Top (Using the GNU Compiler Collection (GCC))

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “Funding Free Software”, the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.

1 Programming Languages Supported by GCC
2 Language Standards Supported by GCC
- 2.1 C Language
- 2.2 C++ Language
- 2.3 Objective-C and Objective-C++ Languages
- 2.4 Go Language
- 2.5 D language
- 2.6 Modula-2 language
- 2.7 References for Other Languages
3 GCC Command Options
- 3.1 Option Summary
- 3.2 Options Controlling the Kind of Output
- 3.3 Compiling C++ Programs
- 3.4 Options Controlling C Dialect
- 3.5 Options Controlling C++ Dialect
- 3.6 Options Controlling Objective-C and Objective-C++ Dialects
- 3.7 Options to Control Diagnostic Messages Formatting
- 3.8 Options to Request or Suppress Warnings
- 3.9 Options That Control Static Analysis
- 3.10 Options for Debugging Your Program
- 3.11 Options That Control Optimization
- 3.12 Program Instrumentation Options
- 3.13 Options Controlling the Preprocessor
- 3.14 Passing Options to the Assembler
- 3.15 Options for Linking
- 3.16 Options for Directory Search
- 3.17 Options for Code Generation Conventions
- 3.18 GCC Developer Options
- 3.19 Machine-Dependent Options
  - 3.19.1 AArch64 Options
    - 3.19.1.1 -march and -mcpu Feature Modifiers
  - 3.19.2 Adapteva Epiphany Options
  - 3.19.3 AMD GCN Options
  - 3.19.4 ARC Options
  - 3.19.5 ARM Options
  - 3.19.6 AVR Options
    - 3.19.6.1 EIND and Devices with More Than 128 Ki Bytes of Flash
    - 3.19.6.2 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers
    - 3.19.6.3 AVR Built-in Macros
    - 3.19.6.4 AVR Internal Options
  - 3.19.7 Blackfin Options
  - 3.19.8 C6X Options
  - 3.19.9 CRIS Options
  - 3.19.10 C-SKY Options
  - 3.19.11 Darwin Options
  - 3.19.12 DEC Alpha Options
  - 3.19.13 eBPF Options
  - 3.19.14 FR30 Options
  - 3.19.15 FT32 Options
  - 3.19.16 FRV Options
  - 3.19.17 GNU/Linux Options
  - 3.19.18 H8/300 Options
  - 3.19.19 HPPA Options
  - 3.19.20 IA-64 Options
  - 3.19.21 LM32 Options
  - 3.19.22 LoongArch Options
  - 3.19.23 M32C Options
  - 3.19.24 M32R/D Options
  - 3.19.25 M680x0 Options
  - 3.19.26 MCore Options
  - 3.19.27 MicroBlaze Options
  - 3.19.28 MIPS Options
  - 3.19.29 MMIX Options
  - 3.19.30 MN10300 Options
  - 3.19.31 Moxie Options
  - 3.19.32 MSP430 Options
  - 3.19.33 NDS32 Options
  - 3.19.34 Nvidia PTX Options
  - 3.19.35 OpenRISC Options
  - 3.19.36 PDP-11 Options
  - 3.19.37 PowerPC Options
  - 3.19.38 PRU Options
  - 3.19.39 RISC-V Options
  - 3.19.40 RL78 Options
  - 3.19.41 IBM RS/6000 and PowerPC Options
  - 3.19.42 RX Options
  - 3.19.43 S/390 and zSeries Options
  - 3.19.44 SH Options
  - 3.19.45 Solaris 2 Options
  - 3.19.46 SPARC Options
  - 3.19.47 Options for System V
  - 3.19.48 V850 Options
  - 3.19.49 VAX Options
  - 3.19.50 Visium Options
  - 3.19.51 VMS Options
  - 3.19.52 VxWorks Options
  - 3.19.53 x86 Options
  - 3.19.54 x86 Windows Options
  - 3.19.55 Cygwin and MinGW Options
  - 3.19.56 Xstormy16 Options
  - 3.19.57 Xtensa Options
  - 3.19.58 zSeries Options
- 3.20 Specifying Subprocesses and the Switches to Pass to Them
- 3.21 Environment Variables Affecting GCC
- 3.22 Using Precompiled Headers
- 3.23 C++ Modules
  - 3.23.1 Module Mapper
  - 3.23.2 Module Preprocessing
  - 3.23.3 Compiled Module Interface
4 C Implementation-Defined Behavior
- 4.1 Translation
- 4.2 Environment
- 4.3 Identifiers
- 4.4 Characters
- 4.5 Integers
- 4.6 Floating Point
- 4.7 Arrays and Pointers
- 4.8 Hints
- 4.9 Structures, Unions, Enumerations, and Bit-Fields
- 4.10 Qualifiers
- 4.11 Declarators
- 4.12 Statements
- 4.13 Preprocessing Directives
- 4.14 Library Functions
- 4.15 Architecture
- 4.16 Locale-Specific Behavior
5 C++ Implementation-Defined Behavior
- 5.1 Conditionally-Supported Behavior
- 5.2 Exception Handling
6 Extensions to the C Language Family
- 6.1 Statements and Declarations in Expressions
- 6.2 Locally Declared Labels
- 6.3 Labels as Values
- 6.4 Nested Functions
- 6.5 Nonlocal Gotos
- 6.6 Constructing Function Calls
- 6.7 Referring to a Type with typeof
- 6.8 Conditionals with Omitted Operands
- 6.9 128-bit Integers
- 6.10 Double-Word Integers
- 6.11 Complex Numbers
- 6.12 Additional Floating Types
- 6.13 Half-Precision Floating Point
- 6.14 Decimal Floating Types
- 6.15 Hex Floats
- 6.16 Fixed-Point Types
- 6.17 Named Address Spaces
  - 6.17.1 AVR Named Address Spaces
  - 6.17.2 M32C Named Address Spaces
  - 6.17.3 PRU Named Address Spaces
  - 6.17.4 RL78 Named Address Spaces
  - 6.17.5 x86 Named Address Spaces
- 6.18 Arrays of Length Zero
- 6.19 Structures with No Members
- 6.20 Unions with Flexible Array Members
- 6.21 Structures with only Flexible Array Members
- 6.22 Arrays of Variable Length
- 6.23 Macros with a Variable Number of Arguments.
- 6.24 Slightly Looser Rules for Escaped Newlines
- 6.25 Non-Lvalue Arrays May Have Subscripts
- 6.26 Arithmetic on void- and Function-Pointers
- 6.27 Pointer Arguments in Variadic Functions
- 6.28 Pointers to Arrays with Qualifiers Work as Expected
- 6.29 Non-Constant Initializers
- 6.30 Compound Literals
- 6.31 Designated Initializers
- 6.32 Case Ranges
- 6.33 Cast to a Union Type
- 6.34 Mixed Declarations, Labels and Code
- 6.35 Declaring Attributes of Functions
  - 6.35.1 Common Function Attributes
  - 6.35.2 AArch64 Function Attributes
    - 6.35.2.1 Inlining rules
  - 6.35.3 AMD GCN Function Attributes
  - 6.35.4 ARC Function Attributes
  - 6.35.5 ARM Function Attributes
  - 6.35.6 AVR Function Attributes
  - 6.35.7 Blackfin Function Attributes
  - 6.35.8 BPF Function Attributes
  - 6.35.9 C-SKY Function Attributes
  - 6.35.10 Epiphany Function Attributes
  - 6.35.11 H8/300 Function Attributes
  - 6.35.12 IA-64 Function Attributes
  - 6.35.13 M32C Function Attributes
  - 6.35.14 M32R/D Function Attributes
  - 6.35.15 m68k Function Attributes
  - 6.35.16 MCORE Function Attributes
  - 6.35.17 MicroBlaze Function Attributes
  - 6.35.18 Microsoft Windows Function Attributes
  - 6.35.19 MIPS Function Attributes
  - 6.35.20 MSP430 Function Attributes
  - 6.35.21 NDS32 Function Attributes
  - 6.35.22 Nvidia PTX Function Attributes
  - 6.35.23 PowerPC Function Attributes
  - 6.35.24 RISC-V Function Attributes
  - 6.35.25 RL78 Function Attributes
  - 6.35.26 RX Function Attributes
  - 6.35.27 S/390 Function Attributes
  - 6.35.28 SH Function Attributes
  - 6.35.29 Symbian OS Function Attributes
  - 6.35.30 V850 Function Attributes
  - 6.35.31 Visium Function Attributes
  - 6.35.32 x86 Function Attributes
    - 6.35.32.1 Inlining rules
  - 6.35.33 Xstormy16 Function Attributes
- 6.36 Specifying Attributes of Variables
  - 6.36.1 Common Variable Attributes
  - 6.36.2 ARC Variable Attributes
  - 6.36.3 AVR Variable Attributes
  - 6.36.4 Blackfin Variable Attributes
  - 6.36.5 H8/300 Variable Attributes
  - 6.36.6 IA-64 Variable Attributes
  - 6.36.7 LoongArch Variable Attributes
  - 6.36.8 M32R/D Variable Attributes
  - 6.36.9 Microsoft Windows Variable Attributes
  - 6.36.10 MSP430 Variable Attributes
  - 6.36.11 Nvidia PTX Variable Attributes
  - 6.36.12 PowerPC Variable Attributes
  - 6.36.13 RL78 Variable Attributes
  - 6.36.14 V850 Variable Attributes
  - 6.36.15 x86 Variable Attributes
  - 6.36.16 Xstormy16 Variable Attributes
- 6.37 Specifying Attributes of Types
  - 6.37.1 Common Type Attributes
  - 6.37.2 ARC Type Attributes
  - 6.37.3 ARM Type Attributes
  - 6.37.4 BPF Type Attributes
  - 6.37.5 PowerPC Type Attributes
  - 6.37.6 x86 Type Attributes
- 6.38 Label Attributes
- 6.39 Enumerator Attributes
- 6.40 Statement Attributes
- 6.41 Attribute Syntax
- 6.42 Prototypes and Old-Style Function Definitions
- 6.43 C++ Style Comments
- 6.44 Dollar Signs in Identifier Names
- 6.45 The Character ESC in Constants
- 6.46 Determining the Alignment of Functions, Types or Variables
- 6.47 An Inline Function is As Fast As a Macro
- 6.48 Const and Volatile Functions
- 6.49 When is a Volatile Object Accessed?
- 6.50 How to Use Inline Assembly Language in C Code
  - 6.50.1 Basic Asm — Assembler Instructions Without Operands
  - 6.50.2 Extended Asm - Assembler Instructions with C Expression Operands
  - 6.50.3 Constraints for asm Operands
  - 6.50.4 Controlling Names Used in Assembler Code
  - 6.50.5 Variables in Specified Registers
    - 6.50.5.1 Defining Global Register Variables
    - 6.50.5.2 Specifying Registers for Local Variables
  - 6.50.6 Size of an asm
- 6.51 Alternate Keywords
- 6.52 Incomplete enum Types
- 6.53 Function Names as Strings
- 6.54 Getting the Return or Frame Address of a Function
- 6.55 Stack scrubbing internal interfaces
- 6.56 Using Vector Instructions through Built-in Functions
- 6.57 Support for offsetof
- 6.58 Legacy __sync Built-in Functions for Atomic Memory Access
- 6.59 Built-in Functions for Memory Model Aware Atomic Operations
- 6.60 Built-in Functions to Perform Arithmetic with Overflow Checking
- 6.61 x86-Specific Memory Model Extensions for Transactional Memory
- 6.62 Object Size Checking
  - 6.62.1 Object Size Checking Built-in Functions
  - 6.62.2 Object Size Checking and Source Fortification
    - 6.62.2.1 Formatted Output Function Checking
- 6.63 Built-in functions for C++ allocations and deallocations
- 6.64 Other Built-in Functions Provided by GCC
- 6.65 Built-in Functions Specific to Particular Target Machines
  - 6.65.1 AArch64 Built-in Functions
  - 6.65.2 Alpha Built-in Functions
  - 6.65.3 ARC Built-in Functions
  - 6.65.4 ARC SIMD Built-in Functions
  - 6.65.5 ARM iWMMXt Built-in Functions
  - 6.65.6 ARM C Language Extensions (ACLE)
  - 6.65.7 ARM Floating Point Status and Control Intrinsics
  - 6.65.8 ARM ARMv8-M Security Extensions
  - 6.65.9 AVR Built-in Functions
  - 6.65.10 Blackfin Built-in Functions
  - 6.65.11 BPF Built-in Functions
  - 6.65.12 FR-V Built-in Functions
    - 6.65.12.1 Argument Types
    - 6.65.12.2 Directly-Mapped Integer Functions
    - 6.65.12.3 Directly-Mapped Media Functions
    - 6.65.12.4 Raw Read/Write Functions
    - 6.65.12.5 Other Built-in Functions
  - 6.65.13 LoongArch Base Built-in Functions
  - 6.65.14 LoongArch SX Vector Intrinsics
  - 6.65.15 LoongArch ASX Vector Intrinsics
  - 6.65.16 MIPS DSP Built-in Functions
  - 6.65.17 MIPS Paired-Single Support
  - 6.65.18 MIPS Loongson Built-in Functions
    - 6.65.18.1 Paired-Single Arithmetic
    - 6.65.18.2 Paired-Single Built-in Functions
    - 6.65.18.3 MIPS-3D Built-in Functions
  - 6.65.19 MIPS SIMD Architecture (MSA) Support
    - 6.65.19.1 MIPS SIMD Architecture Built-in Functions
  - 6.65.20 Other MIPS Built-in Functions
  - 6.65.21 MSP430 Built-in Functions
  - 6.65.22 NDS32 Built-in Functions
  - 6.65.23 Nvidia PTX Built-in Functions
  - 6.65.24 Basic PowerPC Built-in Functions
    - 6.65.24.1 Basic PowerPC Built-in Functions Available on all Configurations
    - 6.65.24.2 Basic PowerPC Built-in Functions Available on ISA 2.05
    - 6.65.24.3 Basic PowerPC Built-in Functions Available on ISA 2.06
    - 6.65.24.4 Basic PowerPC Built-in Functions Available on ISA 2.07
    - 6.65.24.5 Basic PowerPC Built-in Functions Available on ISA 3.0
    - 6.65.24.6 Basic PowerPC Built-in Functions Available on ISA 3.1
  - 6.65.25 PowerPC AltiVec/VSX Built-in Functions
    - 6.65.25.1 PowerPC AltiVec Built-in Functions on ISA 2.05
    - 6.65.25.2 PowerPC AltiVec Built-in Functions Available on ISA 2.06
    - 6.65.25.3 PowerPC AltiVec Built-in Functions Available on ISA 2.07
    - 6.65.25.4 PowerPC AltiVec Built-in Functions Available on ISA 3.0
    - 6.65.25.5 PowerPC AltiVec Built-in Functions Available on ISA 3.1
  - 6.65.26 PowerPC Hardware Transactional Memory Built-in Functions
    - 6.65.26.1 PowerPC HTM Low Level Built-in Functions
    - 6.65.26.2 PowerPC HTM High Level Inline Functions
  - 6.65.27 PowerPC Atomic Memory Operation Functions
  - 6.65.28 PowerPC Matrix-Multiply Assist Built-in Functions
  - 6.65.29 PRU Built-in Functions
  - 6.65.30 RISC-V Built-in Functions
  - 6.65.31 RISC-V Vector Intrinsics
  - 6.65.32 CORE-V Built-in Functions
  - 6.65.33 RX Built-in Functions
  - 6.65.34 S/390 System z Built-in Functions
  - 6.65.35 SH Built-in Functions
  - 6.65.36 SPARC VIS Built-in Functions
  - 6.65.37 TI C6X Built-in Functions
  - 6.65.38 x86 Built-in Functions
  - 6.65.39 x86 Transactional Memory Intrinsics
  - 6.65.40 x86 Control-Flow Protection Intrinsics
- 6.66 Format Checks Specific to Particular Target Machines
  - 6.66.1 Solaris Format Checks
  - 6.66.2 Darwin Format Checks
- 6.67 Pragmas Accepted by GCC
  - 6.67.1 AArch64 Pragmas
  - 6.67.2 ARM Pragmas
  - 6.67.3 M32C Pragmas
  - 6.67.4 PRU Pragmas
  - 6.67.5 RS/6000 and PowerPC Pragmas
  - 6.67.6 S/390 Pragmas
  - 6.67.7 Darwin Pragmas
  - 6.67.8 Solaris Pragmas
  - 6.67.9 Symbol-Renaming Pragmas
  - 6.67.10 Structure-Layout Pragmas
  - 6.67.11 Weak Pragmas
  - 6.67.12 Diagnostic Pragmas
  - 6.67.13 Visibility Pragmas
  - 6.67.14 Push/Pop Macro Pragmas
  - 6.67.15 Function Specific Option Pragmas
  - 6.67.16 Loop-Specific Pragmas
- 6.68 Unnamed Structure and Union Fields
- 6.69 Thread-Local Storage
  - 6.69.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage
  - 6.69.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage
- 6.70 Binary Constants using the ‘0b’ Prefix
7 Extensions to the C++ Language
- 7.1 When is a Volatile C++ Object Accessed?
- 7.2 Restricting Pointer Aliasing
- 7.3 Vague Linkage
- 7.4 C++ Interface and Implementation Pragmas
- 7.5 Where’s the Template?
- 7.6 Extracting the Function Pointer from a Bound Pointer to Member Function
- 7.7 C++-Specific Variable, Function, and Type Attributes
- 7.8 Function Multiversioning
- 7.9 Type Traits
- 7.10 Deprecated Features
- 7.11 Backwards Compatibility
8 GNU Objective-C Features
- 8.1 GNU Objective-C Runtime API
  - 8.1.1 Modern GNU Objective-C Runtime API
  - 8.1.2 Traditional GNU Objective-C Runtime API
- 8.2 +load: Executing Code before main
  - 8.2.1 What You Can and Cannot Do in +load
- 8.3 Type Encoding
- 8.4 Garbage Collection
- 8.5 Constant String Objects
- 8.6 compatibility_alias
- 8.7 Exceptions
- 8.8 Synchronization
- 8.9 Fast Enumeration
- 8.10 Messaging with the GNU Objective-C Runtime
  - 8.10.1 Dynamically Registering Methods
  - 8.10.2 Forwarding Hook
9 Binary Compatibility
10 gcov—a Test Coverage Program
- 10.1 Introduction to gcov
- 10.2 Invoking gcov
- 10.3 Using gcov with GCC Optimization
- 10.4 Brief Description of gcov Data Files
- 10.5 Data File Relocation to Support Cross-Profiling
- 10.6 Profiling and Test Coverage in Freestanding Environments
11 gcov-tool—an Offline Gcda Profile Processing Tool
- 11.1 Introduction to gcov-tool
- 11.2 Invoking gcov-tool
12 gcov-dump—an Offline Gcda and Gcno Profile Dump Tool
- 12.1 Introduction to gcov-dump
- 12.2 Invoking gcov-dump
13 lto-dump—Tool for dumping LTO object files.
- 13.1 Introduction to lto-dump
- 13.2 Invoking lto-dump
14 Known Causes of Trouble with GCC
- 14.1 Actual Bugs We Haven’t Fixed Yet
- 14.2 Interoperation
- 14.3 Incompatibilities of GCC
- 14.4 Fixed Header Files
- 14.5 Standard Libraries
- 14.6 Disappointments and Misunderstandings
- 14.7 Common Misunderstandings with GNU C++
- 14.8 Certain Changes We Don’t Want to Make
- 14.9 Warning Messages and Error Messages
15 Reporting Bugs
- 15.1 Have You Found a Bug?
- 15.2 How and Where to Report Bugs
16 How To Get Help with GCC
17 Contributing to GCC Development
Funding Free Software
The GNU Project and GNU/Linux
GNU General Public License
GNU Free Documentation License
- ADDENDUM: How to use this License for your documents
Contributors to GCC
Appendix A Indices
- A.1 Option Index
- A.2 Concept and Symbol Index

Short Table of Contents

Table of Contents

Introduction ¶