diff --git a/drivers/nble/gatt.c b/drivers/nble/gatt.c
index c1c784bac3b..1c7c3f46a4e 100644
--- a/drivers/nble/gatt.c
+++ b/drivers/nble/gatt.c
@@ -214,9 +214,9 @@ struct bt_gatt_attr *bt_gatt_attr_next(const struct bt_gatt_attr *attr)
 	return NULL;
 }
 
-int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t buf_len, uint16_t offset,
-		      const void *value, uint16_t value_len)
+ssize_t bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t buf_len, uint16_t offset,
+			  const void *value, uint16_t value_len)
 {
 	uint16_t len;
 
@@ -238,9 +238,9 @@ int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 	return len;
 }
 
-int bt_gatt_attr_read_service(struct bt_conn *conn,
-			      const struct bt_gatt_attr *attr,
-			      void *buf, uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_service(struct bt_conn *conn,
+				  const struct bt_gatt_attr *attr,
+				  void *buf, uint16_t len, uint16_t offset)
 {
 	struct bt_uuid *uuid = attr->user_data;
 
@@ -255,7 +255,7 @@ int bt_gatt_attr_read_service(struct bt_conn *conn,
 				 BT_UUID_128(uuid)->val, 16);
 }
 
-int bt_gatt_attr_read_included(struct bt_conn *conn,
+ssize_t bt_gatt_attr_read_included(struct bt_conn *conn,
 			       const struct bt_gatt_attr *attr,
 			       void *buf, uint16_t len, uint16_t offset)
 {
@@ -271,9 +271,9 @@ struct gatt_chrc {
 	};
 } __packed;
 
-int bt_gatt_attr_read_chrc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_chrc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_chrc *chrc = attr->user_data;
 	struct gatt_chrc pdu;
@@ -297,16 +297,16 @@ int bt_gatt_attr_read_chrc(struct bt_conn *conn,
 	return bt_gatt_attr_read(conn, attr, buf, len, offset, &pdu, value_len);
 }
 
-int bt_gatt_attr_read_ccc(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_ccc(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	return BT_GATT_ERR(BT_ATT_ERR_NOT_SUPPORTED);
 }
 
-int bt_gatt_attr_write_ccc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, const void *buf,
-			   uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_write_ccc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr,
+			       const void *buf, uint16_t len, uint16_t offset)
 {
 	struct _bt_gatt_ccc *ccc = attr->user_data;
 	const uint16_t *data = buf;
@@ -331,9 +331,9 @@ int bt_gatt_attr_write_ccc(struct bt_conn *conn,
 	return len;
 }
 
-int bt_gatt_attr_read_cep(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cep(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_cep *value = attr->user_data;
 	uint16_t props = sys_cpu_to_le16(value->properties);
@@ -342,16 +342,16 @@ int bt_gatt_attr_read_cep(struct bt_conn *conn,
 				 sizeof(props));
 }
 
-int bt_gatt_attr_read_cud(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cud(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	return BT_GATT_ERR(BT_ATT_ERR_NOT_SUPPORTED);
 }
 
-int bt_gatt_attr_read_cpf(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cpf(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	return BT_GATT_ERR(BT_ATT_ERR_NOT_SUPPORTED);
 }
diff --git a/include/bluetooth/gatt.h b/include/bluetooth/gatt.h
index e93fbccbca9..542f3d3f784 100644
--- a/include/bluetooth/gatt.h
+++ b/include/bluetooth/gatt.h
@@ -26,6 +26,7 @@ extern "C" {
 
 #if defined(CONFIG_BLUETOOTH_CENTRAL) || defined(CONFIG_BLUETOOTH_PERIPHERAL)
 #include <stddef.h>
+#include <sys/types.h>
 #include <misc/util.h>
 #include <bluetooth/conn.h>
 #include <bluetooth/uuid.h>
@@ -115,7 +116,7 @@ struct bt_gatt_attr {
 	 *  @return Number fo bytes read, or in case of an error
 	 *          BT_GATT_ERR() with a specific ATT error code.
 	 */
-	int			(*read)(struct bt_conn *conn,
+	ssize_t			(*read)(struct bt_conn *conn,
 					const struct bt_gatt_attr *attr,
 					void *buf, uint16_t len,
 					uint16_t offset);
@@ -131,7 +132,7 @@ struct bt_gatt_attr {
 	 *  @return Number of bytes written, or in case of an error
 	 *          BT_GATT_ERR() with a specific ATT error code.
 	 */
-	int			(*write)(struct bt_conn *conn,
+	ssize_t			(*write)(struct bt_conn *conn,
 					 const struct bt_gatt_attr *attr,
 					 const void *buf, uint16_t len,
 					 uint16_t offset);
@@ -150,7 +151,7 @@ struct bt_gatt_attr {
 	 *  @return Number of bytes flushed, or in case of an error
 	 *          BT_GATT_ERR() with a specific ATT error code.
 	 */
-	int			(*flush)(struct bt_conn *conn,
+	ssize_t			(*flush)(struct bt_conn *conn,
 					 const struct bt_gatt_attr *attr,
 					 uint8_t flags);
 
@@ -358,9 +359,9 @@ struct bt_gatt_attr *bt_gatt_attr_next(const struct bt_gatt_attr *attr);
  *  @return int number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t buf_len, uint16_t offset,
-		      const void *value, uint16_t value_len);
+ssize_t bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t buf_len, uint16_t offset,
+			  const void *value, uint16_t value_len);
 
 /** @brief Read Service Attribute helper.
  *
@@ -377,9 +378,9 @@ int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
  *  @return int number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_service(struct bt_conn *conn,
-			      const struct bt_gatt_attr *attr,
-			      void *buf, uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_service(struct bt_conn *conn,
+				  const struct bt_gatt_attr *attr,
+				  void *buf, uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_SERVICE
  *  @brief Generic Service Declaration Macro.
@@ -442,9 +443,9 @@ int bt_gatt_attr_read_service(struct bt_conn *conn,
  *  @return int number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_included(struct bt_conn *conn,
-			       const struct bt_gatt_attr *attr,
-			       void *buf, uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_included(struct bt_conn *conn,
+				   const struct bt_gatt_attr *attr,
+				   void *buf, uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_INCLUDE_SERVICE
  *  @brief Include Service Declaration Macro.
@@ -476,9 +477,9 @@ int bt_gatt_attr_read_included(struct bt_conn *conn,
  *  @return number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_chrc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_chrc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_CHARACTERISTIC
  *  @brief Characteristic Declaration Macro.
@@ -530,9 +531,9 @@ struct _bt_gatt_ccc {
  *  @return number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_ccc(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_ccc(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset);
 
 /** @brief Write Client Characteristic Configuration Attribute helper.
  *
@@ -548,9 +549,9 @@ int bt_gatt_attr_read_ccc(struct bt_conn *conn,
  *  @return number of bytes written in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_write_ccc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, const void *buf,
-			   uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_write_ccc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, const void *buf,
+			       uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_CCC
  *  @brief Client Characteristic Configuration Declaration Macro.
@@ -586,9 +587,9 @@ int bt_gatt_attr_write_ccc(struct bt_conn *conn,
  *  @return number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_cep(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_cep(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_CEP
  *  @brief Characteristic Extended Properties Declaration Macro.
@@ -620,9 +621,9 @@ int bt_gatt_attr_read_cep(struct bt_conn *conn,
  *  @return number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_cud(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_cud(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_CUD
  *  @brief Characteristic User Format Descriptor Declaration Macro.
@@ -655,9 +656,9 @@ int bt_gatt_attr_read_cud(struct bt_conn *conn,
  *  @return number of bytes read in case of success or negative values in
  *  case of error.
  */
-int bt_gatt_attr_read_cpf(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset);
+ssize_t bt_gatt_attr_read_cpf(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset);
 
 /** @def BT_GATT_CPF
  *  @brief Characteristic Presentation Format Descriptor Declaration Macro.
diff --git a/net/bluetooth/gatt.c b/net/bluetooth/gatt.c
index 9b8bc2a4193..974fa54290f 100644
--- a/net/bluetooth/gatt.c
+++ b/net/bluetooth/gatt.c
@@ -120,9 +120,9 @@ populate:
 	return 0;
 }
 
-int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t buf_len, uint16_t offset,
-		      const void *value, uint16_t value_len)
+ssize_t bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t buf_len, uint16_t offset,
+			  const void *value, uint16_t value_len)
 {
 	uint16_t len;
 
@@ -140,9 +140,9 @@ int bt_gatt_attr_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 	return len;
 }
 
-int bt_gatt_attr_read_service(struct bt_conn *conn,
-			      const struct bt_gatt_attr *attr,
-			      void *buf, uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_service(struct bt_conn *conn,
+				  const struct bt_gatt_attr *attr,
+				  void *buf, uint16_t len, uint16_t offset)
 {
 	struct bt_uuid *uuid = attr->user_data;
 
@@ -163,9 +163,9 @@ struct gatt_incl {
 	uint16_t uuid16;
 } __packed;
 
-int bt_gatt_attr_read_included(struct bt_conn *conn,
-			       const struct bt_gatt_attr *attr,
-			       void *buf, uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_included(struct bt_conn *conn,
+				   const struct bt_gatt_attr *attr,
+				   void *buf, uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_include *incl = attr->user_data;
 	struct gatt_incl pdu;
@@ -197,9 +197,9 @@ struct gatt_chrc {
 	};
 } __packed;
 
-int bt_gatt_attr_read_chrc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_chrc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_chrc *chrc = attr->user_data;
 	struct gatt_chrc pdu;
@@ -261,9 +261,9 @@ struct bt_gatt_attr *bt_gatt_attr_next(const struct bt_gatt_attr *attr)
 #endif /* CONFIG_BLUETOOTH_GATT_DYNAMIC_DB */
 }
 
-int bt_gatt_attr_read_ccc(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_ccc(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	struct _bt_gatt_ccc *ccc = attr->user_data;
 	uint16_t value;
@@ -318,9 +318,9 @@ static bool is_bonded(const bt_addr_le_t *addr)
 #endif /* defined(CONFIG_BLUETOOTH_SMP) */
 }
 
-int bt_gatt_attr_write_ccc(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, const void *buf,
-			   uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_write_ccc(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, const void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	struct _bt_gatt_ccc *ccc = attr->user_data;
 	const uint16_t *data = buf;
@@ -370,9 +370,9 @@ int bt_gatt_attr_write_ccc(struct bt_conn *conn,
 	return len;
 }
 
-int bt_gatt_attr_read_cep(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cep(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_cep *value = attr->user_data;
 	uint16_t props = sys_cpu_to_le16(value->properties);
@@ -381,18 +381,18 @@ int bt_gatt_attr_read_cep(struct bt_conn *conn,
 				 sizeof(props));
 }
 
-int bt_gatt_attr_read_cud(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cud(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	char *value = attr->user_data;
 
 	return bt_gatt_attr_read(conn, attr, buf, len, offset, value, strlen(value));
 }
 
-int bt_gatt_attr_read_cpf(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, void *buf,
-			  uint16_t len, uint16_t offset)
+ssize_t bt_gatt_attr_read_cpf(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	struct bt_gatt_cpf *value = attr->user_data;
 
diff --git a/samples/bluetooth/ipsp/src/ipss.c b/samples/bluetooth/ipsp/src/ipss.c
index c1081b14666..eb80cf17112 100644
--- a/samples/bluetooth/ipsp/src/ipss.c
+++ b/samples/bluetooth/ipsp/src/ipss.c
@@ -48,8 +48,8 @@
 
 #define UDP_PORT		4242
 
-static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *name = attr->user_data;
 
@@ -57,9 +57,9 @@ static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(name));
 }
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(UNKNOWN_APPEARANCE);
 
@@ -67,8 +67,8 @@ static int read_appearance(struct bt_conn *conn,
 				 sizeof(appearance));
 }
 
-static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		   void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -76,8 +76,8 @@ static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(value));
 }
 
-static int read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
diff --git a/samples/bluetooth/peripheral/src/main.c b/samples/bluetooth/peripheral/src/main.c
index 08ab9a494d9..558db4efd80 100644
--- a/samples/bluetooth/peripheral/src/main.c
+++ b/samples/bluetooth/peripheral/src/main.c
@@ -34,8 +34,8 @@
 #define DEVICE_NAME_LEN		(sizeof(DEVICE_NAME) - 1)
 #define HEART_RATE_APPEARANCE	0x0341
 
-static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *name = attr->user_data;
 
@@ -43,9 +43,9 @@ static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(name));
 }
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(HEART_RATE_APPEARANCE);
 
@@ -61,8 +61,8 @@ static void hrmc_ccc_cfg_changed(uint16_t value)
 	simulate_hrm = (value == BT_GATT_CCC_NOTIFY) ? 1 : 0;
 }
 
-static int read_blsc(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blsc(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	uint8_t value = 0x01;
 
@@ -80,8 +80,8 @@ static void blvl_ccc_cfg_changed(uint16_t value)
 	simulate_blvl = (value == BT_GATT_CCC_NOTIFY) ? 1 : 0;
 }
 
-static int read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -126,8 +126,8 @@ static void ct_ccc_cfg_changed(uint16_t value)
 static uint8_t ct[10];
 static uint8_t ct_update = 0;
 
-static int read_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		   void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+		       void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -135,8 +135,8 @@ static int read_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 sizeof(ct));
 }
 
-static int write_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		    const void *buf, uint16_t len, uint16_t offset)
+static ssize_t write_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			const void *buf, uint16_t len, uint16_t offset)
 {
 	uint8_t *value = attr->user_data;
 
@@ -150,8 +150,8 @@ static int write_ct(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 	return len;
 }
 
-static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		   void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -159,8 +159,8 @@ static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(value));
 }
 
-static int read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -183,8 +183,8 @@ static struct bt_uuid_128 vnd_auth_uuid = BT_UUID_INIT_128(
 
 static uint8_t vnd_value[] = { 'V', 'e', 'n', 'd', 'o', 'r' };
 
-static int read_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		   void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -192,8 +192,8 @@ static int read_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(value));
 }
 
-static int write_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     const void *buf, uint16_t len, uint16_t offset)
+static ssize_t write_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 const void *buf, uint16_t len, uint16_t offset)
 {
 	uint8_t *value = attr->user_data;
 
@@ -216,9 +216,9 @@ static struct vnd_long_value {
 	.data = { 'V', 'e', 'n', 'd', 'o', 'r' },
 };
 
-static int read_long_vnd(struct bt_conn *conn,
-			 const struct bt_gatt_attr *attr, void *buf,
-			 uint16_t len, uint16_t offset)
+static ssize_t read_long_vnd(struct bt_conn *conn,
+			     const struct bt_gatt_attr *attr, void *buf,
+			     uint16_t len, uint16_t offset)
 {
 	struct vnd_long_value *value = attr->user_data;
 
@@ -226,9 +226,9 @@ static int read_long_vnd(struct bt_conn *conn,
 				 sizeof(value->data));
 }
 
-static int write_long_vnd(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, const void *buf,
-			  uint16_t len, uint16_t offset)
+static ssize_t write_long_vnd(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, const void *buf,
+			      uint16_t len, uint16_t offset)
 {
 	struct vnd_long_value *value = attr->user_data;
 
@@ -242,8 +242,8 @@ static int write_long_vnd(struct bt_conn *conn,
 	return len;
 }
 
-static int flush_long_vnd(struct bt_conn *conn,
-			  const struct bt_gatt_attr *attr, uint8_t flags)
+static ssize_t flush_long_vnd(struct bt_conn *conn,
+			      const struct bt_gatt_attr *attr, uint8_t flags)
 {
 	struct vnd_long_value *value = attr->user_data;
 
@@ -271,8 +271,8 @@ static struct bt_gatt_cep vnd_long_cep = {
 
 static int signed_value;
 
-static int read_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			   void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -280,8 +280,8 @@ static int read_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 sizeof(signed_value));
 }
 
-static int write_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-			const void *buf, uint16_t len, uint16_t offset)
+static ssize_t write_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			    const void *buf, uint16_t len, uint16_t offset)
 {
 	uint8_t *value = attr->user_data;
 
diff --git a/samples/bluetooth/peripheral_csc/src/main.c b/samples/bluetooth/peripheral_csc/src/main.c
index 1bdb8035a23..a7c48bf5d32 100644
--- a/samples/bluetooth/peripheral_csc/src/main.c
+++ b/samples/bluetooth/peripheral_csc/src/main.c
@@ -88,9 +88,9 @@
 
 /* Generic Access Profile Service declaration */
 
-static int read_device_name(struct bt_conn *conn,
-			    const struct bt_gatt_attr *attr,
-			    void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_device_name(struct bt_conn *conn,
+				const struct bt_gatt_attr *attr,
+				void *buf, uint16_t len, uint16_t offset)
 {
 	const char *name = attr->user_data;
 
@@ -98,9 +98,9 @@ static int read_device_name(struct bt_conn *conn,
 				 strlen(name));
 }
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(CSC_APPEARANCE);
 
@@ -120,7 +120,7 @@ static struct bt_gatt_attr gap_attrs[] = {
 
 /* Device Information Service declaration */
 
-static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+static ssize_t read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
@@ -129,7 +129,7 @@ static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(value));
 }
 
-static int read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+static ssize_t read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
@@ -160,8 +160,8 @@ static void blvl_ccc_cfg_changed(uint16_t value)
 	bas_simulate = value == BT_GATT_CCC_NOTIFY;
 }
 
-static int read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -198,7 +198,7 @@ static void ctrl_point_ccc_cfg_changed(uint16_t value)
 	ctrl_point_configured = value == BT_GATT_CCC_INDICATE;
 }
 
-static int read_location(struct bt_conn *conn,
+static ssize_t read_location(struct bt_conn *conn,
 			     const struct bt_gatt_attr *attr, void *buf,
 			     uint16_t len, uint16_t offset)
 {
@@ -208,9 +208,9 @@ static int read_location(struct bt_conn *conn,
 				 sizeof(*value));
 }
 
-static int read_csc_feature(struct bt_conn *conn,
-			    const struct bt_gatt_attr *attr, void *buf,
-			    uint16_t len, uint16_t offset)
+static ssize_t read_csc_feature(struct bt_conn *conn,
+				const struct bt_gatt_attr *attr, void *buf,
+				uint16_t len, uint16_t offset)
 {
 	uint16_t csc_feature = CSC_FEATURE;
 
@@ -229,9 +229,9 @@ struct write_sc_ctrl_point_req {
 	};
 } __packed;
 
-static int write_ctrl_point(struct bt_conn *conn,
-			    const struct bt_gatt_attr *attr, const void *buf,
-			    uint16_t len, uint16_t offset)
+static ssize_t write_ctrl_point(struct bt_conn *conn,
+				const struct bt_gatt_attr *attr,
+				const void *buf, uint16_t len, uint16_t offset)
 {
 	const struct write_sc_ctrl_point_req *req = buf;
 	uint8_t status;
diff --git a/samples/bluetooth/peripheral_dis/src/main.c b/samples/bluetooth/peripheral_dis/src/main.c
index b5bc3e6fae3..e1821531069 100644
--- a/samples/bluetooth/peripheral_dis/src/main.c
+++ b/samples/bluetooth/peripheral_dis/src/main.c
@@ -34,9 +34,9 @@
 #define DEVICE_NAME_LEN	(sizeof(DEVICE_NAME) - 1)
 #define APPEARANCE	0x0000
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(APPEARANCE);
 
@@ -44,8 +44,9 @@ static int read_appearance(struct bt_conn *conn,
 				 sizeof(appearance));
 }
 
-static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_string(struct bt_conn *conn,
+			   const struct bt_gatt_attr *attr, void *buf,
+			   uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
diff --git a/samples/bluetooth/peripheral_esp/src/main.c b/samples/bluetooth/peripheral_esp/src/main.c
index a557410603c..b9bbb62bc2c 100644
--- a/samples/bluetooth/peripheral_esp/src/main.c
+++ b/samples/bluetooth/peripheral_esp/src/main.c
@@ -73,8 +73,9 @@ static inline uint32_t le24_to_int(const uint8_t *u24)
 		(uint32_t)u24[2] << 16);
 }
 
-static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_string(struct bt_conn *conn,
+			   const struct bt_gatt_attr *attr, void *buf,
+			   uint16_t len, uint16_t offset)
 {
 	const char *string = attr->user_data;
 
@@ -82,8 +83,8 @@ static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(string));
 }
 
-static int read_u16(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		    void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_u16(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			void *buf, uint16_t len, uint16_t offset)
 {
 	const uint16_t *u16 = attr->user_data;
 	uint16_t value = sys_cpu_to_le16(*u16);
@@ -94,9 +95,9 @@ static int read_u16(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 
 /* Generic Access Profile Service Declaration */
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t value = sys_cpu_to_le16(APPEARANCE_THERMOMETER);
 
@@ -126,8 +127,8 @@ static void blvl_ccc_cfg_changed(uint16_t value)
 	simulate_blvl = value == BT_GATT_CCC_NOTIFY;
 }
 
-static int read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const uint8_t *value = attr->user_data;
 
@@ -238,9 +239,9 @@ struct read_es_measurement_rp {
 	uint8_t measurement_uncertainty;
 } __packed;
 
-static int read_es_measurement(struct bt_conn *conn,
-			       const struct bt_gatt_attr *attr, void *buf,
-			       uint16_t len, uint16_t offset)
+static ssize_t read_es_measurement(struct bt_conn *conn,
+				   const struct bt_gatt_attr *attr, void *buf,
+				   uint16_t len, uint16_t offset)
 {
 	const struct es_measurement *value = attr->user_data;
 	struct read_es_measurement_rp rsp;
@@ -256,9 +257,9 @@ static int read_es_measurement(struct bt_conn *conn,
 				 sizeof(rsp));
 }
 
-static int read_temp_valid_range(struct bt_conn *conn,
-				 const struct bt_gatt_attr *attr, void *buf,
-				 uint16_t len, uint16_t offset)
+static ssize_t read_temp_valid_range(struct bt_conn *conn,
+				     const struct bt_gatt_attr *attr, void *buf,
+				     uint16_t len, uint16_t offset)
 {
 	const struct temperature_sensor *sensor = attr->user_data;
 	uint16_t tmp[] = {sys_cpu_to_le16(sensor->lower_limit),
@@ -278,9 +279,10 @@ struct es_trigger_setting_reference {
 	int16_t ref_val;
 } __packed;
 
-static int read_temp_trigger_setting(struct bt_conn *conn,
-				     const struct bt_gatt_attr *attr, void *buf,
-				     uint16_t len, uint16_t offset)
+static ssize_t read_temp_trigger_setting(struct bt_conn *conn,
+					 const struct bt_gatt_attr *attr,
+					 void *buf, uint16_t len,
+					 uint16_t offset)
 {
 	const struct temperature_sensor *sensor = attr->user_data;
 
@@ -322,10 +324,10 @@ struct write_es_trigger_setting_req {
 	uint8_t operand[];
 } __packed;
 
-static int write_temp_trigger_setting(struct bt_conn *conn,
-				      const struct bt_gatt_attr *attr,
-				      const void *buf, uint16_t len,
-				      uint16_t offset)
+static ssize_t write_temp_trigger_setting(struct bt_conn *conn,
+					  const struct bt_gatt_attr *attr,
+					  const void *buf, uint16_t len,
+					  uint16_t offset)
 {
 	const struct write_es_trigger_setting_req *req = buf;
 	const struct es_trigger_setting_reference *ref;
diff --git a/samples/bluetooth/peripheral_hr/src/main.c b/samples/bluetooth/peripheral_hr/src/main.c
index 87243007f1e..f5164f302c1 100644
--- a/samples/bluetooth/peripheral_hr/src/main.c
+++ b/samples/bluetooth/peripheral_hr/src/main.c
@@ -36,8 +36,8 @@
 
 struct bt_conn *default_conn;
 
-static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *name = attr->user_data;
 
@@ -45,9 +45,9 @@ static int read_name(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(name));
 }
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(HEART_RATE_APPEARANCE);
 
@@ -63,8 +63,8 @@ static void hrmc_ccc_cfg_changed(uint16_t value)
 	simulate_hrm = (value == BT_GATT_CCC_NOTIFY) ? 1 : 0;
 }
 
-static int read_blsc(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blsc(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	uint8_t value = 0x01;
 
@@ -82,8 +82,8 @@ static void blvl_ccc_cfg_changed(uint16_t value)
 	simulate_blvl = (value == BT_GATT_CCC_NOTIFY) ? 1 : 0;
 }
 
-static int read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		     void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			 void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -91,8 +91,8 @@ static int read_blvl(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 sizeof(*value));
 }
 
-static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		   void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
@@ -100,8 +100,8 @@ static int read_model(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 strlen(value));
 }
 
-static int read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_manuf(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
diff --git a/samples/bluetooth/peripheral_sc_only/src/main.c b/samples/bluetooth/peripheral_sc_only/src/main.c
index 59fd7422e5a..a5b218b4982 100644
--- a/samples/bluetooth/peripheral_sc_only/src/main.c
+++ b/samples/bluetooth/peripheral_sc_only/src/main.c
@@ -34,9 +34,9 @@
 #define DEVICE_NAME_LEN	(sizeof(DEVICE_NAME) - 1)
 #define APPEARANCE	0x0000
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(APPEARANCE);
 
@@ -44,8 +44,9 @@ static int read_appearance(struct bt_conn *conn,
 				 sizeof(appearance));
 }
 
-static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_string(struct bt_conn *conn,
+			   const struct bt_gatt_attr *attr, void *buf,
+			   uint16_t len, uint16_t offset)
 {
 	const char *value = attr->user_data;
 
diff --git a/tests/bluetooth/shell/src/main.c b/tests/bluetooth/shell/src/main.c
index e8f41bd6fe2..31489bb5024 100644
--- a/tests/bluetooth/shell/src/main.c
+++ b/tests/bluetooth/shell/src/main.c
@@ -206,8 +206,9 @@ static struct bt_conn_cb conn_callbacks = {
 	.security_changed = security_changed,
 };
 
-static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_string(struct bt_conn *conn,
+			   const struct bt_gatt_attr *attr, void *buf,
+			   uint16_t len, uint16_t offset)
 {
 	const char *str = attr->user_data;
 
@@ -217,9 +218,9 @@ static int read_string(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 
 static uint16_t appearance_value = 0x0001;
 
-static int read_appearance(struct bt_conn *conn,
-			   const struct bt_gatt_attr *attr, void *buf,
-			   uint16_t len, uint16_t offset)
+static ssize_t read_appearance(struct bt_conn *conn,
+			       const struct bt_gatt_attr *attr, void *buf,
+			       uint16_t len, uint16_t offset)
 {
 	uint16_t appearance = sys_cpu_to_le16(appearance_value);
 
diff --git a/tests/bluetooth/tester/src/gatt.c b/tests/bluetooth/tester/src/gatt.c
index 566c4eb537d..ae2610f797c 100644
--- a/tests/bluetooth/tester/src/gatt.c
+++ b/tests/bluetooth/tester/src/gatt.c
@@ -224,8 +224,8 @@ struct gatt_value {
 	bool has_ccc;
 };
 
-static int read_value(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		      void *buf, uint16_t len, uint16_t offset)
+static ssize_t read_value(struct bt_conn *conn, const struct bt_gatt_attr *attr,
+			  void *buf, uint16_t len, uint16_t offset)
 {
 	const struct gatt_value *value = attr->user_data;
 
@@ -238,8 +238,9 @@ static int read_value(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 				 value->len);
 }
 
-static int write_value(struct bt_conn *conn, const struct bt_gatt_attr *attr,
-		       const void *buf, uint16_t len, uint16_t offset)
+static ssize_t write_value(struct bt_conn *conn,
+			   const struct bt_gatt_attr *attr, const void *buf,
+			   uint16_t len, uint16_t offset)
 {
 	struct gatt_value *value = attr->user_data;
 
@@ -266,8 +267,8 @@ static int write_value(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 	return len;
 }
 
-static int flush_value(struct bt_conn *conn,
-		       const struct bt_gatt_attr *attr, uint8_t flags)
+static ssize_t flush_value(struct bt_conn *conn,
+			    const struct bt_gatt_attr *attr, uint8_t flags)
 {
 	struct gatt_value *value = attr->user_data;