Libraries and Floating Point Support Guide

Use of static data in the C libraries

ARM Compiler toolchain v5.02 for µVision Using ARM C and C++ Libraries and Floating-Point Support

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Use of static data in the C libraries

Static data refers to persistent read/write data that is not stored on the stack or the heap. This persistent data can be external or internal in scope, and is:

at a fixed address, when compiled with --apcs /norwpi
at a fixed address relative to the static base, register r9, when compiled with --apcs /rwpi.

Libraries that use static data might be reentrant, but this depends on their use of the __user_libspace static data area, and on the build options you choose:

When compiled with --apcs /norwpi, read/write static data is addressed in a position-dependent fashion. This is the default. Code from these variants is single-threaded because it uses read/write static data.
When compiled with --apcs /rwpi, read/write static data is addressed in a position-independent fashion using offsets from the static base register sb. Code from these variants is reentrant and can be multithreaded if each thread uses a different static base value.

The following describes how the C libraries use static data:

The default floating-point arithmetic libraries fz_* and fj_* do not use static data and are always reentrant. However, the f_* and g_* libraries do use static data.
All statically-initialized data in the C libraries is read-only.
All writable static data is zero initialized.
Most C library functions use no writable static data and are reentrant whether built with default build options, --apcs /norwpi or reentrant build options, --apcs /rwpi.
Some functions have static data implicit in their definitions. You must not use these in a reentrant application unless you build it with --apcs /rwpi and the callers use different values in sb.

Note

Exactly which functions use static data in their definitions might change in future releases.

Callouts from the C library enable access to static data. C library functions that use static data can be categorized as:

functions that do not use any static data of any kind, for example fprintf()
functions that manage a static state, such as malloc(), rand(), and strtok()
functions that do not manage a static state, but use static data in a way that is specific to the implementation in the ARM compiler, for example isalpha().

When the C library does something that requires implicit static data, it uses a callout to a function you can replace. These functions are shown in Table 1. They do not use semihosting.

Table 1. C library callouts

Function	Description
`__rt_errno_addr()`	Called to get the address of the variable `errno`
`__rt_fp_status_addr()`	Called by the floating-point support code to get the address of the floating-point status word
`locale` functions	The function `__user_libspace()` creates a block of private static data for the library

The default implementation of __user_libspace creates a 96-byte block in the ZI region. Even if your application does not have a main() function, the __user_libspace() function does not normally have to be redefined.

Note

Exactly which functions use static data in their definitions might change in future releases.

Libraries and Floating Point Support Guide

Use of static data in the C libraries

Use of static data in the C libraries

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