fix(modem): Truncate C-API out buffer len to API_STR_MAX

And document the use of C-API and min buffer requirements

components/esp_modem/Kconfig

@@ -75,6 +75,9 @@ menu "esp-modem"
         help
             Some AT commands returns textual values which C-API copy as c-string to user allocated space,
             it also truncates the output data to this size. Increase this if some AT answers are truncated.
+            Callers of esp_modem_get_imsi(), esp_modem_get_imei(), esp_modem_get_operator_name(),
+            esp_modem_get_module_name(), esp_modem_at(), and esp_modem_at_raw() must allocate output
+            buffers of at least this many bytes (see ESP_MODEM_C_API_STR_BUF_SIZE).
 
     config ESP_MODEM_URC_HANDLER
         bool "Enable support for adding URC handler"

components/esp_modem/command/include/esp_modem_api.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2021-2025 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2021-2026 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -21,7 +21,7 @@ extern "C" {
 esp_err_t esp_modem_sync(esp_modem_dce_t *dce);
 /**
  * @brief Reads the operator name
- * @param[out] name operator name
+ * @param[out] name operator name; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @param[out] act access technology
  * @return OK, FAIL or TIMEOUT
  */
@@ -47,7 +47,7 @@ esp_err_t esp_modem_reset_pin(esp_modem_dce_t *dce, const char *puk, const char
 /**
  * @brief Execute the supplied AT command in raw mode (doesn't append '\r' to command, returns everything)
  * @param[in] cmd String command that's send to DTE
- * @param[out] out Raw output from DTE
+ * @param[out] out Raw output from DTE; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @param[in] pass Pattern in response for the API to return OK
  * @param[in] fail Pattern in response for the API to return FAIL
  * @param[in] timeout AT command timeout in milliseconds
@@ -57,7 +57,7 @@ esp_err_t esp_modem_at_raw(esp_modem_dce_t *dce, const char *cmd, char *out, con
 /**
  * @brief Execute the supplied AT command
  * @param[in] cmd AT command
- * @param[out] out Command output string
+ * @param[out] out Command output string; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @param[in] timeout AT command timeout in milliseconds
  * @return OK, FAIL or TIMEOUT
  */
@@ -121,19 +121,19 @@ esp_err_t esp_modem_set_command_mode(esp_modem_dce_t *dce);
 esp_err_t esp_modem_set_cmux(esp_modem_dce_t *dce);
 /**
  * @brief Reads the IMSI number
- * @param[out] imsi Module's IMSI number
+ * @param[out] imsi Module's IMSI number; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @return OK, FAIL or TIMEOUT
  */
 esp_err_t esp_modem_get_imsi(esp_modem_dce_t *dce, char *imsi);
 /**
  * @brief Reads the IMEI number
- * @param[out] imei Module's IMEI number
+ * @param[out] imei Module's IMEI number; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @return OK, FAIL or TIMEOUT
  */
 esp_err_t esp_modem_get_imei(esp_modem_dce_t *dce, char *imei);
 /**
  * @brief Reads the module name
- * @param[out] name module name
+ * @param[out] name module name; must point to a buffer of at least @c ESP_MODEM_C_API_STR_BUF_SIZE bytes
  * @return OK, FAIL or TIMEOUT
  */
 esp_err_t esp_modem_get_module_name(esp_modem_dce_t *dce, char *name);

components/esp_modem/examples/pppos_client/main/pppos_client_main.c

@@ -260,7 +260,7 @@ void app_main(void)
 
 #ifdef CONFIG_EXAMPLE_MODEM_DEVICE_CUSTOM
     {
-        char time[64];
+        char time[ESP_MODEM_C_API_STR_BUF_SIZE];
         err = esp_modem_get_time(dce, time);
         if (err != ESP_OK) {
             ESP_LOGE(TAG, "esp_modem_get_time failed with %d %s", err, esp_err_to_name(err));
@@ -332,7 +332,7 @@ void app_main(void)
         ESP_LOGE(TAG, "esp_modem_set_mode(ESP_MODEM_MODE_COMMAND) failed with %d", err);
         return;
     }
-    char imsi[32];
+    char imsi[ESP_MODEM_C_API_STR_BUF_SIZE];
     err = esp_modem_get_imsi(dce, imsi);
     if (err != ESP_OK) {
         ESP_LOGE(TAG, "esp_modem_get_imsi failed with %d", err);

components/esp_modem/include/esp_modem_c_api_types.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include "sdkconfig.h"
 #include "esp_modem_config.h"
 #include "esp_netif.h"
 
@@ -25,6 +26,15 @@ typedef struct esp_modem_PdpContext_t {
     const char *apn;
 } esp_modem_PdpContext_t;
 
+/**
+ * @brief Minimum byte size of caller-allocated buffers for C-API string output parameters.
+ *
+ * Buffers passed to esp_modem_get_imsi(), esp_modem_get_imei(),
+ * esp_modem_get_operator_name(), esp_modem_get_module_name(), esp_modem_at(), and
+ * esp_modem_at_raw() must be at least this many bytes (including space for a NUL terminator).
+ */
+#define ESP_MODEM_C_API_STR_BUF_SIZE CONFIG_ESP_MODEM_C_API_STR_MAX
+
 /**
  * @brief SIM PIN status reported by AT+CPIN?
  */

components/esp_modem/src/esp_modem_c_api.cpp

@@ -20,6 +20,14 @@
 size_t strlcpy(char *dest, const char *src, size_t len);
 #endif
 
+static void truncate_to_c_api_max(std::string &s)
+{
+    const size_t max_len = CONFIG_ESP_MODEM_C_API_STR_MAX - 1;
+    if (s.size() > max_len) {
+        s.resize(max_len);
+    }
+}
+
 #ifdef CONFIG_ESP_MODEM_ADD_CUSTOM_MODULE
 #include CONFIG_ESP_MODEM_CUSTOM_MODULE_HEADER
 #endif
@@ -239,6 +247,7 @@ extern "C" esp_err_t esp_modem_at(esp_modem_dce_t *dce_wrap, const char *at, cha
     std::string at_str(at);
     auto ret = command_response_to_esp_err(dce_wrap->dce->at(at_str, out, timeout));
     if ((p_out != NULL)) {
+        truncate_to_c_api_max(out);
         strlcpy(p_out, out.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
     }
     return ret;
@@ -270,6 +279,7 @@ extern "C" esp_err_t esp_modem_get_imsi(esp_modem_dce_t *dce_wrap, char *p_imsi)
     std::string imsi;
     auto ret = command_response_to_esp_err(dce_wrap->dce->get_imsi(imsi));
     if (ret == ESP_OK) {
+        truncate_to_c_api_max(imsi);
         strlcpy(p_imsi, imsi.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
     }
     return ret;
@@ -283,6 +293,7 @@ extern "C" esp_err_t esp_modem_at_raw(esp_modem_dce_t *dce_wrap, const char *cmd
     std::string out;
     auto ret = command_response_to_esp_err(dce_wrap->dce->at_raw(cmd, out, pass, fail, timeout));
     if ((p_out != NULL)) {
+        truncate_to_c_api_max(out);
         strlcpy(p_out, out.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
     }
     return ret;
@@ -313,6 +324,7 @@ extern "C" esp_err_t esp_modem_get_imei(esp_modem_dce_t *dce_wrap, char *p_imei)
     std::string imei;
     auto ret = command_response_to_esp_err(dce_wrap->dce->get_imei(imei));
     if (ret == ESP_OK) {
+        truncate_to_c_api_max(imei);
         strlcpy(p_imei, imei.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
     }
     return ret;
@@ -328,6 +340,7 @@ extern "C" esp_err_t esp_modem_get_operator_name(esp_modem_dce_t *dce_wrap, char
     auto ret = command_response_to_esp_err(dce_wrap->dce->get_operator_name(name, act));
     if (ret == ESP_OK) {
         if (p_name != nullptr) {
+            truncate_to_c_api_max(name);
             strlcpy(p_name, name.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
         }
         if (p_act != nullptr) {
@@ -345,6 +358,7 @@ extern "C" esp_err_t esp_modem_get_module_name(esp_modem_dce_t *dce_wrap, char *
     std::string name;
     auto ret = command_response_to_esp_err(dce_wrap->dce->get_module_name(name));
     if (ret == ESP_OK) {
+        truncate_to_c_api_max(name);
         strlcpy(p_name, name.c_str(), CONFIG_ESP_MODEM_C_API_STR_MAX);
     }
     return ret;