diff --git a/lib/BUILD.bazel b/lib/BUILD.bazel
index 5d0502f..a5e4b65 100644
--- a/lib/BUILD.bazel
+++ b/lib/BUILD.bazel
@@ -5,6 +5,7 @@ cc_library(
     name = "lib",
     srcs = [
         "crc32.c",
+        "strbuf.c",
         "toolbox.c",
         "util.c",
     ],
@@ -13,6 +14,7 @@ cc_library(
         "defs.h",
         "endian.h",
         "error.h",
+        "strbuf.h",
         "util.h",
     ],
     copts = COPTS,
@@ -59,3 +61,15 @@ cc_test(
         ":lib",
     ],
 )
+
+cc_test(
+    name = "strbuf_test",
+    size = "small",
+    srcs = [
+        "strbuf_test.c",
+    ],
+    copts = COPTS,
+    deps = [
+        ":lib",
+    ],
+)
diff --git a/lib/strbuf.c b/lib/strbuf.c
new file mode 100644
index 0000000..98db8d0
--- /dev/null
+++ b/lib/strbuf.c
@@ -0,0 +1,41 @@
+// Copyright 2022 Dietrich Epp.
+// This file is part of SyncFiles. SyncFiles is licensed under the terms of the
+// Mozilla Public License, version 2.0. See LICENSE.txt for details.
+#include "lib/strbuf.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+bool StrbufAlloc(struct Strbuf *restrict b, size_t n)
+{
+	size_t req = n + 1;
+	if (req > b->alloc) {
+		size_t nalloc = (b->alloc + 16) * 3 / 2;
+		if (req > nalloc) {
+			nalloc = req;
+		}
+		char *nbuf = realloc(b->buf, nalloc);
+		if (nbuf == NULL) {
+			return false;
+		}
+		b->buf = nbuf;
+		b->alloc = nalloc;
+	}
+	return true;
+}
+
+bool StrbufReserve(struct Strbuf *restrict b, size_t n)
+{
+	return StrbufAlloc(b, b->len + n);
+}
+
+bool StrbufAppendMem(struct Strbuf *restrict b, const char *s, size_t n)
+{
+	if (!StrbufReserve(b, n)) {
+		return false;
+	}
+	memcpy(b->buf + b->len, s, n);
+	b->buf[b->len + n] = '\0';
+	b->len += n;
+	return true;
+}
diff --git a/lib/strbuf.h b/lib/strbuf.h
new file mode 100644
index 0000000..78bfc14
--- /dev/null
+++ b/lib/strbuf.h
@@ -0,0 +1,29 @@
+// Copyright 2022 Dietrich Epp.
+// This file is part of SyncFiles. SyncFiles is licensed under the terms of the
+// Mozilla Public License, version 2.0. See LICENSE.txt for details.
+#ifndef LIB_STRBUF_H
+#define LIB_STRBUF_H
+#include <stddef.h>
+
+#include "lib/defs.h"
+
+// A strbuf is a buffer for building a string. The structure can be
+// zero-initialized.
+struct Strbuf {
+	char *buf;    // String data. May be NULL if strbuf is empty.
+	size_t len;   // String length, not including nul terminator.
+	size_t alloc; // Data reserved for string, not including terminator.
+};
+
+// Reserve enough space in the strbuf to store a string which is n bytes long.
+// Also reserve enough space for the nul byte after. Return true on success.
+bool StrbufAlloc(struct Strbuf *restrict b, size_t n);
+
+// Reserve enough space in the strbuf to append n bytes to the buffer. Also
+// reserve enough space for the nul byte after. Return true on success.
+bool StrbufReserve(struct Strbuf *restrict b, size_t n);
+
+// Append the given data to the buffer. Return true on success.
+bool StrbufAppendMem(struct Strbuf *restrict b, const char *s, size_t n);
+
+#endif
diff --git a/lib/strbuf_test.c b/lib/strbuf_test.c
new file mode 100644
index 0000000..d6a0f2d
--- /dev/null
+++ b/lib/strbuf_test.c
@@ -0,0 +1,45 @@
+// Copyright 2022 Dietrich Epp.
+// This file is part of SyncFiles. SyncFiles is licensed under the terms of the
+// Mozilla Public License, version 2.0. See LICENSE.txt for details.
+#include "lib/strbuf.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct Segment {
+	const char *buf;
+	size_t size;
+};
+
+#define S "0123456789"
+#define S8 S S S S S S S S
+
+static const struct Segment kSegments[] = {
+	{"abc", 3},
+	{S8, 80},
+	{"def", 3},
+	{0, 0},
+};
+
+static const char kOut[] = "abc" S8 "def";
+
+int main(int argc, char **argv)
+{
+	(void)argc;
+	(void)argv;
+	struct Strbuf s = {0, 0, 0};
+	for (int i = 0; kSegments[i].buf != NULL; i++) {
+		bool r = StrbufAppendMem(&s, kSegments[i].buf, kSegments[i].size);
+		if (!r) {
+			fputs("StrbufAppendMem returned false\n", stderr);
+			exit(1);
+		}
+	}
+	if (s.len != sizeof(kOut) - 1 || memcmp(s.buf, kOut, sizeof(kOut)) != 0) {
+		fputs("Invalid result\n", stderr);
+		exit(1);
+	}
+	free(s.buf);
+	return 0;
+}