From 6843ab569a6b9d6cbd30f30aeb75f106eae05316 Mon Sep 17 00:00:00 2001
From: Alex Kotov <kotovalexarian@gmail.com>
Date: Sun, 13 Dec 2020 17:04:16 +0500
Subject: [PATCH] Truncate shared memory object

---
 configure.ac     |  4 ++++
 include/shmemq.h |  2 ++
 src/main.c       | 22 ++++++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/configure.ac b/configure.ac
index 3001647..f64d772 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,8 +36,10 @@ AC_CHECK_HEADERS([stdlib.h])
 AC_CHECK_HEADERS([string.h])
 AC_CHECK_HEADERS([sys/mman.h])
 AC_CHECK_HEADERS([sys/stat.h])
+AC_CHECK_HEADERS([unistd.h])
 
 AC_CHECK_FUNCS([ftruncate])
+AC_CHECK_FUNCS([fstat])
 AC_CHECK_FUNCS([munmap])
 AC_CHECK_FUNCS([strcpy])
 AC_CHECK_FUNCS([strlen])
@@ -45,4 +47,6 @@ AC_CHECK_FUNCS([strlen])
 AC_SEARCH_LIBS([shm_open], [rt])
 AC_SEARCH_LIBS([shm_unlink], [rt])
 
+AC_CHECK_MEMBER([struct stat.st_size], [], [], [#include <sys/stat.h>])
+
 AC_OUTPUT
diff --git a/include/shmemq.h b/include/shmemq.h
index 0b1ae70..87b821f 100644
--- a/include/shmemq.h
+++ b/include/shmemq.h
@@ -21,6 +21,8 @@ enum Shmemq_Error {
     SHMEMQ_ERROR_MALLOC = 1,
     SHMEMQ_ERROR_INVALID_NAME = 2,
     SHMEMQ_ERROR_SHARED_MEMORY = 3,
+    SHMEMQ_ERROR_FSTAT = 4,
+    SHMEMQ_ERROR_FTRUNCATE = 5,
 };
 
 struct Shmemq_FrameHeader {
diff --git a/src/main.c b/src/main.c
index cec77f4..0c15e3c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2,6 +2,8 @@
 #include "config.h"
 #endif
 
+#define _POSIX_C_SOURCE 200809L
+
 #include <shmemq.h>
 
 #include <fcntl.h>
@@ -9,6 +11,7 @@
 #include <string.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
+#include <unistd.h>
 
 struct Shmemq *shmemq_new(
     const char *const name,
@@ -59,6 +62,25 @@ enum Shmemq_Error shmemq_init(
 
     if (shmemq->shm_id == -1) return SHMEMQ_ERROR_SHARED_MEMORY;
 
+    struct stat statbuf;
+
+    if (fstat(shmemq->shm_id, &statbuf) != 0) return SHMEMQ_ERROR_FSTAT;
+
+    const size_t min_size =
+        shmemq->is_consumer ? sizeof(struct Shmemq_BufferHeader) : size;
+
+    if ((size_t)statbuf.st_size < min_size) {
+        if (ftruncate(shmemq->shm_id, min_size) != 0) {
+            return SHMEMQ_ERROR_FTRUNCATE;
+        }
+    }
+
+    if (fstat(shmemq->shm_id, &statbuf) != 0) return SHMEMQ_ERROR_FSTAT;
+
+    if ((size_t)statbuf.st_size < size && !shmemq->is_consumer) {
+        if (ftruncate(shmemq->shm_id, size) != 0) return SHMEMQ_ERROR_FTRUNCATE;
+    }
+
     shmemq->buffer = NULL;
 
     return SHMEMQ_ERROR_NONE;