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C Language Support

C is a general-purpose low-level programming language that is widely used for writing code for embedded systems.

Zephyr is primarily written in C and natively supports applications written in the C language. All Zephyr API functions and macros are implemented in C and available as part of the C header files under the include directory, so writing Zephyr applications in C gives the developers access to the most features.

The main() function must have the return type of int as Zephyr applications run in a “hosted” environment as defined by the C standard. Applications must return zero (0) from main. All non-zero return values are reserved.

Language Standards

Zephyr does not target a specific version of the C standards; however, the Zephyr codebase makes extensive use of the features newly introduced in the 1999 release of the ISO C standard (ISO/IEC 9899:1999, hereinafter referred to as C99) such as those listed below, effectively requiring the use of a compiler toolchain that supports the C99 standard and above:

• inline functions

• standard boolean types (bool in <stdbool.h>)

• fixed-width integer types ([u]intN_t in <stdint.h>)

• designated initializers

• variadic macros

• restrict qualification

Some Zephyr components make use of the features newly introduced in the 2011 release of the ISO C standard (ISO/IEC 9899:2011, hereinafter referred to as C11) such as the type-generic expressions using the _Generic keyword. For example, the cbprintf() component, used as the default formatted output processor for Zephyr, makes use of the C11 type-generic expressions, and this effectively requires most Zephyr applications to be compiled using a compiler toolchain that supports the C11 standard and above.

In summary, it is recommended to use a compiler toolchain that supports at least the C11 standard for developing with Zephyr. It is, however, important to note that some optional Zephyr components and external modules may make use of the C language features that have been introduced in more recent versions of the standards, in which case it will be necessary to use a more up-to-date compiler toolchain that supports such standards.

Standard Library

The C Standard Library is an integral part of any C program, and Zephyr provides the support for a number of different C libraries for the applications to choose from, depending on the compiler toolchain being used to build the application.

• Common C library code
  • Time function
  • Dynamic Memory Management
• Minimal libc
  • Functions
  • Formatted Output
  • Dynamic Memory Management
  • Error numbers
• Newlib
  • Types of Newlib
  • Formatted Output
  • Dynamic Memory Management
• Picolibc
  • Picolibc Module
  • Toolchain Picolibc

Formatted Output

C defines standard formatted output functions such as printf and sprintf and these functions are implemented by the C standard libraries.

Each C standard library has its own set of requirements and configurations for selecting the formatted output modes and capabilities. Refer to each C standard library documentation for more details.

Dynamic Memory Management

C defines a standard dynamic memory management interface (for example, malloc() and free()) and these functions are implemented by the C standard libraries.

While the details of the dynamic memory management implementation varies across different C standard libraries, all supported libraries must conform to the following conventions. Every supported C standard library shall:

• manage its own memory heap either internally or by invoking the hook functions (for example, sbrk()) implemented in libc-hooks.c.

• maintain the architecture- and memory region-specific alignment requirements for the memory blocks allocated by the standard dynamic memory allocation interface (for example, malloc()).

• allocate memory blocks inside the z_malloc_partition memory partition when userspace is enabled. See Pre-defined Memory Partitions.

For more details regarding the C standard library-specific memory management implementation, refer to each C standard library documentation.

Note

Native Zephyr applications should use the memory management API supported by the Zephyr kernel such as k_malloc() in order to take advantage of the advanced features that they offer.

C standard dynamic memory management interface functions such as malloc() should be used only by the portable applications and libraries that target multiple operating systems.