3.5. Coding Guidelines

This document provides some additional guidelines to consider when writing TF-A code. These are not intended to be strictly-enforced rules like the contents of the Coding Style.

3.5.1. Automatic Editor Configuration

Many of the rules given below (such as indentation size, use of tabs, and newlines) can be set automatically using the EditorConfig configuration file in the root of the repository: .editorconfig. With a supported editor, the rules set out in this file can be automatically applied when you are editing files in the TF-A repository.

Several editors include built-in support for EditorConfig files, and many others support its functionality through plugins.

Use of the EditorConfig file is suggested but is not required.

3.5.2. Automatic Compliance Checking

To assist with coding style compliance, the project Makefile contains two targets which both utilise the checkpatch.pl script that ships with the Linux source tree. The project also defines certain checkpatch options in the .checkpatch.conf file in the top-level directory.


Checkpatch errors will gate upstream merging of pull requests. Checkpatch warnings will not gate merging but should be reviewed and fixed if possible.

To check the entire source tree, you must first download copies of checkpatch.pl, spelling.txt and const_structs.checkpatch available in the Linux master tree scripts directory, then set the CHECKPATCH environment variable to point to checkpatch.pl (with the other 2 files in the same directory) and build the checkcodebase target:

make CHECKPATCH=<path-to-linux>/linux/scripts/checkpatch.pl checkcodebase

To just check the style on the files that differ between your local branch and the remote master, use:

make CHECKPATCH=<path-to-linux>/linux/scripts/checkpatch.pl checkpatch

If you wish to check your patch against something other than the remote master, set the BASE_COMMIT variable to your desired branch. By default, BASE_COMMIT is set to origin/master. Ignored Checkpatch Warnings

Some checkpatch warnings in the TF codebase are deliberately ignored. These include:

  • **WARNING: line over 80 characters**: Although the codebase should generally conform to the 80 character limit this is overly restrictive in some cases.

  • **WARNING: Use of volatile is usually wrong: see Why the “volatile” type class should not be used . Although this document contains some very useful information, there are several legimate uses of the volatile keyword within the TF codebase.

3.5.3. Performance considerations Avoid printf and use logging macros

debug.h provides logging macros (for example, WARN and ERROR) which wrap tf_log and which allow the logging call to be compiled-out depending on the make command. Use these macros to avoid print statements being compiled unconditionally into the binary.

Each logging macro has a numerical log level:

#define LOG_LEVEL_NONE    0
#define LOG_LEVEL_ERROR   10
#define LOG_LEVEL_NOTICE  20
#define LOG_LEVEL_INFO    40

By default, all logging statements with a log level <= LOG_LEVEL_INFO will be compiled into debug builds and all statements with a log level <= LOG_LEVEL_NOTICE will be compiled into release builds. This can be overridden from the command line or by the platform makefile (although it may be necessary to clean the build directory first).

For example, to enable VERBOSE logging on FVP:

make PLAT=fvp LOG_LEVEL=50 all Use const data where possible

For example, the following code:

struct my_struct {
        int arg1;
        int arg2;

void init(struct my_struct *ptr);

void main(void)
        struct my_struct x;
        x.arg1 = 1;
        x.arg2 = 2;

is better written as:

struct my_struct {
        int arg1;
        int arg2;

void init(const struct my_struct *ptr);

void main(void)
        const struct my_struct x = { 1, 2 };

This allows the linker to put the data in a read-only data section instead of a writeable data section, which may result in a smaller and faster binary. Note that this may require dependent functions (init() in the above example) to have const arguments, assuming they don’t need to modify the data.

3.5.4. Libc functions that are banned or to be used with caution

Below is a list of functions that present security risks and either must not be used (Banned) or are discouraged from use and must be used with care (Caution).

libc function



strcpy, wcscpy, strncpy


use strlcpy instead

strcat, wcscat, strncat


use strlcat instead

sprintf, vsprintf


use snprintf, vsnprintf instead



ensure result fits in buffer i.e : snprintf(buf,size…) < size



inspect va_list match types specified in format string



use strtok_r or strsep instead

strtok_r, strsep


inspect for terminated input buffer



use equivalent strto* functions



Use snprintf instead

The libc component in the codebase will not add support for the banned APIs.

3.5.5. Error handling and robustness Using CASSERT to check for compile time data errors

Where possible, use the CASSERT macro to check the validity of data known at compile time instead of checking validity at runtime, to avoid unnecessary runtime code.

For example, this can be used to check that the assembler’s and compiler’s views of the size of an array is the same.

#include <cassert.h>

define MY_STRUCT_SIZE 8 /* Used by assembler source files */

struct my_struct {
    uint32_t arg1;
    uint32_t arg2;

CASSERT(MY_STRUCT_SIZE == sizeof(struct my_struct), assert_my_struct_size_mismatch);

If MY_STRUCT_SIZE in the above example were wrong then the compiler would emit an error like this:

my_struct.h:10:1: error: size of array ‘assert_my_struct_size_mismatch’ is negative Using assert() to check for programming errors

In general, each secure world TF image (BL1, BL2, BL31 and BL32) should be treated as a tightly integrated package; the image builder should be aware of and responsible for all functionality within the image, even if code within that image is provided by multiple entities. This allows us to be more aggressive in interpreting invalid state or bad function arguments as programming errors using assert(), including arguments passed across platform porting interfaces. This is in contrast to code in a Linux environment, which is less tightly integrated and may attempt to be more defensive by passing the error back up the call stack.

Where possible, badly written TF code should fail early using assert(). This helps reduce the amount of untested conditional code. By default these statements are not compiled into release builds, although this can be overridden using the ENABLE_ASSERTIONS build flag.


  • Bad argument supplied to library function

  • Bad argument provided by platform porting function

  • Internal secure world image state is inconsistent Handling integration errors

Each secure world image may be provided by a different entity (for example, a Trusted Boot vendor may provide the BL2 image, a TEE vendor may provide the BL32 image and the OEM/SoC vendor may provide the other images).

An image may contain bugs that are only visible when the images are integrated. The system integrator may not even have access to the debug variants of all the images in order to check if asserts are firing. For example, the release variant of BL1 may have already been burnt into the SoC. Therefore, TF code that detects an integration error should _not_ consider this a programming error, and should always take action, even in release builds.

If an integration error is considered non-critical it should be treated as a recoverable error. If the error is considered critical it should be treated as an unexpected unrecoverable error. Handling recoverable errors

The secure world must not crash when supplied with bad data from an external source. For example, data from the normal world or a hardware device. Similarly, the secure world must not crash if it detects a non-critical problem within itself or the system. It must make every effort to recover from the problem by emitting a WARN message, performing any necessary error handling and continuing.


  • Secure world receives SMC from normal world with bad arguments.

  • Secure world receives SMC from normal world at an unexpected time.

  • BL31 receives SMC from BL32 with bad arguments.

  • BL31 receives SMC from BL32 at unexpected time.

  • Secure world receives recoverable error from hardware device. Retrying the operation may help here.

  • Non-critical secure world service is not functioning correctly.

  • BL31 SPD discovers minor configuration problem with corresponding SP. Handling unrecoverable errors

In some cases it may not be possible for the secure world to recover from an error. This situation should be handled in one of the following ways:

  1. If the unrecoverable error is unexpected then emit an ERROR message and call panic(). This will end up calling the platform-specific function plat_panic_handler().

  2. If the unrecoverable error is expected to occur in certain circumstances, then emit an ERROR message and call the platform-specific function plat_error_handler().

Cases 1 and 2 are subtly different. A platform may implement plat_panic_handler and plat_error_handler in the same way (for example, by waiting for a secure watchdog to time-out or by invoking an interface on the platform’s power controller to reset the platform). However, plat_error_handler may take additional action for some errors (for example, it may set a flag so the platform resets into a different mode). Also, plat_panic_handler() may implement additional debug functionality (for example, invoking a hardware breakpoint).

Examples of unexpected unrecoverable errors:

  • BL32 receives an unexpected SMC response from BL31 that it is unable to recover from.

  • BL31 Trusted OS SPD code discovers that BL2 has not loaded the corresponding Trusted OS, which is critical for platform operation.

  • Secure world discovers that a critical hardware device is an unexpected and unrecoverable state.

  • Secure world receives an unexpected and unrecoverable error from a critical hardware device.

  • Secure world discovers that it is running on unsupported hardware.

Examples of expected unrecoverable errors:

  • BL1/BL2 fails to load the next image due to missing/corrupt firmware on disk.

  • BL1/BL2 fails to authenticate the next image due to an invalid certificate.

  • Secure world continuously receives recoverable errors from a hardware device but is unable to proceed without a valid response. Handling critical unresponsiveness

If the secure world is waiting for a response from an external source (for example, the normal world or a hardware device) which is critical for continued operation, it must not wait indefinitely. It must have a mechanism (for example, a secure watchdog) for resetting itself and/or the external source to prevent the system from executing in this state indefinitely.


  • BL1 is waiting for the normal world to raise an SMC to proceed to the next stage of the secure firmware update process.

  • A Trusted OS is waiting for a response from a proxy in the normal world that is critical for continued operation.

  • Secure world is waiting for a hardware response that is critical for continued operation.

3.5.6. Use of built-in C and libc data types

The TF-A codebase should be kept as portable as possible, especially since both 64-bit and 32-bit platforms are supported. To help with this, the following data type usage guidelines should be followed:

  • Where possible, use the built-in C data types for variable storage (for example, char, int, long long, etc) instead of the standard C99 types. Most code is typically only concerned with the minimum size of the data stored, which the built-in C types guarantee.

  • Avoid using the exact-size standard C99 types in general (for example, uint16_t, uint32_t, uint64_t, etc) since they can prevent the compiler from making optimizations. There are legitimate uses for them, for example to represent data of a known structure. When using them in struct definitions, consider how padding in the struct will work across architectures. For example, extra padding may be introduced in AArch32 systems if a struct member crosses a 32-bit boundary.

  • Use int as the default integer type - it’s likely to be the fastest on all systems. Also this can be assumed to be 32-bit as a consequence of the Procedure Call Standard for the Arm Architecture and the Procedure Call Standard for the Arm 64-bit Architecture .

  • Avoid use of short as this may end up being slower than int in some systems. If a variable must be exactly 16-bit, use int16_t or uint16_t.

  • Avoid use of long. This is guaranteed to be at least 32-bit but, given that int is 32-bit on Arm platforms, there is no use for it. For integers of at least 64-bit, use long long.

  • Use char for storing text. Use uint8_t for storing other 8-bit data.

  • Use unsigned for integers that can never be negative (counts, indices, sizes, etc). TF intends to comply with MISRA “essential type” coding rules (10.X), where signed and unsigned types are considered different essential types. Choosing the correct type will aid this. MISRA static analysers will pick up any implicit signed/unsigned conversions that may lead to unexpected behaviour.

  • For pointer types:

    • If an argument in a function declaration is pointing to a known type then simply use a pointer to that type (for example: struct my_struct *).

    • If a variable (including an argument in a function declaration) is pointing to a general, memory-mapped address, an array of pointers or another structure that is likely to require pointer arithmetic then use uintptr_t. This will reduce the amount of casting required in the code. Avoid using unsigned long or unsigned long long for this purpose; it may work but is less portable.

    • For other pointer arguments in a function declaration, use void *. This includes pointers to types that are abstracted away from the known API and pointers to arbitrary data. This allows the calling function to pass a pointer argument to the function without any explicit casting (the cast to void * is implicit). The function implementation can then do the appropriate casting to a specific type.

    • Avoid pointer arithmetic generally (as this violates MISRA C 2012 rule 18.4) and especially on void pointers (as this is only supported via language extensions and is considered non-standard). In TF-A, setting the W build flag to W=3 enables the -Wpointer-arith compiler flag and this will emit warnings where pointer arithmetic is used.

    • Use ptrdiff_t to compare the difference between 2 pointers.

  • Use size_t when storing the sizeof() something.

  • Use ssize_t when returning the sizeof() something from a function that can also return an error code; the signed type allows for a negative return code in case of error. This practice should be used sparingly.

  • Use u_register_t when it’s important to store the contents of a register in its native size (32-bit in AArch32 and 64-bit in AArch64). This is not a standard C99 type but is widely available in libc implementations, including the FreeBSD version included with the TF codebase. Where possible, cast the variable to a more appropriate type before interpreting the data. For example, the following struct in ep_info.h could use this type to minimize the storage required for the set of registers:

typedef struct aapcs64_params {
        u_register_t arg0;
        u_register_t arg1;
        u_register_t arg2;
        u_register_t arg3;
        u_register_t arg4;
        u_register_t arg5;
        u_register_t arg6;
        u_register_t arg7;
} aapcs64_params_t;

If some code wants to operate on arg0 and knows that it represents a 32-bit unsigned integer on all systems, cast it to unsigned int.

These guidelines should be updated if additional types are needed.

3.5.7. Favor C language over assembly language

Generally, prefer code written in C over assembly. Assembly code is less portable, harder to understand, maintain and audit security wise. Also, static analysis tools generally don’t analyze assembly code.

If specific system-level instructions must be used (like cache maintenance operations), please consider using inline assembly. The arch_helpers.h files already define inline functions for a lot of these.

There are, however, legitimate uses of assembly language. These are usually early boot (eg. cpu reset sequences) and exception handling code before the C runtime environment is set up.

When writing assembly please note that a wide variety of common instruction sequences have helper macros in asm_macros.S which are preferred over writing them directly. This is especially important for debugging purposes as debug symbols must manually be included. Please use the func_prologue and func_epilogue macros if you need to use the stack. Also, obeying the Procedure Call Standard (PCS) is generally recommended.

3.5.8. Do not use weak functions


The following guideline applies more strongly to common, platform-independent code. For plaform code (under plat/ directory), it is up to each platform maintainer to decide whether this should be striclty enforced or not.

The use of weak functions is highly discouraged in the TF-A codebase. Newly introduced platform interfaces should be strongly defined, wherever possible. In the rare cases where this is not possible or where weak functions appear as the best tool to solve the problem at hand, this should be discussed with the project’s maintainers and justified in the code.

For the purpose of providing a default implementation of a platform interface, an alternative to weak functions is to provide a strongly-defined implementation under the plat/common/ directory. Then platforms have two options to pull in this implementation:

  • They can include the source file through the platform’s makefile. Note that this method is suitable only if the platform wants all default implementations defined in this file, else either the file should be refactored or the next approach should be used.

  • They access the platform interface through a constant function pointer.

In both cases, what matters is that platforms include the default implementation as a conscious decision.


Weak functions may sound useful to simplify the initial porting effort to a new platform, such that one can quickly get the firmware to build and link, without implementing all platform interfaces from the beginning. For this reason, the TF-A project used to make heavy use of weak functions and there are still many outstanding usages of them across the code base today. We intend to convert them to strongly-defined functions over time.

However, weak functions also have major drawbacks, which we consider outweighing their benefits. They can make it hard to identify which implementation gets built into the firmware, especially when using multiple levels of “weakness”. This has resulted in bugs in the past.

Weak functions are also forbidden by MISRA coding guidelines, which TF-A aims to comply with.

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