diff -r ec62453fc8a6 -r 1ecf4dbbc60c src/buffer.c
--- a/src/buffer.c	Fri Dec 20 21:25:33 2024 +0100
+++ b/src/buffer.c	Sat Dec 21 21:03:28 2024 +0100
@@ -30,6 +30,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <errno.h>
 
 static int buffer_copy_on_write(CxBuffer* buffer) {
     if (0 == (buffer->flags & CX_BUFFER_COPY_ON_WRITE)) return 0;
@@ -136,6 +137,7 @@
     npos += offset;
 
     if ((offset > 0 && npos < opos) || (offset < 0 && npos > opos)) {
+        errno = EOVERFLOW;
         return -1;
     }
 
@@ -247,6 +249,7 @@
     size_t len;
     size_t nitems_out = nitems;
     if (cx_szmul(size, nitems, &len)) {
+        errno = EOVERFLOW;
         return 0;
     }
     size_t required = buffer->pos + len;
@@ -285,6 +288,7 @@
     if (perform_flush) {
         size_t flush_max;
         if (cx_szmul(buffer->flush_blkmax, buffer->flush_blksize, &flush_max)) {
+            errno = EOVERFLOW;
             return 0;
         }
         size_t flush_pos = buffer->flush_func == NULL || buffer->flush_target == NULL
@@ -385,6 +389,7 @@
 ) {
     size_t len;
     if (cx_szmul(size, nitems, &len)) {
+        errno = EOVERFLOW;
         return 0;
     }
     if (buffer->pos + len > buffer->size) {
@@ -436,6 +441,10 @@
         CxBuffer *buffer,
         size_t shift
 ) {
+    if (buffer->size > SIZE_MAX - shift) {
+        errno = EOVERFLOW;
+        return -1;
+    }
     size_t req_capacity = buffer->size + shift;
     size_t movebytes;