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Libraries and Floating Point Support Guide

Preface The ARM C and C++ Libraries Mandatory linkage with the C library C and C++ runtime libraries Summary of the C and C++ runtime libraries Compliance with the Application Binary Interface ( Increasing portability of object files to other CL ARM C and C++ library directory structure Selection of ARM C and C++ library variants based Thumb C libraries C and C++ library features C++ and C libraries and the std namespace Multithreaded support in ARM C libraries ARM C libraries and multithreading ARM C libraries and reentrant functions ARM C libraries and thread-safe functions Use of static data in the C libraries Use of the __user_libspace static data area by the C library functions to access subsections of the _ Re-implementation of legacy function __user_libspa Management of locks in multithreaded applications How to ensure re-implemented mutex functions are c Using the ARM C library in a multithreaded environ Thread safety in the ARM C library Thread safety in the ARM C++ library The floating-point status word in a multithreaded Support for building an application with the C lib Using the C library with an application Using the C and C++ libraries with an application Using $Sub$$ to mix semihosted and nonsemihosted I Using the libraries in a nonsemihosting environmen C++ exceptions in a non-semihosting environment Direct semihosting C library function dependencies Indirect semihosting C library function dependenci C library API definitions for targeting a differen Support for building an application without the C Building an application without the C library Creating an application as bare machine C without Integer and floating-point compiler functions and Bare machine integer C Bare machine C with floating-point processing Customized C library startup code and access to C Using low-level functions when exploiting the C li Using high-level functions when exploiting the C l Using malloc() when exploiting the C library Tailoring the C library to a new execution environ Initialization of the execution environment and ex C++ initialization, construction and destruction Exceptions system initialization Emergency buffer memory for exceptions Library functions called from main() Program exit and the assert macro Assembler macros that tailor locale functions in t Link time selection of the locale subsystem in the Runtime selection of the locale subsystem in the C Definition of locale data blocks in the C library LC_CTYPE data block LC_COLLATE data block LC_MONETARY data block LC_NUMERIC data block LC_TIME data block Modification of C library functions for error sign Stack and heap memory allocation and the ARM C and Library heap usage requirements of the ARM C and C Choosing a heap implementation for memory allocati Stack pointer initialization and heap bounds Legacy support for __user_initial_stackheap() Avoiding the heap and heap-using library functions Tailoring input/output functions in the C and C++ Target dependencies on low-level functions in the The C library printf family of functions The C library scanf family of functions Redefining low-level library functions to enable d The C library functions fread(), fgets() and gets( Re-implementing __backspace() in the C library Re-implementing __backspacewc() in the C library Redefining target-dependent system I/O functions i Tailoring non-input/output C library functions Real-time integer division in the ARM libraries ISO C library implementation definition How the ARM C library fulfills ISO C specification mathlib error handling ISO-compliant implementation of signals supported ISO-compliant C library input/output characteristi Standard C++ library implementation definition C library functions and extensions Compiler generated and library-resident helper fun C and C++ library naming conventions Using macro__ARM_WCHAR_NO_IO to disable FILE decla Using library functions with execute-only memory The ARM C Micro-library Floating-point Support The C and C++ Library Functions reference Floating-point Support Functions Reference

Re-implementing __backspace() in the C library

1.18 Re-implementing __backspace() in the C library

The function __backspace() is used by the scanf family of functions, and must be re-implemented if you retarget the stdio arrangements at the fgetc() level.


Normally, you are not required to call __backspace() directly, unless you are implementing your own scanf-like function.
The syntax is:
int __backspace(FILE *stream);
__backspace(stream) must only be called after reading a character from the stream. You must not call it after a write, a seek, or immediately after opening the file, for example. It returns to the stream the last character that was read from the stream, so that the same character can be read from the stream again by the next read operation. This means that a character that was read from the stream by scanf but that is not required (that is, it terminates the scanf operation) is read correctly by the next function that reads from the stream.
__backspace is separate from ungetc(). This is to guarantee that a single character can be pushed back after the scanf family of functions has finished.
The value returned by __backspace() is either 0 (success) or EOF (failure). It returns EOF only if used incorrectly, for example, if no characters have been read from the stream. When used correctly, __backspace() must always return 0, because the scanf family of functions do not check the error return.
The interaction between __backspace() and ungetc() is:
  • If you apply __backspace() to a stream and then ungetc() a character into the same stream, subsequent calls to fgetc() must return first the character returned by ungetc(), and then the character returned by __backspace().
  • If you ungetc() a character back to a stream, then read it with fgetc(), and then backspace it, the next character read by fgetc() must be the same character that was returned to the stream. That is the __backspace() operation must cancel the effect of the fgetc() operation. However, another call to ungetc() after the call to __backspace() is not required to succeed.
  • The situation where you ungetc() a character into a stream and then __backspace() another one immediately, with no intervening read, never arises. __backspace() must only be called after fgetc(), so this sequence of calls is illegal. If you are writing __backspace() implementations, you can assume that the ungetc() of a character into a stream followed immediately by a __backspace() with no intervening read, never occurs.
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