implicit declaration of function ‘time’

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SystemVerilog for Design(2nd edition)

这本书，超赞。强烈推荐！！！！！有很多很好的用例！ Foreword ................................................................................................................. xxi Preface ................................................................................................................... xxiii Target audience...................................................................................................................... xxiii Topics covered........................................................................................................................xxiv About the examples in this book..............................................................................................xxv Obtaining copies of the examples...........................................................................................xxvi Example testing.......................................................................................................................xxvi Other sources of information .................................................................................................xxvii Acknowledgements..................................................................................................................xxx Chapter 1: Introduction to SystemVerilog...............................................................1 1.1 SystemVerilog origins.......................................................................................................1 1.1.1 Generations of the SystemVerilog standard.......................................................2 1.1.2 Donations to SystemVerilog ..............................................................................4 1.2 Key SystemVerilog enhancements for hardware design...................................................5 1.3 Summary ...........................................................................................................................6 Chapter 2: SystemVerilog Declaration Spaces ........................................................7 2.1 Packages ...........................................................................................................................8 2.1.1 Package definitions ............................................................................................9 2.1.2 Referencing package contents..........................................................................10 2.1.3 Synthesis guidelines .........................................................................................14 2.2 $unit compilation-unit declarations.................................................................................14 2.2.1 Coding guidelines.............................................................................................17 2.2.2 SystemVerilog identifier search rules ..............................................................17 2.2.3 Source code order.............................................................................................17 2.2.4 Coding guidelines for importing packages into $unit ......................................19 2.2.5 Synthesis guidelines .........................................................................................25 2.3 Declarations in unnamed statement blocks .....................................................................26 2.3.1 Local variables in unnamed blocks ..................................................................27 2.4 Simulation time units and precision ................................................................................28 2.4.1 Verilog’s timescale directive............................................................................28 2.4.2 Time values with time units .............................................................................30 2.4.3 Scope-level time unit and precision .................................................................31 viii 2.4.4 Compilation-unit time units and precision.......................................................32 2.5 Summary .........................................................................................................................34 Chapter 3: SystemVerilog Literal Values and Built-in Data Types.....................37 3.1 Enhanced literal value assignments.................................................................................38 3.2 ‘define enhancements ......................................................................................................39 3.2.1 Macro argument substitution within strings.....................................................39 3.2.2 Constructing identifier names from macros.....................................................41 3.3 SystemVerilog variables..................................................................................................42 3.3.1 Object types and data types..............................................................................42 3.3.2 SystemVerilog 4-state variables.......................................................................43 3.3.3 SystemVerilog 2-state variables.......................................................................44 3.3.4 Explicit and implicit variable and net data types .............................................47 3.3.5 Synthesis guidelines .........................................................................................48 3.4 Using 2-state types in RTL models .................................................................................48 3.4.1 2-state type characteristics ...............................................................................49 3.4.2 2-state types versus 2-state simulation.............................................................49 3.4.3 Using 2-state types with case statements .........................................................51 3.5 Relaxation of type rules...................................................................................................52 3.6 Signed and unsigned modifiers .......................................................................................55 3.7 Static and automatic variables.........................................................................................56 3.7.1 Static and automatic variable initialization ......................................................59 3.7.2 Synthesis guidelines for automatic variables ...................................................60 3.7.3 Guidelines for using static and automatic variables.........................................61 3.8 Deterministic variable initialization ................................................................................61 3.8.1 Initialization determinism ................................................................................61 3.8.2 Initializing sequential logic asynchronous inputs ............................................65 3.9 Type casting ....................................................................................................................67 3.9.1 Static (compile time) casting............................................................................67 3.9.2 Dynamic casting...............................................................................................69 3.9.3 Synthesis guidelines .........................................................................................70 3.10 Constants .........................................................................................................................71 3.11 Summary .........................................................................................................................72 Chapter 4: SystemVerilog User-Defined and Enumerated Types .......................75 4.1 User-defined types...........................................................................................................75 4.1.1 Local typedef definitions..................................................................................76 4.1.2 Shared typedef definitions................................................................................76 4.1.3 Naming convention for user-defined types ......................................................78 4.2 Enumerated types ............................................................................................................79 4.2.1 Enumerated type label sequences.....................................................................83 ix 4.2.2 Enumerated type label scope............................................................................83 4.2.3 Enumerated type values ...................................................................................84 4.2.4 Base type of enumerated types.........................................................................85 4.2.5 Typed and anonymous enumerations...............................................................86 4.2.6 Strong typing on enumerated type operations..................................................86 4.2.7 Casting expressions to enumerated types.........................................................88 4.2.8 Special system tasks and methods for enumerated types.................................89 4.2.9 Printing enumerated types................................................................................92 4.3 Summary .........................................................................................................................93 Chapter 5: SystemVerilog Arrays, Structures and Unions ..................................95 5.1 Structures.........................................................................................................................96 5.1.1 Structure declarations.......................................................................................97 5.1.2 Assigning values to structures..........................................................................98 5.1.3 Packed and unpacked structures.....................................................................101 5.1.4 Passing structures through ports.....................................................................104 5.1.5 Passing structures as arguments to tasks and functions .................................105 5.1.6 Synthesis guidelines .......................................................................................105 5.2 Unions ...........................................................................................................................105 5.2.1 Unpacked unions............................................................................................106 5.2.2 Tagged unions ................................................................................................108 5.2.3 Packed unions.................................................................................................109 5.2.4 Synthesis guidelines .......................................................................................111 5.2.5 An example of using structures and unions ...................................................111 5.3 Arrays ............................................................................................................................113 5.3.1 Unpacked arrays.............................................................................................113 5.3.2 Packed arrays .................................................................................................116 5.3.3 Using packed and unpacked arrays ................................................................118 5.3.4 Initializing arrays at declaration.....................................................................119 5.3.5 Assigning values to arrays .............................................................................121 5.3.6 Copying arrays ...............................................................................................123 5.3.7 Copying arrays and structures using bit-stream casting.................................124 5.3.8 Arrays of arrays..............................................................................................125 5.3.9 Using user-defined types with arrays .............................................................126 5.3.10 Passing arrays through ports and to tasks and functions................................127 5.3.11 Arrays of structures and unions......................................................................128 5.3.12 Arrays in structures and unions......................................................................128 5.3.13 Synthesis guidelines .......................................................................................128 5.3.14 An example of using arrays............................................................................129 5.4 The foreach array looping construct..............................................................................130 x 5.5 Array querying system functions ..................................................................................132 5.6 The $bits “sizeof” system function ...............................................................................134 5.7 Dynamic arrays, associative arrays, sparse arrays and strings ......................................135 5.8 Summary .......................................................................................................................136 Chapter 6: SystemVerilog Procedural Blocks, Tasks and Functions ................137 6.1 Verilog general purpose always procedural block ........................................................138 6.2 SystemVerilog specialized procedural blocks...............................................................142 6.2.1 Combinational logic procedural blocks .........................................................142 6.2.2 Latched logic procedural blocks ....................................................................150 6.2.3 Sequential logic procedural blocks ................................................................152 6.2.4 Synthesis guidelines .......................................................................................152 6.3 Enhancements to tasks and functions............................................................................153 6.3.1 Implicit task and function statement grouping...............................................153 6.3.2 Returning function values ..............................................................................153 6.3.3 Returning before the end of tasks and functions............................................154 6.3.4 Void functions................................................................................................155 6.3.5 Passing task/function arguments by name .....................................................156 6.3.6 Enhanced function formal arguments ............................................................157 6.3.7 Functions with no formal arguments..............................................................158 6.3.8 Default formal argument direction and type ..................................................158 6.3.9 Default formal argument values.....................................................................159 6.3.10 Arrays, structures and unions as formal arguments .......................................160 6.3.11 Passing argument values by reference instead of copy..................................161 6.3.12 Named task and function ends .......................................................................165 6.3.13 Empty tasks and functions .............................................................................166 6.4 Summary .......................................................................................................................166 Chapter 7: SystemVerilog Procedural Statements..............................................169 7.1 New operators................................................................................................................170 7.1.1 Increment and decrement operators ...............................................................170 7.1.2 Assignment operators.....................................................................................173 7.1.3 Equality operators with don’t care wildcards.................................................176 7.1.4 Set membership operator — inside ................................................................178 7.2 Operand enhancements..................................................................................................180 7.2.1 Operations on 2-state and 4-state types..........................................................180 7.2.2 Type casting ...................................................................................................180 7.2.3 Size casting.....................................................................................................181 7.2.4 Sign casting ....................................................................................................182 7.3 Enhanced for loops........................................................................................................182 7.3.1 Local variables within for loop declarations..................................................183 xi 7.3.2 Multiple for loop assignments........................................................................185 7.3.3 Hierarchically referencing variables declared in for loops ............................185 7.3.4 Synthesis guidelines .......................................................................................186 7.4 Bottom testing do...while loop ......................................................................................186 7.4.1 Synthesis guidelines .......................................................................................188 7.5 The foreach array looping construct..............................................................................188 7.6 New jump statements — break, continue, return ..........................................................188 7.6.1 The continue statement ..................................................................................190 7.6.2 The break statement .......................................................................................190 7.6.3 The return statement.......................................................................................191 7.6.4 Synthesis guidelines .......................................................................................192 7.7 Enhanced block names ..................................................................................................192 7.8 Statement labels.............................................................................................................194 7.9 Enhanced case statements .............................................................................................195 7.9.1 Unique case decisions ....................................................................................196 7.9.2 Priority case statements..................................................................................199 7.9.3 Unique and priority versus parallel_case and full_case .................................201 7.10 Enhanced if...else decisions...........................................................................................203 7.10.1 Unique if...else decisions ...............................................................................203 7.10.2 Priority if decisions ........................................................................................205 7.11 Summary .......................................................................................................................206 Chapter 8: Modeling Finite State Machines with SystemVerilog......................207 8.1 Modeling state machines with enumerated types..........................................................208 8.1.1 Representing state encoding with enumerated types .....................................210 8.1.2 Reversed case statements with enumerated types..........................................211 8.1.3 Enumerated types and unique case statements...............................................213 8.1.4 Specifying unused state values.......................................................................214 8.1.5 Assigning state values to enumerated type variables.....................................216 8.1.6 Performing operations on enumerated type variables....................................218 8.2 Using 2-state types in FSM models...............................................................................219 8.2.1 Resetting FSMs with 2-state and enumerated types ......................................219 8.3 Summary .......................................................................................................................221 Chapter 9: SystemVerilog Design Hierarchy.......................................................223 9.1 Module prototypes.........................................................................................................224 9.1.1 Prototype and actual definition ......................................................................225 9.1.2 Avoiding port declaration redundancy...........................................................225 9.2 Named ending statements..............................................................................................226 9.2.1 Named module ends.......................................................................................226 9.2.2 Named code block ends .................................................................................226 xii 9.3 Nested (local) module declarations ...............................................................................227 9.3.1 Nested module name visibility.......................................................................230 9.3.2 Instantiating nested modules ..........................................................................231 9.3.3 Nested module name search rules ..................................................................232 9.4 Simplified netlists of module instances.........................................................................233 9.4.1 Implicit .name port connections....................................................................238 9.4.2 Implicit .* port connection ...........................................................................242 9.5 Net aliasing....................................................................................................................244 9.5.1 Alias rules.......................................................................................................245 9.5.2 Implicit net declarations.................................................................................246 9.5.3 Using aliases with .name and .* .....................................................................247 9.6 Passing values through module ports ............................................................................251 9.6.1 All types can be passed through ports............................................................251 9.6.2 Module port restrictions in SystemVerilog ....................................................252 9.7 Reference ports..............................................................................................................255 9.7.1 Reference ports as shared variables ...............................................................256 9.7.2 Synthesis guidelines .......................................................................................256 9.8 Enhanced port declarations ...........................................................................................257 9.8.1 Verilog-1995 port declarations ......................................................................257 9.8.2 Verilog-2001 port declarations ......................................................................257 9.8.3 SystemVerilog port declarations ....................................................................258 9.9 Parameterized types.......................................................................................................260 9.10 Summary .......................................................................................................................261 Chapter 10: SystemVerilog Interfaces..................................................................263 10.1 Interface concepts..........................................................................................................264 10.1.1 Disadvantages of Verilog’s module ports ......................................................268 10.1.2 Advantages of SystemVerilog interfaces .......................................................269 10.1.3 SystemVerilog interface contents ..................................................................273 10.1.4 Differences between modules and interfaces .................................................273 10.2 Interface declarations ....................................................................................................274 10.2.1 Source code declaration order........................................................................276 10.2.2 Global and local interface definitions ............................................................276 10.3 Using interfaces as module ports...................................................................................277 10.3.1 Explicitly named interface ports ....................................................................277 10.3.2 Generic interface ports ...................................................................................278 10.3.3 Synthesis guidelines .......................................................................................278 10.4 Instantiating and connecting interfaces .........................................................................278 10.5 Referencing signals within an interface ........................................................................279 10.6 Interface modports.........................................................................................................281 xiii 10.6.1 Specifying which modport view to use..........................................................282 10.6.2 Using modports to define different sets of connections.................................286 10.7 Using tasks and functions in interfaces .........................................................................288 10.7.1 Interface methods ...........................................................................................289 10.7.2 Importing interface methods ..........................................................................289 10.7.3 Synthesis guidelines for interface methods....................................................292 10.7.4 Exporting tasks and functions ........................................................................293 10.8 Using procedural blocks in interfaces ...........................................................................296 10.9 Reconfigurable interfaces..............................................................................................296 10.10 Verification with interfaces ...........................................................................................298 10.11 Summary .......................................................................................................................299 Chapter 11: A Complete Design Modeled with SystemVerilog..........................301 11.1 SystemVerilog ATM example.......................................................................................301 11.2 Data abstraction.............................................................................................................302 11.3 Interface encapsulation..................................................................................................305 11.4 Design top level: squat ..................................................................................................308 11.5 Receivers and transmitters.............................................................................................315 11.5.1 Receiver state machine...................................................................................315 11.5.2 Transmitter state machine ..............................................................................318 11.6 Testbench.......................................................................................................................321 11.7 Summary .......................................................................................................................327 Chapter 12: Behavioral and Transaction Level Modeling .................................329 12.1 Behavioral modeling .....................................................................................................330 12.2 What is a transaction?....................................................................................................330 12.3 Transaction level modeling in SystemVerilog ..............................................................332 12.3.1 Memory subsystem example..........................................................................333 12.4 Transaction level models via interfaces ........................................................................335 12.5 Bus arbitration ...............................................................................................................337 12.6 Transactors, adapters, and bus functional models.........................................................341 12.6.1 Master adapter as module...............................................................................341 12.6.2 Adapter in an interface ...................................................................................348 12.7 More complex transactions ...........................................................................................353 12.8 Summary .......................................................................................................................354 Appendix A: The SystemVerilog Formal Definition (BNF) ...............................355 Appendix B: Verilog and SystemVerilog Reserved Keywords...........................395 Appendix C: A History of SUPERLOG, the Beginning of SystemVerilog .......401 Index ........................................................................................................................415

在C语言中，当我们使用一个函数而没有提前声明它时，编译器会默认为该函数进行隐式声明。这就是所谓的"implicit declaration"。在你提到的情况中，编译器发现了对函数"time"的调用，但在该文件中没有找到对该函数的显式声明。函数"time"是C语言中的一个标准库函数，用于获取当前时间。为了解决隐式声明的问题，我们需要在代码中包含头文件"time.h"，该头文件中包含了对函数"time"的声明。可以通过添加以下代码来解决问题： ```c #include <time.h> int main() { time_t currentTime = time(NULL); // 其他代码... return 0; } ``` 这样，编译器就能够正确识别并使用函数"time"了。

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implicit declaration of function ‘time’

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