C Support
ESP-IDF is primarily written in C and provides C APIs. Newlib is used as standard C library (the Newlib version can be found in newlib/sbom.yml). In general, all C features supported by the compiler, currently GCC, should be available 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, 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.