862,37 +867,40 @@
             break;
         default:
             qemuio_command_usage(&readv_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind > argc - 2) {
         qemuio_command_usage(&readv_cmd);
-        return;
+        return -EINVAL;
     }
 
 
     offset = cvtnum(argv[optind]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[optind]);
-        return;
+        return offset;
     }
     optind++;
 
     nr_iov = argc - optind;
     buf = create_iovec(blk, &qiov, &argv[optind], nr_iov, 0xab);
     if (buf == NULL) {
-        return;
+        return -EINVAL;
     }
 
     gettimeofday(&t1, NULL);
-    cnt = do_aio_readv(blk, &qiov, offset, &total);
+    ret = do_aio_readv(blk, &qiov, offset, &total);
     gettimeofday(&t2, NULL);
 
-    if (cnt < 0) {
-        printf("readv failed: %s\n", strerror(-cnt));
+    if (ret < 0) {
+        printf("readv failed: %s\n", strerror(-ret));
         goto out;
     }
+    cnt = ret;
+
+    ret = 0;
 
     if (Pflag) {
         void *cmp_buf = g_malloc(qiov.size);
@@ -900,6 +908,7 @@
         if (memcmp(buf, cmp_buf, qiov.size)) {
             printf("Pattern verification failed at offset %"
                    PRId64 ", %zd bytes\n", offset, qiov.size);
+            ret = -EINVAL;
         }
         g_free(cmp_buf);
     }
@@ -919,6 +928,7 @@
 out:
     qemu_iovec_destroy(&qiov);
     qemu_io_free(buf);
+    return ret;
 }
 
 static void write_help(void)
@@ -944,7 +954,7 @@
 "\n");
 }
 
-static void write_f(BlockBackend *blk, int argc, char **argv);
+static int write_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t write_cmd = {
     .name       = "write",
@@ -958,13 +968,13 @@
     .help       = write_help,
 };
 
-static void write_f(BlockBackend *blk, int argc, char **argv)
+static int write_f(BlockBackend *blk, int argc, char **argv)
 {
     struct timeval t1, t2;
     bool Cflag = false, qflag = false, bflag = false;
     bool Pflag = false, zflag = false, cflag = false;
     int flags = 0;
-    int c, cnt;
+    int c, cnt, ret;
     char *buf = NULL;
     int64_t offset;
     int64_t count;
@@ -993,7 +1003,7 @@
             Pflag = true;
             pattern = parse_pattern(optarg);
             if (pattern < 0) {
-                return;
+                return -EINVAL;
             }
             break;
         case 'q':
@@ -1007,63 +1017,63 @@
             break;
         default:
             qemuio_command_usage(&write_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind != argc - 2) {
         qemuio_command_usage(&write_cmd);
-        return;
+        return -EINVAL;
     }
 
     if (bflag && zflag) {
         printf("-b and -z cannot be specified at the same time\n");
-        return;
+        return -EINVAL;
     }
 
     if ((flags & BDRV_REQ_FUA) && (bflag || cflag)) {
         printf("-f and -b or -c cannot be specified at the same time\n");
-        return;
+        return -EINVAL;
     }
 
     if ((flags & BDRV_REQ_MAY_UNMAP) && !zflag) {
         printf("-u requires -z to be specified\n");
-        return;
+        return -EINVAL;
     }
 
     if (zflag && Pflag) {
         printf("-z and -P cannot be specified at the same time\n");
-        return;
+        return -EINVAL;
     }
 
     offset = cvtnum(argv[optind]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[optind]);
-        return;
+        return offset;
     }
 
     optind++;
     count = cvtnum(argv[optind]);
     if (count < 0) {
         print_cvtnum_err(count, argv[optind]);
-        return;
+        return count;
     } else if (count > BDRV_REQUEST_MAX_BYTES) {
         printf("length cannot exceed %" PRIu64 ", given %s\n",
                (uint64_t)BDRV_REQUEST_MAX_BYTES, argv[optind]);
-        return;
+        return -EINVAL;
     }
 
     if (bflag || cflag) {
         if (!QEMU_IS_ALIGNED(offset, BDRV_SECTOR_SIZE)) {
             printf("%" PRId64 " is not a sector-aligned value for 'offset'\n",
                    offset);
-            return;
+            return -EINVAL;
         }
 
         if (!QEMU_IS_ALIGNED(count, BDRV_SECTOR_SIZE)) {
             printf("%"PRId64" is not a sector-aligned value for 'count'\n",
                    count);
-            return;
+            return -EINVAL;
         }
     }
 
@@ -1073,20 +1083,23 @@
 
     gettimeofday(&t1, NULL);
     if (bflag) {
-        cnt = do_save_vmstate(blk, buf, offset, count, &total);
+        ret = do_save_vmstate(blk, buf, offset, count, &total);
     } else if (zflag) {
-        cnt = do_co_pwrite_zeroes(blk, offset, count, flags, &total);
+        ret = do_co_pwrite_zeroes(blk, offset, count, flags, &total);
     } else if (cflag) {
-        cnt = do_write_compressed(blk, buf, offset, count, &total);
+        ret = do_write_compressed(blk, buf, offset, count, &total);
     } else {
-        cnt = do_pwrite(blk, buf, offset, count, flags, &total);
+        ret = do_pwrite(blk, buf, offset, count, flags, &total);
     }
     gettimeofday(&t2, NULL);
 
-    if (cnt < 0) {
-        printf("write failed: %s\n", strerror(-cnt));
+    if (ret < 0) {
+        printf("write failed: %s\n", strerror(-ret));
         goto out;
     }
+    cnt = ret;
+
+    ret = 0;
 
     if (qflag) {
         goto out;
@@ -1100,6 +1113,7 @@
     if (!zflag) {
         qemu_io_free(buf);
     }
+    return ret;
 }
 
 static void
@@ -1121,7 +1135,7 @@
 "\n");
 }
 
-static void writev_f(BlockBackend *blk, int argc, char **argv);
+static int writev_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t writev_cmd = {
     .name       = "writev",
@@ -1134,12 +1148,12 @@
     .help       = writev_help,
 };
 
-static void writev_f(BlockBackend *blk, int argc, char **argv)
+static int writev_f(BlockBackend *blk, int argc, char **argv)
 {
     struct timeval t1, t2;
     bool Cflag = false, qflag = false;
     int flags = 0;
-    int c, cnt;
+    int c, cnt, ret;
     char *buf;
     int64_t offset;
     /* Some compilers get confused and warn if this is not initialized.  */
@@ -1162,41 +1176,44 @@
         case 'P':
             pattern = parse_pattern(optarg);
             if (pattern < 0) {
-                return;
+                return -EINVAL;
             }
             break;
         default:
             qemuio_command_usage(&writev_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind > argc - 2) {
         qemuio_command_usage(&writev_cmd);
-        return;
+        return -EINVAL;
     }
 
     offset = cvtnum(argv[optind]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[optind]);
-        return;
+        return offset;
     }
     optind++;
 
     nr_iov = argc - optind;
     buf = create_iovec(blk, &qiov, &argv[optind], nr_iov, pattern);
     if (buf == NULL) {
-        return;
+        return -EINVAL;
     }
 
     gettimeofday(&t1, NULL);
-    cnt = do_aio_writev(blk, &qiov, offset, flags, &total);
+    ret = do_aio_writev(blk, &qiov, offset, flags, &total);
     gettimeofday(&t2, NULL);
 
-    if (cnt < 0) {
-        printf("writev failed: %s\n", strerror(-cnt));
+    if (ret < 0) {
+        printf("writev failed: %s\n", strerror(-ret));
         goto out;
     }
+    cnt = ret;
+
+    ret = 0;
 
     if (qflag) {
         goto out;
@@ -1208,6 +1225,7 @@
 out:
     qemu_iovec_destroy(&qiov);
     qemu_io_free(buf);
+    return ret;
 }
 
 struct aio_ctx {
@@ -1314,6 +1332,9 @@
 " standard output stream (with -v option) for subsequent inspection.\n"
 " The read is performed asynchronously and the aio_flush command must be\n"
 " used to ensure all outstanding aio requests have been completed.\n"
+" Note that due to its asynchronous nature, this command will be\n"
+" considered successful once the request is submitted, independently\n"
+" of potential I/O errors or pattern mismatches.\n"
 " -C, -- report statistics in a machine parsable format\n"
 " -P, -- use a pattern to verify read data\n"
 " -i, -- treat request as invalid, for exercising stats\n"
@@ -1322,7 +1343,7 @@
 "\n");
 }
 
-static void aio_read_f(BlockBackend *blk, int argc, char **argv);
+static int aio_read_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t aio_read_cmd = {
     .name       = "aio_read",
@@ -1334,7 +1355,7 @@
     .help       = aio_read_help,
 };
 
-static void aio_read_f(BlockBackend *blk, int argc, char **argv)
+static int aio_read_f(BlockBackend *blk, int argc, char **argv)
 {
     int nr_iov, c;
     struct aio_ctx *ctx = g_new0(struct aio_ctx, 1);
@@ -1350,14 +1371,14 @@
             ctx->pattern = parse_pattern(optarg);
             if (ctx->pattern < 0) {
                 g_free(ctx);
-                return;
+                return -EINVAL;
             }
             break;
         case 'i':
             printf("injecting invalid read request\n");
             block_acct_invalid(blk_get_stats(blk), BLOCK_ACCT_READ);
             g_free(ctx);
-            return;
+            return 0;
         case 'q':
             ctx->qflag = true;
             break;
@@ -1367,21 +1388,22 @@
         default:
             g_free(ctx);
             qemuio_command_usage(&aio_read_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind > argc - 2) {
         g_free(ctx);
         qemuio_command_usage(&aio_read_cmd);
-        return;
+        return -EINVAL;
     }
 
     ctx->offset = cvtnum(argv[optind]);
     if (ctx->offset < 0) {
-        print_cvtnum_err(ctx->offset, argv[optind]);
+        int ret = ctx->offset;
+        print_cvtnum_err(ret, argv[optind]);
         g_free(ctx);
-        return;
+        return ret;
     }
     optind++;
 
@@ -1390,13 +1412,14 @@
     if (ctx->buf == NULL) {
         block_acct_invalid(blk_get_stats(blk), BLOCK_ACCT_READ);
         g_free(ctx);
-        return;
+        return -EINVAL;
     }
 
     gettimeofday(&ctx->t1, NULL);
     block_acct_start(blk_get_stats(blk), &ctx->acct, ctx->qiov.size,
                      BLOCK_ACCT_READ);
     blk_aio_preadv(blk, ctx->offset, &ctx->qiov, 0, aio_read_done, ctx);
+    return 0;
 }
 
 static void aio_write_help(void)
@@ -1413,6 +1436,9 @@
 " filled with a set pattern (0xcdcdcdcd).\n"
 " The write is performed asynchronously and the aio_flush command must be\n"
 " used to ensure all outstanding aio requests have been completed.\n"
+" Note that due to its asynchronous nature, this command will be\n"
+" considered successful once the request is submitted, independently\n"
+" of potential I/O errors or pattern mismatches.\n"
 " -P, -- use different pattern to fill file\n"
 " -C, -- report statistics in a machine parsable format\n"
 " -f, -- use Force Unit Access semantics\n"
@@ -1423,7 +1449,7 @@
 "\n");
 }
 
-static void aio_write_f(BlockBackend *blk, int argc, char **argv);
+static int aio_write_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t aio_write_cmd = {
     .name       = "aio_write",
@@ -1436,7 +1462,7 @@
     .help       = aio_write_help,
 };
 
-static void aio_write_f(BlockBackend *blk, int argc, char **argv)
+static int aio_write_f(BlockBackend *blk, int argc, char **argv)
 {
     int nr_iov, c;
     int pattern = 0xcd;
@@ -1462,53 +1488,54 @@
             pattern = parse_pattern(optarg);
             if (pattern < 0) {
                 g_free(ctx);
-                return;
+                return -EINVAL;
             }
             break;
         case 'i':
             printf("injecting invalid write request\n");
             block_acct_invalid(blk_get_stats(blk), BLOCK_ACCT_WRITE);
             g_free(ctx);
-            return;
+            return 0;
         case 'z':
             ctx->zflag = true;
             break;
         default:
             g_free(ctx);
             qemuio_command_usage(&aio_write_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind > argc - 2) {
         g_free(ctx);
         qemuio_command_usage(&aio_write_cmd);
-        return;
+        return -EINVAL;
     }
 
     if (ctx->zflag && optind != argc - 2) {
         printf("-z supports only a single length parameter\n");
         g_free(ctx);
-        return;
+        return -EINVAL;
     }
 
     if ((flags & BDRV_REQ_MAY_UNMAP) && !ctx->zflag) {
         printf("-u requires -z to be specified\n");
         g_free(ctx);
-        return;
+        return -EINVAL;
     }
 
     if (ctx->zflag && ctx->Pflag) {
         printf("-z and -P cannot be specified at the same time\n");
         g_free(ctx);
-        return;
+        return -EINVAL;
     }
 
     ctx->offset = cvtnum(argv[optind]);
     if (ctx->offset < 0) {
-        print_cvtnum_err(ctx->offset, argv[optind]);
+        int ret = ctx->offset;
+        print_cvtnum_err(ret, argv[optind]);
         g_free(ctx);
-        return;
+        return ret;
     }
     optind++;
 
@@ -1517,7 +1544,7 @@
         if (count < 0) {
             print_cvtnum_err(count, argv[optind]);
             g_free(ctx);
-            return;
+            return count;
         }
 
         ctx->qiov.size = count;
@@ -1530,7 +1557,7 @@
         if (ctx->buf == NULL) {
             block_acct_invalid(blk_get_stats(blk), BLOCK_ACCT_WRITE);
             g_free(ctx);
-            return;
+            return -EINVAL;
         }
 
         gettimeofday(&ctx->t1, NULL);
@@ -1540,14 +1567,17 @@
         blk_aio_pwritev(blk, ctx->offset, &ctx->qiov, flags, aio_write_done,
                         ctx);
     }
+
+    return 0;
 }
 
-static void aio_flush_f(BlockBackend *blk, int argc, char **argv)
+static int aio_flush_f(BlockBackend *blk, int argc, char **argv)
 {
     BlockAcctCookie cookie;
     block_acct_start(blk_get_stats(blk), &cookie, 0, BLOCK_ACCT_FLUSH);
     blk_drain_all();
     block_acct_done(blk_get_stats(blk), &cookie);
+    return 0;
 }
 
 static const cmdinfo_t aio_flush_cmd = {
@@ -1556,9 +1586,9 @@
     .oneline    = "completes all outstanding aio requests"
 };
 
-static void flush_f(BlockBackend *blk, int argc, char **argv)
+static int flush_f(BlockBackend *blk, int argc, char **argv)
 {
-    blk_flush(blk);
+    return blk_flush(blk);
 }
 
 static const cmdinfo_t flush_cmd = {
@@ -1568,7 +1598,7 @@
     .oneline    = "flush all in-core file state to disk",
 };
 
-static void truncate_f(BlockBackend *blk, int argc, char **argv)
+static int truncate_f(BlockBackend *blk, int argc, char **argv)
 {
     Error *local_err = NULL;
     int64_t offset;
@@ -1577,14 +1607,16 @@
     offset = cvtnum(argv[1]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[1]);
-        return;
+        return offset;
     }
 
     ret = blk_truncate(blk, offset, PREALLOC_MODE_OFF, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
-        return;
+        return ret;
     }
+
+    return 0;
 }
 
 static const cmdinfo_t truncate_cmd = {
@@ -1598,7 +1630,7 @@
     .oneline    = "truncates the current file at the given offset",
 };
 
-static void length_f(BlockBackend *blk, int argc, char **argv)
+static int length_f(BlockBackend *blk, int argc, char **argv)
 {
     int64_t size;
     char s1[64];
@@ -1606,11 +1638,12 @@
     size = blk_getlength(blk);
     if (size < 0) {
         printf("getlength: %s\n", strerror(-size));
-        return;
+        return size;
     }
 
     cvtstr(size, s1, sizeof(s1));
     printf("%s\n", s1);
+    return 0;
 }
 
 
@@ -1622,7 +1655,7 @@
 };
 
 
-static void info_f(BlockBackend *blk, int argc, char **argv)
+static int info_f(BlockBackend *blk, int argc, char **argv)
 {
     BlockDriverState *bs = blk_bs(blk);
     BlockDriverInfo bdi;
@@ -1639,7 +1672,7 @@
 
     ret = bdrv_get_info(bs, &bdi);
     if (ret) {
-        return;
+        return ret;
     }
 
     cvtstr(bdi.cluster_size, s1, sizeof(s1));
@@ -1654,6 +1687,8 @@
         bdrv_image_info_specific_dump(fprintf, stdout, spec_info);
         qapi_free_ImageInfoSpecific(spec_info);
     }
+
+    return 0;
 }
 
 
@@ -1680,7 +1715,7 @@
 "\n");
 }
 
-static void discard_f(BlockBackend *blk, int argc, char **argv);
+static int discard_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t discard_cmd = {
     .name       = "discard",
@@ -1694,7 +1729,7 @@
     .help       = discard_help,
 };
 
-static void discard_f(BlockBackend *blk, int argc, char **argv)
+static int discard_f(BlockBackend *blk, int argc, char **argv)
 {
     struct timeval t1, t2;
     bool Cflag = false, qflag = false;
@@ -1711,31 +1746,31 @@
             break;
         default:
             qemuio_command_usage(&discard_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind != argc - 2) {
         qemuio_command_usage(&discard_cmd);
-        return;
+        return -EINVAL;
     }
 
     offset = cvtnum(argv[optind]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[optind]);
-        return;
+        return offset;
     }
 
     optind++;
     bytes = cvtnum(argv[optind]);
     if (bytes < 0) {
         print_cvtnum_err(bytes, argv[optind]);
-        return;
+        return bytes;
     } else if (bytes >> BDRV_SECTOR_BITS > BDRV_REQUEST_MAX_SECTORS) {
         printf("length cannot exceed %"PRIu64", given %s\n",
                (uint64_t)BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS,
                argv[optind]);
-        return;
+        return -EINVAL;
     }
 
     gettimeofday(&t1, NULL);
@@ -1744,7 +1779,7 @@
 
     if (ret < 0) {
         printf("discard failed: %s\n", strerror(-ret));
-        return;
+        return ret;
     }
 
     /* Finally, report back -- -C gives a parsable format */
@@ -1752,9 +1787,11 @@
         t2 = tsub(t2, t1);
         print_report("discard", &t2, offset, bytes, bytes, 1, Cflag);
     }
+
+    return 0;
 }
 
-static void alloc_f(BlockBackend *blk, int argc, char **argv)
+static int alloc_f(BlockBackend *blk, int argc, char **argv)
 {
     BlockDriverState *bs = blk_bs(blk);
     int64_t offset, start, remaining, count;
@@ -1765,14 +1802,14 @@
     start = offset = cvtnum(argv[1]);
     if (offset < 0) {
         print_cvtnum_err(offset, argv[1]);
-        return;
+        return offset;
     }
 
     if (argc == 3) {
         count = cvtnum(argv[2]);
         if (count < 0) {
             print_cvtnum_err(count, argv[2]);
-            return;
+            return count;
         }
     } else {
         count = BDRV_SECTOR_SIZE;
@@ -1784,7 +1821,7 @@
         ret = bdrv_is_allocated(bs, offset, remaining, &num);
         if (ret < 0) {
             printf("is_allocated failed: %s\n", strerror(-ret));
-            return;
+            return ret;
         }
         offset += num;
         remaining -= num;
@@ -1801,6 +1838,7 @@
 
     printf("%"PRId64"/%"PRId64" bytes allocated at offset %s\n",
            sum_alloc, count, s1);
+    return 0;
 }
 
 static const cmdinfo_t alloc_cmd = {
@@ -1846,7 +1884,7 @@
     return firstret;
 }
 
-static void map_f(BlockBackend *blk, int argc, char **argv)
+static int map_f(BlockBackend *blk, int argc, char **argv)
 {
     int64_t offset, bytes;
     char s1[64], s2[64];
@@ -1858,17 +1896,17 @@
     bytes = blk_getlength(blk);
     if (bytes < 0) {
         error_report("Failed to query image length: %s", strerror(-bytes));
-        return;
+        return bytes;
     }
 
     while (bytes) {
         ret = map_is_allocated(blk_bs(blk), offset, bytes, &num);
         if (ret < 0) {
             error_report("Failed to get allocation status: %s", strerror(-ret));
-            return;
+            return ret;
         } else if (!num) {
             error_report("Unexpected end of image");
-            return;
+            return -EIO;
         }
 
         retstr = ret ? "    allocated" : "not allocated";
@@ -1880,6 +1918,8 @@
         offset += num;
         bytes -= num;
     }
+
+    return 0;
 }
 
 static const cmdinfo_t map_cmd = {
@@ -1907,7 +1947,7 @@
 "\n");
 }
 
-static void reopen_f(BlockBackend *blk, int argc, char **argv);
+static int reopen_f(BlockBackend *blk, int argc, char **argv);
 
 static QemuOptsList reopen_opts = {
     .name = "reopen",
@@ -1929,7 +1969,7 @@
        .help           = reopen_help,
 };
 
-static void reopen_f(BlockBackend *blk, int argc, char **argv)
+static int reopen_f(BlockBackend *blk, int argc, char **argv)
 {
     BlockDriverState *bs = blk_bs(blk);
     QemuOpts *qopts;
@@ -1947,19 +1987,19 @@
         case 'c':
             if (bdrv_parse_cache_mode(optarg, &flags, &writethrough) < 0) {
                 error_report("Invalid cache option: %s", optarg);
-                return;
+                return -EINVAL;
             }
             break;
         case 'o':
             if (!qemu_opts_parse_noisily(&reopen_opts, optarg, 0)) {
                 qemu_opts_reset(&reopen_opts);
-                return;
+                return -EINVAL;
             }
             break;
         case 'r':
             if (has_rw_option) {
                 error_report("Only one -r/-w option may be given");
-                return;
+                return -EINVAL;
             }
             flags &= ~BDRV_O_RDWR;
             has_rw_option = true;
@@ -1967,7 +2007,7 @@
         case 'w':
             if (has_rw_option) {
                 error_report("Only one -r/-w option may be given");
-                return;
+                return -EINVAL;
             }
             flags |= BDRV_O_RDWR;
             has_rw_option = true;
@@ -1975,14 +2015,14 @@
         default:
             qemu_opts_reset(&reopen_opts);
             qemuio_command_usage(&reopen_cmd);
-            return;
+            return -EINVAL;
         }
     }
 
     if (optind != argc) {
         qemu_opts_reset(&reopen_opts);
         qemuio_command_usage(&reopen_cmd);
-        return;
+        return -EINVAL;
     }
 
     if (writethrough != blk_enable_write_cache(blk) &&
@@ -1990,7 +2030,7 @@
     {
         error_report("Cannot change cache.writeback: Device attached");
         qemu_opts_reset(&reopen_opts);
-        return;
+        return -EBUSY;
     }
 
     if (!(flags & BDRV_O_RDWR)) {
@@ -2016,29 +2056,37 @@
 
     if (local_err) {
         error_report_err(local_err);
-    } else {
-        blk_set_enable_write_cache(blk, !writethrough);
+        return -EINVAL;
     }
+
+    blk_set_enable_write_cache(blk, !writethrough);
+    return 0;
 }
 
-static void break_f(BlockBackend *blk, int argc, char **argv)
+static int break_f(BlockBackend *blk, int argc, char **argv)
 {
     int ret;
 
     ret = bdrv_debug_breakpoint(blk_bs(blk), argv[1], argv[2]);
     if (ret < 0) {
         printf("Could not set breakpoint: %s\n", strerror(-ret));
+        return ret;
     }
+
+    return 0;
 }
 
-static void remove_break_f(BlockBackend *blk, int argc, char **argv)
+static int remove_break_f(BlockBackend *blk, int argc, char **argv)
 {
     int ret;
 
     ret = bdrv_debug_remove_breakpoint(blk_bs(blk), argv[1]);
     if (ret < 0) {
         printf("Could not remove breakpoint %s: %s\n", argv[1], strerror(-ret));
+        return ret;
     }
+
+    return 0;
 }
 
 static const cmdinfo_t break_cmd = {
@@ -2060,14 +2108,17 @@
        .oneline        = "remove a breakpoint by tag",
 };
 
-static void resume_f(BlockBackend *blk, int argc, char **argv)
+static int resume_f(BlockBackend *blk, int argc, char **argv)
 {
     int ret;
 
     ret = bdrv_debug_resume(blk_bs(blk), argv[1]);
     if (ret < 0) {
         printf("Could not resume request: %s\n", strerror(-ret));
+        return ret;
     }
+
+    return 0;
 }
 
 static const cmdinfo_t resume_cmd = {
@@ -2079,11 +2130,12 @@
        .oneline        = "resumes the request tagged as tag",
 };
 
-static void wait_break_f(BlockBackend *blk, int argc, char **argv)
+static int wait_break_f(BlockBackend *blk, int argc, char **argv)
 {
     while (!bdrv_debug_is_suspended(blk_bs(blk), argv[1])) {
         aio_poll(blk_get_aio_context(blk), true);
     }
+    return 0;
 }
 
 static const cmdinfo_t wait_break_cmd = {
@@ -2095,7 +2147,7 @@
        .oneline        = "waits for the suspension of a request",
 };
 
-static void abort_f(BlockBackend *blk, int argc, char **argv)
+static int abort_f(BlockBackend *blk, int argc, char **argv)
 {
     abort();
 }
@@ -2121,7 +2173,7 @@
 "\n", SIGTERM);
 }
 
-static void sigraise_f(BlockBackend *blk, int argc, char **argv);
+static int sigraise_f(BlockBackend *blk, int argc, char **argv);
 
 static const cmdinfo_t sigraise_cmd = {
     .name       = "sigraise",
@@ -2134,16 +2186,16 @@
     .help       = sigraise_help,
 };
 
-static void sigraise_f(BlockBackend *blk, int argc, char **argv)
+static int sigraise_f(BlockBackend *blk, int argc, char **argv)
 {
     int64_t sig = cvtnum(argv[1]);
     if (sig < 0) {
         print_cvtnum_err(sig, argv[1]);
-        return;
+        return sig;
     } else if (sig > NSIG) {
         printf("signal argument '%s' is too large to be a valid signal\n",
                argv[1]);
-        return;
+        return -EINVAL;
     }
 
     /* Using raise() to kill this process does not necessarily flush all open
@@ -2153,6 +2205,8 @@
     fflush(stderr);
 
     raise(sig);
+
+    return 0;
 }
 
 static void sleep_cb(void *opaque)
@@ -2161,7 +2215,7 @@
     *expired = true;
 }
 
-static void sleep_f(BlockBackend *blk, int argc, char **argv)
+static int sleep_f(BlockBackend *blk, int argc, char **argv)
 {
     char *endptr;
     long ms;
@@ -2171,7 +2225,7 @@
     ms = strtol(argv[1], &endptr, 0);
     if (ms < 0 || *endptr != '\0') {
         printf("%s is not a valid number\n", argv[1]);
-        return;
+        return -EINVAL;
     }
 
     timer = timer_new_ns(QEMU_CLOCK_HOST, sleep_cb, &expired);
@@ -2182,6 +2236,7 @@
     }
 
     timer_free(timer);
+    return 0;
 }
 
 static const cmdinfo_t sleep_cmd = {
@@ -2228,22 +2283,23 @@
     printf("\nUse 'help commandname' for extended help.\n");
 }
 
-static void help_f(BlockBackend *blk, int argc, char **argv)
+static int help_f(BlockBackend *blk, int argc, char **argv)
 {
     const cmdinfo_t *ct;
 
     if (argc == 1) {
         help_all();
-        return;
+        return 0;
     }
 
     ct = find_command(argv[1]);
     if (ct == NULL) {
         printf("command %s not found\n", argv[1]);
-        return;
+        return -EINVAL;
     }
 
     help_onecmd(argv[1], ct);
+    return 0;
 }
 
 static const cmdinfo_t help_cmd = {
@@ -2257,13 +2313,14 @@
     .oneline    = "help for one or all commands",
 };
 
-void qemuio_command(BlockBackend *blk, const char *cmd)
+int qemuio_command(BlockBackend *blk, const char *cmd)
 {
     AioContext *ctx;
     char *input;
     const cmdinfo_t *ct;
     char **v;
     int c;
+    int ret = 0;
 
     input = g_strdup(cmd);
     v = breakline(input, &c);
@@ -2272,14 +2329,17 @@
         if (ct) {
             ctx = blk ? blk_get_aio_context(blk) : qemu_get_aio_context();
             aio_context_acquire(ctx);
-            command(blk, ct, c, v);
+            ret = command(blk, ct, c, v);
             aio_context_release(ctx);
         } else {
             fprintf(stderr, "command \"%s\" not found\n", v[0]);
+            ret = -EINVAL;
         }
     }
     g_free(input);
     g_free(v);
+
+    return ret;
 }
 
 static void __attribute((constructor)) init_qemuio_commands(void)