docs: Added C documentation page

Closes https://github.com/espressif/esp-idf/issues/12439
Closes https://github.com/espressif/esp-idf/issues/12001

docs/en/api-guides/c.rst

@@ -0,0 +1,36 @@
+C Support
+===========
+
+:link_to_translation:`zh_CN:[中文]`
+
+.. highlight:: cpp
+
+ESP-IDF is primarily written in C and provides C APIs. `Newlib <https://sourceware.org/newlib/>`_ is used as standard C library (the Newlib version can be found in :component_file:`newlib/sbom.yml`). In general, all C features supported by the compiler, currently GCC, should be available in ESP-IDF, unless specified in :ref:`unsupported_c_features` below.
+
+.. _c_version:
+
+C Version
+---------
+
+**GNU dialect of ISO C17** (``--std=gnu17``) is the current default C version in ESP-IDF.
+
+To compile the source code of a certain component using a different language standard, set the desired compiler flag in the component's ``CMakeLists.txt`` file:
+
+.. code-block:: cmake
+
+    idf_component_register( ... )
+    target_compile_options(${COMPONENT_LIB} PRIVATE -std=gnu11)
+
+If the public header files of the component also need to be compiled with the same language standard, replace the flag ``PRIVATE`` with ``PUBLIC``.
+
+.. _unsupported_c_features:
+
+Unsupported C Features
+----------------------
+
+The following features are not supported in ESP-IDF.
+
+Nested Function Pointers
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The **GNU dialect of ISO C17** supports `nested functions <https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html>`_. However, they do not work in ESP-IDF when referenced as function pointer because the compiler generates a trampoline on the stack, while the stack is not executable in ESP-IDF. Hence, do not use function pointers to nested functions.

docs/en/api-guides/index.rst

@@ -12,8 +12,9 @@ API Guides
    bootloader
    build-system
    :SOC_SUPPORT_COEXISTENCE: coexist
-   core_dump
+   c
    cplusplus
+   core_dump
    current-consumption-measurement-modules
    :SOC_RTC_MEM_SUPPORTED: deep-sleep-stub
    :SOC_USB_OTG_SUPPORTED: dfu

docs/zh_CN/api-guides/c.rst

@@ -0,0 +1 @@
+.. include:: ../../en/api-guides/c.rst

docs/zh_CN/api-guides/index.rst

@@ -12,8 +12,9 @@ API 指南
    bootloader
    build-system
    :SOC_SUPPORT_COEXISTENCE: coexist
-   core_dump
+   c
    cplusplus
+   core_dump
    current-consumption-measurement-modules
    :SOC_RTC_MEM_SUPPORTED: deep-sleep-stub
    :SOC_USB_OTG_SUPPORTED: dfu

tools/cmake/build.cmake

@@ -129,7 +129,7 @@ function(__build_set_lang_version)
     if(NOT IDF_TARGET STREQUAL "linux")
         # Building for chip targets: we use a known version of the toolchain.
         # Use latest supported versions.
-        # Please update docs/en/api-guides/cplusplus.rst and
+        # Please update docs/en/api-guides/c.rst, docs/en/api-guides/cplusplus.rst and
         # tools/test_apps/system/cxx_build_test/main/test_cxx_standard.cpp when changing this.
         set(c_std gnu17)
         set(cxx_std gnu++2b)