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This is Info file ./gdb.info, produced by Makeinfo-1.52 from the input file gdb.texinfo. START-INFO-DIR-ENTRY * Gdb:: The GNU debugger. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is Edition 4.12, January 1994, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB Version 4.12. Copyright (C) 1988, '89, '90, '91, '92, '93 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. File: gdb.info, Node: Print Settings, Next: Value History, Prev: Auto Display, Up: Data Print settings ============== GDB provides the following ways to control how arrays, structures, and symbols are printed. These settings are useful for debugging programs in any language: `set print address' `set print address on' GDB prints memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses. The default is `on'. For example, this is what a stack frame display looks like, with `set print address on': (gdb) f #0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") at input.c:530 530 if (lquote != def_lquote) `set print address off' Do not print addresses when displaying their contents. For example, this is the same stack frame displayed with `set print address off': (gdb) set print addr off (gdb) f #0 set_quotes (lq="<<", rq=">>") at input.c:530 530 if (lquote != def_lquote) You can use `set print address off' to eliminate all machine dependent displays from the GDB interface. For example, with `print address off', you should get the same text for backtraces on all machines--whether or not they involve pointer arguments. `show print address' Show whether or not addresses are to be printed. When GDB prints a symbolic address, it normally prints the closest earlier symbol plus an offset. If that symbol does not uniquely identify the address (for example, it is a name whose scope is a single source file), you may need to disambiguate. One way to do this is with `info line', for example `info line *0x4537'. Alternately, you can set GDB to print the source file and line number when it prints a symbolic address: `set print symbol-filename on' Tell GDB to print the source file name and line number of a symbol in the symbolic form of an address. `set print symbol-filename off' Do not print source file name and line number of a symbol. This is the default. `show print symbol-filename' Show whether or not GDB will print the source file name and line number of a symbol in the symbolic form of an address. Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction. Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol: `set print max-symbolic-offset MAX-OFFSET' Tell GDB to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than MAX-OFFSET. The default is 0, which means to always print the symbolic form of an address, if any symbol precedes it. `show print max-symbolic-offset' Ask how large the maximum offset is that GDB prints in a symbolic address. Sometimes GDB can tell you more about an address if it does an extensive search of its symbol information. The default is to provide a quick symbolic display that is usually correct, but which may not give the most useful answer when working in some object file formats. If you are not getting the information you need, try: `set print fast-symbolic-addr off' Search all symbol information when displaying an address symbolically. This setting may display more information about static variables, for example, but also takes longer. `set print fast-symbolic-addr' `set print fast-symbolic-addr on' Search only the "minimal symbol information" when displaying symbolic information about an address. This is the default. `show print fast-symbolic-addr' Ask whether GDB is using a fast or slow method of printing symbolic address. If you have a pointer and you are not sure where it points, try `set print symbol-filename on' and `set print fast-symbolic-addr off'. Then you can determine the name and source file location of the variable where it points, using `p/a POINTER'. This interprets the address in symbolic form. For example, here GDB shows that a variable `ptt' points at another variable `t', defined in `hi2.c': (gdb) set print fast-symbolic-addr off (gdb) set print symbol-filename on (gdb) p/a ptt $4 = 0xe008 <t in hi2.c> *Warning:* For pointers that point to a local variable, `p/a' does not show the symbol name and filename of the referent, even with the appropriate `set print' options turned on. Other settings control how different kinds of objects are printed: `set print array' `set print array on' Pretty-print arrays. This format is more convenient to read, but uses more space. The default is off. `set print array off' Return to compressed format for arrays. `show print array' Show whether compressed or pretty format is selected for displaying arrays. `set print elements NUMBER-OF-ELEMENTS' If GDB is printing a large array, it stops printing after it has printed the number of elements set by the `set print elements' command. This limit also applies to the display of strings. Setting the number of elements to zero means that the printing is unlimited. `show print elements' Display the number of elements of a large array that GDB prints before losing patience. `set print pretty on' Cause GDB to print structures in an indented format with one member per line, like this: $1 = { next = 0x0, flags = { sweet = 1, sour = 1 }, meat = 0x54 "Pork" } `set print pretty off' Cause GDB to print structures in a compact format, like this: $1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \ meat = 0x54 "Pork"} This is the default format. `show print pretty' Show which format GDB is using to print structures. `set print sevenbit-strings on' Print using only seven-bit characters; if this option is set, GDB displays any eight-bit characters (in strings or character values) using the notation `\'NNN. This setting is best if you are working in English (ASCII) and you use the high-order bit of characters as a marker or "meta" bit. `set print sevenbit-strings off' Print full eight-bit characters. This allows the use of more international character sets, and is the default. `show print sevenbit-strings' Show whether or not GDB is printing only seven-bit characters. `set print union on' Tell GDB to print unions which are contained in structures. This is the default setting. `set print union off' Tell GDB not to print unions which are contained in structures. `show print union' Ask GDB whether or not it will print unions which are contained in structures. For example, given the declarations typedef enum {Tree, Bug} Species; typedef enum {Big_tree, Acorn, Seedling} Tree_forms; typedef enum {Caterpillar, Cocoon, Butterfly} Bug_forms; struct thing { Species it; union { Tree_forms tree; Bug_forms bug; } form; }; struct thing foo = {Tree, {Acorn}}; with `set print union on' in effect `p foo' would print $1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}} and with `set print union off' in effect it would print $1 = {it = Tree, form = {...}} These settings are of interest when debugging C++ programs: `set print demangle' `set print demangle on' Print C++ names in their source form rather than in the encoded ("mangled") form passed to the assembler and linker for type-safe linkage. The default is `on'. `show print demangle' Show whether C++ names are printed in mangled or demangled form. `set print asm-demangle' `set print asm-demangle on' Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off. `show print asm-demangle' Show whether C++ names in assembly listings are printed in mangled or demangled form. `set demangle-style STYLE' Choose among several encoding schemes used by different compilers to represent C++ names. The choices for STYLE are currently: `auto' Allow GDB to choose a decoding style by inspecting your program. `gnu' Decode based on the GNU C++ compiler (`g++') encoding algorithm. `lucid' Decode based on the Lucid C++ compiler (`lcc') encoding algorithm. `arm' Decode using the algorithm in the `C++ Annotated Reference Manual'. *Warning:* this setting alone is not sufficient to allow debugging `cfront'-generated executables. GDB would require further enhancement to permit that. `show demangle-style' Display the encoding style currently in use for decoding C++ symbols. `set print object' `set print object on' When displaying a pointer to an object, identify the *actual* (derived) type of the object rather than the *declared* type, using the virtual function table. `set print object off' Display only the declared type of objects, without reference to the virtual function table. This is the default setting. `show print object' Show whether actual, or declared, object types are displayed. `set print vtbl' `set print vtbl on' Pretty print C++ virtual function tables. The default is off. `set print vtbl off' Do not pretty print C++ virtual function tables. `show print vtbl' Show whether C++ virtual function tables are pretty printed, or not. File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Print Settings, Up: Data Value history ============= Values printed by the `print' command are saved in the GDB "value history" so that you can refer to them in other expressions. Values are kept until the symbol table is re-read or discarded (for example with the `file' or `symbol-file' commands). When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table. The values printed are given "history numbers" by which you can refer to them. These are successive integers starting with one. `print' shows you the history number assigned to a value by printing `$NUM = ' before the value; here NUM is the history number. To refer to any previous value, use `$' followed by the value's history number. The way `print' labels its output is designed to remind you of this. Just `$' refers to the most recent value in the history, and `$$' refers to the value before that. `$$N' refers to the Nth value from the end; `$$2' is the value just prior to `$$', `$$1' is equivalent to `$$', and `$$0' is equivalent to `$'. For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type p *$ If you have a chain of structures where the component `next' points to the next one, you can print the contents of the next one with this: p *$.next You can print successive links in the chain by repeating this command--which you can do by just typing RET. Note that the history records values, not expressions. If the value of `x' is 4 and you type these commands: print x set x=5 then the value recorded in the value history by the `print' command remains 4 even though the value of `x' has changed. `show values' Print the last ten values in the value history, with their item numbers. This is like `p $$9' repeated ten times, except that `show values' does not change the history. `show values N' Print ten history values centered on history item number N. `show values +' Print ten history values just after the values last printed. If no more values are available, produces no display. Pressing RET to repeat `show values N' has exactly the same effect as `show values +'. File: gdb.info, Node: Convenience Vars, Next: Registers, Prev: Value History, Up: Data Convenience variables ===================== GDB provides "convenience variables" that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely. Convenience variables are prefixed with `$'. Any name preceded by `$' can be used for a convenience variable, unless it is one of the predefined machine-specific register names (*note Registers::.). (Value history references, in contrast, are *numbers* preceded by `$'. *Note Value history: Value History.) You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example: set $foo = *object_ptr would save in `$foo' the value contained in the object pointed to by `object_ptr'. Using a convenience variable for the first time creates it, but its value is `void' until you assign a new value. You can alter the value with another assignment at any time. Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value. `show convenience' Print a list of convenience variables used so far, and their values. Abbreviated `show con'. One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures: set $i = 0 print bar[$i++]->contents ... repeat that command by typing RET. Some convenience variables are created automatically by GDB and given values likely to be useful. `$_' The variable `$_' is automatically set by the `x' command to the last address examined (*note Examining memory: Memory.). Other commands which provide a default address for `x' to examine also set `$_' to that address; these commands include `info line' and `info breakpoint'. The type of `$_' is `void *' except when set by the `x' command, in which case it is a pointer to the type of `$__'. `$__' The variable `$__' is automatically set by the `x' command to the value found in the last address examined. Its type is chosen to match the format in which the data was printed. File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Vars, Up: Data Registers ========= You can refer to machine register contents, in expressions, as variables with names starting with `$'. The names of registers are different for each machine; use `info registers' to see the names used on your machine. `info registers' Print the names and values of all registers except floating-point registers (in the selected stack frame). `info all-registers' Print the names and values of all registers, including floating-point registers. `info registers REGNAME ...' Print the relativized value of each specified register REGNAME. rEGNAME may be any register name valid on the machine you are using, with or without the initial `$'. GDB has four "standard" register names that are available (in expressions) on most machines--whenever they do not conflict with an architecture's canonical mnemonics for registers. The register names `$pc' and `$sp' are used for the program counter register and the stack pointer. `$fp' is used for a register that contains a pointer to the current stack frame, and `$ps' is used for a register that contains the processor status. For example, you could print the program counter in hex with p/x $pc or print the instruction to be executed next with x/i $pc or add four to the stack pointer(1) with set $sp += 4 Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict. The `info registers' command shows the canonical names. For example, on the SPARC, `info registers' displays the processor status register as `$psr' but you can also refer to it as `$ps'. GDB always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can *print* it as a floating point value with `print/f $REGNAME'). Some registers have distinct "raw" and "virtual" data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in "extended" (raw) format, but all C programs expect to work with "double" (virtual) format. In such cases, GDB normally works with the virtual format only (the format that makes sense for your program), but the `info registers' command prints the data in both formats. Normally, register values are relative to the selected stack frame (*note Selecting a frame: Selection.). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with `frame 0'). However, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference. `set rstack_high_address ADDRESS' On AMD 29000 family processors, registers are saved in a separate "register stack". There is no way for GDB to determine the extent of this stack. Normally, GDB just assumes that the stack is "large enough". This may result in GDB referencing memory locations that do not exist. If necessary, you can get around this problem by specifying the ending address of the register stack with the `set rstack_high_address' command. The argument should be an address, which you probably want to precede with `0x' to specify in hexadecimal. `show rstack_high_address' Display the current limit of the register stack, on AMD 29000 family processors. ---------- Footnotes ---------- (1) This is a way of removing one word from the stack, on machines where stacks grow downward in memory (most machines, nowadays). This assumes that the innermost stack frame is selected; setting `$sp' is not allowed when other stack frames are selected. To pop entire frames off the stack, regardless of machine architecture, use `return'; *note Returning from a function: Returning.. File: gdb.info, Node: Floating Point Hardware, Prev: Registers, Up: Data Floating point hardware ======================= Depending on the host machine architecture, GDB may be able to give you more information about the status of the floating point hardware. `info float' Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip; on some platforms, `info float' is not available at all. File: gdb.info, Node: Languages, Next: Symbols, Prev: Data, Up: Top Using GDB with Different Languages ********************************** Although programming languages generally have common aspects, they are rarely expressed in the same manner. For instance, in ANSI C, dereferencing a pointer `p' is accomplished by `*p', but in Modula-2, it is accomplished by `p^'. Values can also be represented (and displayed) differently. Hex numbers in C are written like `0x1ae', while in Modula-2 they appear as `1AEH'. Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions, called the "working language", can be selected manually, or GDB can set it automatically. * Menu: * Setting:: Switching between source languages * Show:: Displaying the language * Checks:: Type and range checks * Support:: Supported languages File: gdb.info, Node: Setting, Next: Show, Up: Languages Switching between source languages ================================== There are two ways to control the working language--either have GDB set it automatically, or select it manually yourself. You can use the `set language' command for either purpose. On startup, GDB defaults to setting the language automatically. * Menu: * Manually:: Setting the working language manually * Automatically:: Having GDB infer the source language File: gdb.info, Node: Manually, Next: Automatically, Up: Setting Setting the working language ---------------------------- If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program. If you wish, you may set the language manually. To do this, issue the command `set language LANG', where LANG is the name of a language, such as `c' or `modula-2'. For a list of the supported languages, type `set language'. Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages--but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as: print a = b + c might not have the effect you intended. In C, this means to add `b' and `c' and place the result in `a'. The result printed would be the value of `a'. In Modula-2, this means to compare `a' to the result of `b+c', yielding a `BOOLEAN' value. File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting Having GDB infer the source language ------------------------------------ To have GDB set the working language automatically, use `set language local' or `set language auto'. GDB then infers the language that a program was written in by looking at the name of its source files, and examining their extensions: `*.mod' Modula-2 source file `*.c' C source file `*.C' `*.cc' C++ source file This information is recorded for each function or procedure in a source file. When your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning. This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using `set language auto' in this case frees you from having to set the working language manually. File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages Displaying the language ======================= The following commands help you find out which language is the working language, and also what language source files were written in. `show language' Display the current working language. This is the language you can use with commands such as `print' to build and compute expressions that may involve variables in your program. `info frame' Among the other information listed here (*note Information about a frame: Frame Info.) is the source language for this frame. This language becomes the working language if you use an identifier from this frame. `info source' Among the other information listed here (*note Examining the Symbol Table: Symbols.) is the source language of this source file. File: gdb.info, Node: Checks, Next: Support, Prev: Show, Up: Languages Type and range checking ======================= *Warning:* In this release, the GDB commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities. Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running. GDB can check for conditions like the above if you wish. Although GDB does not check the statements in your program, it can check expressions entered directly into GDB for evaluation via the `print' command, for example. As with the working language, GDB can also decide whether or not to check automatically based on your program's source language. *Note Supported languages: Support, for the default settings of supported languages. * Menu: * Type Checking:: An overview of type checking * Range Checking:: An overview of range checking File: gdb.info, Node: Type Checking, Next: Range Checking, Up: Checks An overview of type checking ---------------------------- Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example, 1 + 2 => 3 but error--> 1 + 2.3 The second example fails because the `CARDINAL' 1 is not type-compatible with the `REAL' 2.3. For expressions you use in GDB commands, you can tell the GDB type checker to skip checking; to treat any mismatches as errors and abandon the expression; or only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, GDB evaluates expressions like the second example above, but also issues a warning. Even though you may turn type checking off, other type-based reasons may prevent GDB from evaluating an expression. For instance, GDB does not know how to add an `int' and a `struct foo'. These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway. Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. *Note Supported languages: Support, for further details on specific languages. GDB provides some additional commands for controlling the type checker: `set check type auto' Set type checking on or off based on the current working language. *Note Supported languages: Support, for the default settings for each language. `set check type on' `set check type off' Set type checking on or off, overriding the default setting for the current working language. Issue a warning if the setting does not match the language default. If any type mismatches occur in evaluating an expression while typechecking is on, GDB prints a message and aborts evaluation of the expression. `set check type warn' Cause the type checker to issue warnings, but to always attempt to evaluate the expression. Evaluating the expression may still be impossible for other reasons. For example, GDB cannot add numbers and structures. `show type' Show the current setting of the type checker, and whether or not GDB is setting it automatically. File: gdb.info, Node: Range Checking, Prev: Type Checking, Up: Checks An overview of range checking ----------------------------- In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array. For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway. A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to "wrap around" to lower values--for example, if M is the largest integer value, and S is the smallest, then M + 1 => S This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. *Note Supported languages: Support, for further details on specific languages. GDB provides some additional commands for controlling the range checker: `set check range auto' Set range checking on or off based on the current working language. *Note Supported languages: Support, for the default settings for each language. `set check range on' `set check range off' Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs, then a message is printed and evaluation of the expression is aborted. `set check range warn' Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems). `show range' Show the current setting of the range checker, and whether or not it is being set automatically by GDB. File: gdb.info, Node: Support, Prev: Checks, Up: Languages Supported languages =================== GDB 4 supports C, C++, and Modula-2. Some GDB features may be used in expressions regardless of the language you use: the GDB `@' and `::' operators, and the `{type}addr' construct (*note Expressions: Expressions.) can be used with the constructs of any supported language. The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial. * Menu: * C:: C and C++ * Modula-2:: Modula-2 File: gdb.info, Node: C, Next: Modula-2, Up: Support C and C++ --------- Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss both languages together. The C++ debugging facilities are jointly implemented by the GNU C++ compiler and GDB. Therefore, to debug your C++ code effectively, you must compile your C++ programs with the GNU C++ compiler, `g++'. For best results when debugging C++ programs, use the stabs debugging format. You can select that format explicitly with the `g++' command-line options `-gstabs' or `-gstabs+'. See *Note Options for Debugging Your Program or GNU CC: (gcc.info)Debugging Options, for more information. * Menu: * C Operators:: C and C++ operators * C Constants:: C and C++ constants * Cplus expressions:: C++ expressions * C Defaults:: Default settings for C and C++ * C Checks:: C and C++ type and range checks * Debugging C:: GDB and C * Debugging C plus plus:: Special features for C++ File: gdb.info, Node: C Operators, Next: C Constants, Up: C C and C++ operators ------------------- Operators must be defined on values of specific types. For instance, `+' is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of C and C++, the following definitions hold: * *Integral types* include `int' with any of its storage-class specifiers; `char'; and `enum'. * *Floating-point types* include `float' and `double'. * *Pointer types* include all types defined as `(TYPE *)'. * *Scalar types* include all of the above. The following operators are supported. They are listed here in order of increasing precedence: `,' The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated. `=' Assignment. The value of an assignment expression is the value assigned. Defined on scalar types. `OP=' Used in an expression of the form `A OP= B', and translated to `A = A OP B'. `OP=' and `=' have the same precendence. OP is any one of the operators `|', `^', `&', `<<', `>>', `+', `-', `*', `/', `%'. `?:' The ternary operator. `A ? B : C' can be thought of as: if A then B else C. A should be of an integral type. `||' Logical OR. Defined on integral types. `&&' Logical AND. Defined on integral types. `|' Bitwise OR. Defined on integral types. `^' Bitwise exclusive-OR. Defined on integral types. `&' Bitwise AND. Defined on integral types. `==, !=' Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true. `<, >, <=, >=' Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true. `<<, >>' left shift, and right shift. Defined on integral types. `@' The GDB "artificial array" operator (*note Expressions: Expressions.). `+, -' Addition and subtraction. Defined on integral types, floating-point types and pointer types. `*, /, %' Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types. `++, --' Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place. `*' Pointer dereferencing. Defined on pointer types. Same precedence as `++'. `&' Address operator. Defined on variables. Same precedence as `++'. For debugging C++, GDB implements a use of `&' beyond what is allowed in the C++ language itself: you can use `&(&REF)' (or, if you prefer, simply `&&REF') to examine the address where a C++ reference variable (declared with `&REF') is stored. `-' Negative. Defined on integral and floating-point types. Same precedence as `++'. `!' Logical negation. Defined on integral types. Same precedence as `++'. `~' Bitwise complement operator. Defined on integral types. Same precedence as `++'. `., ->' Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on `struct' and `union' data. `[]' Array indexing. `A[I]' is defined as `*(A+I)'. Same precedence as `->'. `()' Function parameter list. Same precedence as `->'. `::' C++ scope resolution operator. Defined on `struct', `union', and `class' types. `::' Doubled colons also represent the GDB scope operator (*note Expressions: Expressions.). Same precedence as `::', above. File: gdb.info, Node: C Constants, Next: Cplus expressions, Prev: C Operators, Up: C C and C++ constants ------------------- GDB allows you to express the constants of C and C++ in the following ways: * Integer constants are a sequence of digits. Octal constants are specified by a leading `0' (ie. zero), and hexadecimal constants by a leading `0x' or `0X'. Constants may also end with a letter `l', specifying that the constant should be treated as a `long' value. * Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent. An exponent is of the form: `e[[+]|-]NNN', where NNN is another sequence of digits. The `+' is optional for positive exponents. * Enumerated constants consist of enumerated identifiers, or their integral equivalents. * Character constants are a single character surrounded by single quotes (`''), or a number--the ordinal value of the corresponding character (usually its ASCII value). Within quotes, the single character may be represented by a letter or by "escape sequences", which are of the form `\NNN', where NNN is the octal representation of the character's ordinal value; or of the form `\X', where `X' is a predefined special character--for example, `\n' for newline. * String constants are a sequence of character constants surrounded by double quotes (`"'). * Pointer constants are an integral value. You can also write pointers to constants using the C operator `&'. * Array constants are comma-separated lists surrounded by braces `{' and `}'; for example, `{1,2,3}' is a three-element array of integers, `{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and `{&"hi", &"there", &"fred"}' is a three-element array of pointers. File: gdb.info, Node: Cplus expressions, Next: C Defaults, Prev: C Constants, Up: C C++ expressions --------------- GDB expression handling has a number of extensions to interpret a significant subset of C++ expressions. *Warning:* GDB can only debug C++ code if you compile with the GNU C++ compiler. Moreover, C++ debugging depends on the use of additional debugging information in the symbol table, and thus requires special support. GDB has this support *only* with the stabs debug format. In particular, if your compiler generates a.out, MIPS ECOFF, RS/6000 XCOFF, or ELF with stabs extensions to the symbol table, these facilities are all available. (With GNU CC, you can use the `-gstabs' option to request stabs debugging extensions explicitly.) Where the object code format is standard COFF or DWARF in ELF, on the other hand, most of the C++ support in GDB does *not* work. 1. Member function calls are allowed; you can use expressions like count = aml->GetOriginal(x, y) 2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDB allows implicit references to the class instance pointer `this' following the same rules as C++. 3. You can call overloaded functions; GDB resolves the function call to the right definition, with one restriction--you must use arguments of the type required by the function that you want to call. GDB does not perform conversions requiring constructors or user-defined type operators. 4. GDB understands variables declared as C++ references; you can use them in expressions just as you do in C++ source--they are automatically dereferenced. In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The *address* of a reference variable is always shown, unless you have specified `set print address off'. 5. GDB supports the C++ name resolution operator `::'--your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use `::' repeatedly if necessary, for example in an expression like `SCOPE1::SCOPE2::NAME'. GDB also allows resolving name scope by reference to source files, in both C and C++ debugging (*note Program variables: Variables.). File: gdb.info, Node: C Defaults, Next: C Checks, Prev: Cplus expressions, Up: C C and C++ defaults ------------------ If you allow GDB to set type and range checking automatically, they both default to `off' whenever the working language changes to C or C++. This happens regardless of whether you, or GDB, selected the working language. If you allow GDB to set the language automatically, it sets the working language to C or C++ on entering code compiled from a source file whose name ends with `.c', `.C', or `.cc'. *Note Having GDB infer the source language: Automatically, for further details. File: gdb.info, Node: C Checks, Next: Debugging C, Prev: C Defaults, Up: C C and C++ type and range checks ------------------------------- By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB considers two variables type equivalent if: * The two variables are structured and have the same structure, union, or enumerated tag. * Two two variables have the same type name, or types that have been declared equivalent through `typedef'. Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array. File: gdb.info, Node: Debugging C, Next: Debugging C plus plus, Prev: C Checks, Up: C GDB and C --------- The `set print union' and `show print union' commands apply to the `union' type. When set to `on', any `union' that is inside a `struct' or `class' is also printed. Otherwise, it appears as `{...}'. The `@' operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function. *Note Expressions: Expressions. File: gdb.info, Node: Debugging C plus plus, Prev: Debugging C, Up: C GDB features for C++ -------------------- Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary: `breakpoint menus' When you want a breakpoint in a function whose name is overloaded, GDB breakpoint menus help you specify which function definition you want. *Note Breakpoint menus: Breakpoint Menus. `rbreak REGEX' Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. *Note Setting breakpoints: Set Breaks. `catch EXCEPTIONS' `info catch' Debug C++ exception handling using these commands. *Note Breakpoints and exceptions: Exception Handling. `ptype TYPENAME' Print inheritance relationships as well as other information for type TYPENAME. *Note Examining the Symbol Table: Symbols. `set print demangle' `show print demangle' `set print asm-demangle' `show print asm-demangle' Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. *Note Print settings: Print Settings. `set print object' `show print object' Choose whether to print derived (actual) or declared types of objects. *Note Print settings: Print Settings. `set print vtbl' `show print vtbl' Control the format for printing virtual function tables. *Note Print settings: Print Settings. `Overloaded symbol names' You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type `SYMBOL(TYPES)' rather than just SYMBOL. You can also use the GDB command-line word completion facilities to list the available choices, or to finish the type list for you. *Note Command completion: Completion, for details on how to do this. File: gdb.info, Node: Modula-2, Prev: C, Up: Support Modula-2 -------- The extensions made to GDB to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDB reads in the executable's symbol table. * Menu: * M2 Operators:: Built-in operators * Built-In Func/Proc:: Built-in functions and procedures * M2 Constants:: Modula-2 constants * M2 Defaults:: Default settings for Modula-2 * Deviations:: Deviations from standard Modula-2 * M2 Checks:: Modula-2 type and range checks * M2 Scope:: The scope operators `::' and `.' * GDB/M2:: GDB and Modula-2 File: gdb.info, Node: M2 Operators, Next: Built-In Func/Proc, Up: Modula-2 Operators --------- Operators must be defined on values of specific types. For instance, `+' is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of Modula-2, the following definitions hold: * *Integral types* consist of `INTEGER', `CARDINAL', and their subranges. * *Character types* consist of `CHAR' and its subranges. * *Floating-point types* consist of `REAL'. * *Pointer types* consist of anything declared as `POINTER TO TYPE'. * *Scalar types* consist of all of the above. * *Set types* consist of `SET' and `BITSET' types. * *Boolean types* consist of `BOOLEAN'. The following operators are supported, and appear in order of increasing precedence: `,' Function argument or array index separator. `:=' Assignment. The value of VAR `:=' VALUE is VALUE. `<, >' Less than, greater than on integral, floating-point, or enumerated types. `<=, >=' Less than, greater than, less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types. Same precedence as `<'. `=, <>, #' Equality and two ways of expressing inequality, valid on scalar types. Same precedence as `<'. In GDB scripts, only `<>' is available for inequality, since `#' conflicts with the script comment character. `IN' Set membership. Defined on set types and the types of their members. Same precedence as `<'. `OR' Boolean disjunction. Defined on boolean types. `AND, &' Boolean conjuction. Defined on boolean types. `@' The GDB "artificial array" operator (*note Expressions: Expressions.). `+, -' Addition and subtraction on integral and floating-point types, or union and difference on set types. `*' Multiplication on integral and floating-point types, or set intersection on set types. `/' Division on floating-point types, or symmetric set difference on set types. Same precedence as `*'. `DIV, MOD' Integer division and remainder. Defined on integral types. Same precedence as `*'. `-' Negative. Defined on `INTEGER' and `REAL' data. `^' Pointer dereferencing. Defined on pointer types. `NOT' Boolean negation. Defined on boolean types. Same precedence as `^'. `.' `RECORD' field selector. Defined on `RECORD' data. Same precedence as `^'. `[]' Array indexing. Defined on `ARRAY' data. Same precedence as `^'. `()' Procedure argument list. Defined on `PROCEDURE' objects. Same precedence as `^'. `::, .' GDB and Modula-2 scope operators. *Warning:* Sets and their operations are not yet supported, so GDB treats the use of the operator `IN', or the use of operators `+', `-', `*', `/', `=', , `<>', `#', `<=', and `>=' on sets as an error.