C Support
ESP-IDF is primarily written in C and provides C APIs. ESP-IDF uses Newlib as its C Standard Library implementation (the Newlib version is specified in newlib/sbom.yml). In general, all C features that are supported by the compiler (currently GCC) can be used in ESP-IDF, unless specified in Unsupported C Features below.
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:
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
The following features are not supported in ESP-IDF.
Nested Function Pointers
The GNU dialect of ISO C17 supports nested functions. However, ESP-IDF does not support referencing nested functions as pointers. This is due to the fact that the GCC compiler generates a trampoline (i.e., small piece of executable code) on the stack when a pointer to a nested function is referenced. ESP-IDF does not permit executing code from a stack, thus use of pointers to nested functions is not supported.