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Info file ./gdb.info, produced by Makeinfo, -*- Text -*- from input file gdb-all.texi. START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is Edition 4.04, March 1992, of `Using GDB: A Guide to the GNU Source-Level Debugger' for GDB Version 4.5. Copyright (C) 1988, 1989, 1990, 1991, 1992 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 section entitled "GNU General Public License" is included exactly as in the original, and provided 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, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: gdb.info, Node: Manually, Next: Automatically, Prev: Setting, Up: Setting Setting the working language ---------------------------- To set the language, issue the command `set language LANG', where LANG is the name of a language: `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. If you allow GDB to set the language automatically, then you can count on expressions evaluating the same way in your debugging session and in your program. 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' `*.cc' C or 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 will 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 is the language that will become the working language if you ever use an identifier that is in 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 will 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, Prev: Checks, 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's 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 ignore range errors; to always treat them as errors and abandon the expression; or to issue warnings when a range error occurs but evaluate the expression anyway. A range error can result from numerical overflow, from exceeding an array index bound, or when you type in 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's 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. The syntax for C and C++ is so closely related that GDB does not distinguish the two. 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 of the supported languages. The following sections detail to what degree each of these source languages 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 will accept, 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, Prev: Support, Up: Support C and C++ --------- Since C and C++ are so closely related, GDB does not distinguish between them when interpreting the expressions recognized in GDB commands. 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++'. * Menu: * C Operators:: C and C++ Operators * C Constants:: C and C++ Constants * Cplusplus 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, Prev: C, 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's. * *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's 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's and `union's. `[]' 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. `::' The GDB scope operator (*note Expressions: Expressions.). Same precedence as `::', above. File: gdb.info, Node: C Constants, Next: Cplusplus 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. File: gdb.info, Node: Cplusplus expressions, Next: C Defaults, Prev: C Constants, Up: C C++ Expressions ............... GDB's expression handling has the following extensions to interpret a significant subset of C++ expressions: 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 will resolve 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 will 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: Cplusplus 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/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/C++ on entering code compiled from a source file whose name ends with `.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 ............................... *Warning:* in this release, GDB does not yet perform type or range checking. By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB will consider 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' will also be printed. Otherwise, it will appear 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 Commands 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's breakpoint menus help you specify which function definition you want. *Note 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. File: gdb.info, Node: Modula-2, Prev: C, Up: Support Modula-2 -------- The extensions made to GDB to support Modula-2 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 will most likely result in 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, Prev: Modula-2, 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's and `BITSET's. * *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's and `REAL's. `^' Pointer dereferencing. Defined on pointer types. `NOT' Boolean negation. Defined on boolean types. Same precedence as `^'. `.' `RECORD' field selector. Defined on `RECORD's. Same precedence as `^'. `[]' Array indexing. Defined on `ARRAY's. Same precedence as `^'. `()' Procedure argument list. Defined on `PROCEDURE's. Same precedence as `^'. `::, .' GDB and Modula-2 scope operators. *Warning:* Sets and their operations are not yet supported, so GDB will treat the use of the operator `IN', or the use of operators `+', `-', `*', `/', `=', , `<>', `#', `<=', and `>=' on sets as an error. File: gdb.info, Node: Built-In Func/Proc, Next: M2 Constants, Prev: M2 Operators, Up: Modula-2 Built-in Functions and Procedures ................................. Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used: A represents an `ARRAY' variable. C represents a `CHAR' constant or variable. I represents a variable or constant of integral type. M represents an identifier that belongs to a set. Generally used in the same function with the metavariable S. The type of S should be `SET OF MTYPE' (where MTYPE is the type of M. N represents a variable or constant of integral or floating-point type. R represents a variable or constant of floating-point type. T represents a type. V represents a variable. X represents a variable or constant of one of many types. See the explanation of the function for details. All Modula-2 built-in procedures also return a result, described below. `ABS(N)' Returns the absolute value of N. `CAP(C)' If C is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument `CHR(I)' Returns the character whose ordinal value is I. `DEC(V)' Decrements the value in the variable V. Returns the new value. `DEC(V,I)' Decrements the value in the variable V by I. Returns the new value. `EXCL(M,S)' Removes the element M from the set S. Returns the new set. `FLOAT(I)' Returns the floating point equivalent of the integer I. `HIGH(A)' Returns the index of the last member of A. `INC(V)' Increments the value in the variable V. Returns the new value. `INC(V,I)' Increments the value in the variable V by I. Returns the new value. `INCL(M,S)' Adds the element M to the set S if it is not already there. Returns the new set. `MAX(T)' Returns the maximum value of the type T. `MIN(T)' Returns the minimum value of the type T. `ODD(I)' Returns boolean TRUE if I is an odd number. `ORD(X)' Returns the ordinal value of its argument. For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set). X must be of an ordered type, which include integral, character and enumerated types. `SIZE(X)' Returns the size of its argument. X can be a variable or a type. `TRUNC(R)' Returns the integral part of R. `VAL(T,I)' Returns the member of the type T whose ordinal value is I. *Warning:* Sets and their operations are not yet supported, so GDB will treat the use of procedures `INCL' and `EXCL' as an error. File: gdb.info, Node: M2 Constants, Next: M2 Defaults, Prev: Built-In Func/Proc, Up: Modula-2 Constants ......... GDB allows you to express the constants of Modula-2 in the following ways: * Integer constants are simply a sequence of digits. When used in an expression, a constant is interpreted to be type-compatible with the rest of the expression. Hexadecimal integers are specified by a trailing `H', and octal integers by a trailing `B'. * Floating point constants appear as a sequence of digits, followed by a decimal point and another sequence of digits. An optional exponent can then be specified, in the form `E[+|-]NNN', where `[+|-]NNN' is the desired exponent. All of the digits of the floating point constant must be valid decimal (base 10) digits. * Character constants consist of a single character enclosed by a pair of like quotes, either single (`'') or double (`"'). They may also be expressed by their ordinal value (their ASCII value, usually) followed by a `C'. * String constants consist of a sequence of characters enclosed by a pair of like quotes, either single (`'') or double (`"'). Escape sequences in the style of C are also allowed. *Note C and C++ Constants: C Constants, for a brief explanation of escape sequences. * Enumerated constants consist of an enumerated identifier. * Boolean constants consist of the identifiers `TRUE' and `FALSE'. * Pointer constants consist of integral values only. * Set constants are not yet supported. File: gdb.info, Node: M2 Defaults, Next: Deviations, Prev: M2 Constants, Up: Modula-2 Modula-2 Defaults ................. If type and range checking are set automatically by GDB, they both default to `on' whenever the working language changes to Modula-2. This happens regardless of whether you, or GDB, selected the working language. If you allow GDB to set the language automatically, then entering code compiled from a file whose name ends with `.mod' will set the working language to Modula-2. *Note Having GDB set the language automatically: Automatically, for further details. File: gdb.info, Node: Deviations, Next: M2 Checks, Prev: M2 Defaults, Up: Modula-2 Deviations from Standard Modula-2 ................................. A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness: * Unlike in standard Modula-2, pointer constants can be formed by integers. This allows you to modify pointer variables during debugging. (In standard Modula-2, the actual address contained in a pointer variable is hidden from you; it can only be modified through direct assignment to another pointer variable or expression that returned a pointer.) * C escape sequences can be used in strings and characters to represent non-printable characters. GDB will print out strings with these escape sequences embedded. Single non-printable characters are printed using the `CHR(NNN)' format. * The assignment operator (`:=') returns the value of its right-hand argument. * All built-in procedures both modify *and* return their argument. File: gdb.info, Node: M2 Checks, Next: M2 Scope, Prev: Deviations, Up: Modula-2 Modula-2 Type and Range Checks .............................. *Warning:* in this release, GDB does not yet perform type or range checking. GDB considers two Modula-2 variables type equivalent if: * They are of types that have been declared equivalent via a `TYPE T1 = T2' statement * They have been declared on the same line. (Note: This is true of the GNU Modula-2 compiler, but it may not be true of other compilers.) As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error. Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures. File: gdb.info, Node: M2 Scope, Next: GDB/M2, Prev: M2 Checks, Up: Modula-2 The scope operators `::' and `.' ................................ There are a few subtle differences between the Modula-2 scope operator (`.') and the GDB scope operator (`::'). The two have similar syntax: MODULE . ID SCOPE :: ID where SCOPE is the name of a module or a procedure, MODULE the name of a module, and ID is any declared identifier within your program, except another module. Using the `::' operator makes GDB search the scope specified by SCOPE for the identifier ID. If it is not found in the specified scope, then GDB will search all scopes enclosing the one specified by SCOPE. Using the `.' operator makes GDB search the current scope for the identifier specified by ID that was imported from the definition module specified by MODULE. With this operator, it is an error if the identifier ID was not imported from definition module MODULE, or if ID is not an identifier in MODULE. File: gdb.info, Node: GDB/M2, Prev: M2 Scope, Up: Modula-2 GDB and Modula-2 ................ Some GDB commands have little use when debugging Modula-2 programs. Five subcommands of `set print' and `show print' apply specifically to C and C++: `vtbl', `demangle', `asm-demangle', `object', and `union'. The first four apply to C++, and the last to C's `union' type, which has no direct analogue in Modula-2. The `@' operator (*note Expressions: Expressions.), while available while using any language, is not useful with Modula-2. Its intent is to aid the debugging of "dynamic arrays", which cannot be created in Modula-2 as they can in C or C++. However, because an address can be specified by an integral constant, the construct `{TYPE}ADREXP' is still useful. (*note Expressions: Expressions.) In GDB scripts, the Modula-2 inequality operator `#' is interpreted as the beginning of a comment. Use `<>' instead. File: gdb.info, Node: Symbols, Next: Altering, Prev: Languages, Up: Top Examining the Symbol Table ************************** The commands described in this section allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as your program executes. GDB finds it in your program's symbol table, in the file indicated when you started GDB (*note Choosing Files: File Options.), or by one of the file-management commands (*note Commands to Specify Files: Files.). `info address SYMBOL' Describe where the data for SYMBOL is stored. For a register variable, this says which register it is kept in. For a non-register local variable, this prints the stack-frame offset at which the variable is always stored. Note the contrast with `print &SYMBOL', which does not work at all for a register variables, and for a stack local variable prints the exact address of the current instantiation of the variable. `whatis EXP' Print the data type of expression EXP. EXP is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. *Note Expressions: Expressions. `whatis' Print the data type of `$', the last value in the value history. `ptype TYPENAME' Print a description of data type TYPENAME. TYPENAME may be the name of a type, or for C code it may have the form `struct STRUCT-TAG', `union UNION-TAG' or `enum ENUM-TAG'. `ptype EXP' `ptype' Print a description of the type of expression EXP. `ptype' differs from `whatis' by printing a detailed description, instead of just the name of the type. For example, if your program declares a variable as struct complex {double real; double imag;} v; compare the output of the two commands: (gdb) whatis v type = struct complex (gdb) ptype v type = struct complex { double real; double imag; } As with `whatis', using `ptype' without an argument refers to the type of `$', the last value in the value history. `info types REGEXP' `info types' Print a brief description of all types whose name matches REGEXP (or all types in your program, if you supply no argument). Each complete typename is matched as though it were a complete line; thus, `i type value' gives information on all types in your program whose name includes the string `value', but `i type ^value$' gives information only on types whose complete name is `value'. This command differs from `ptype' in two ways: first, like `whatis', it does not print a detailed description; second, it lists all source files where a type is defined. `info source' Show the name of the current source file--that is, the source file for the function containing the current point of execution--and the language it was written in. `info sources' Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed. `info functions' Print the names and data types of all defined functions. `info functions REGEXP' Print the names and data types of all defined functions whose names contain a match for regular expression REGEXP. Thus, `info fun step' finds all functions whose names include `step'; `info fun ^step' finds those whose names start with `step'. `info variables' Print the names and data types of all variables that are declared outside of functions (i.e., excluding local variables). `info variables REGEXP' Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression REGEXP. `printsyms FILENAME' `printpsyms FILENAME' `printmsyms FILENAME' Write a dump of debugging symbol data into the file FILENAME. These commands are used to debug the GDB symbol-reading code. Only symbols with debugging data are included. If you use `printsyms', GDB includes all the symbols for which it has already collected full details: that is, FILENAME reflects symbols for only those files whose symbols GDB has read. You can use the command `info sources' to find out which files these are. If you use `printpsyms' instead, the dump shows information about symbols that GDB only knows partially--that is, symbols defined in files that GDB has skimmed, but not yet read completely. Finally, `printmsyms' dumos just the minimal symbol information required for each object file from which GDB has read some symbols. The description of `symbol-file' explains how GDB reads symbols; both `info source' and `symbol-file' are described in *Note Commands to Specify Files: Files. File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top Altering Execution ****************** Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the GDB features for altering execution of the program. For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function to its caller. * Menu: * Assignment:: Assignment to Variables * Jumping:: Continuing at a Different Address * Signaling:: Giving your program a Signal * Returning:: Returning from a Function * Calling:: Calling your Program's Functions * Patching:: Patching your Program File: gdb.info, Node: Assignment, Next: Jumping, Prev: Altering, Up: Altering Assignment to Variables ======================= To alter the value of a variable, evaluate an assignment expression. *Note Expressions: Expressions. For example, print x=4 stores the value 4 into the variable `x', and then prints the value of the assignment expression (which is 4). *Note Using GDB with Different Languages: Languages, for more information on operators in supported languages. If you are not interested in seeing the value of the assignment, use the `set' command instead of the `print' command. `set' is really the same as `print' except that the expression's value is not printed and is not put in the value history (*note Value History: Value History.). The expression is evaluated only for its effects. If the beginning of the argument string of the `set' command appears identical to a `set' subcommand, use the `set variable' command instead of just `set'. This command is identical to `set' except for its lack of subcommands. For example, a program might well have a variable `width'--which leads to an error if we try to set a new value with just `set width=13', as we might if `set width' did not happen to be a GDB command: (gdb) whatis width type = double (gdb) p width $4 = 13 (gdb) set width=47 Invalid syntax in expression. The invalid expression, of course, is `=47'. What we can do in order to actually set our program's variable `width' is (gdb) set var width=47 GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and any structure can be converted to any other structure that is the same length or shorter. To store values into arbitrary places in memory, use the `{...}' construct to generate a value of specified type at a specified address (*note Expressions: Expressions.). For example, `{int}0x83040' refers to memory location `0x83040' as an integer (which implies a certain size and representation in memory), and set {int}0x83040 = 4 stores the value 4 into that memory location. File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering Continuing at a Different Address ================================= Ordinarily, when you continue your program, you do so at the place where it stopped, with the `continue' command. You can instead continue at an address of your own choosing, with the following commands: `jump LINESPEC' Resume execution at line LINESPEC. Execution will stop immediately if there is a breakpoint there. *Note Printing Source Lines: List, for a description of the different forms of LINESPEC. The `jump' command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter. If line LINESPEC is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the `jump' command requests confirmation if the specified line is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program. `jump *ADDRESS' Resume execution at the instruction at address ADDRESS. You can get much the same effect as the `jump' command by storing a new value into the register `$pc'. The difference is that this does not start your program running; it only changes the address where it *will* run when it is continued. For example, set $pc = 0x485 causes the next `continue' command or stepping command to execute at address `0x485', rather than at the address where your program stopped. *Note Continuing and Stepping: Continuing and Stepping. The most common occasion to use the `jump' command is to back up, perhaps with more breakpoints set, over a portion of a program that has already executed, in order to examine its execution in more detail. File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering Giving your program a Signal ============================ `signal SIGNALNUM' Resume execution where your program stopped, but give it immediately the signal number SIGNALNUM. Alternatively, if SIGNALNUM is zero, continue execution without giving a signal. This is useful when your program stopped on account of a signal and would ordinary see the signal when resumed with the `continue' command; `signal 0' causes it to resume without a signal. `signal' does not repeat when you press RET a second time after executing the command. File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering Returning from a Function ========================= `return' `return EXPRESSION' You can cancel execution of a function call with the `return' command. If you give an EXPRESSION argument, its value is used as the function's return value. When you use `return', GDB discards the selected stack frame (and all frames within it). You can think of this as making the discarded frame return prematurely. If you wish to specify a value to be returned, give that value as the argument to `return'. This pops the selected stack frame (*note Selecting a Frame: Selection.), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions. The `return' command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned. In contrast, the `finish' command (*note Continuing and Stepping: Continuing and Stepping.) resumes execution until the selected stack frame returns naturally. File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering Calling your Program's Functions ================================ `call EXPR' Evaluate the expression EXPR without displaying `void' returned values. You can use this variant of the `print' command if you want to execute a function from your program, but without cluttering the output with `void' returned values. The result is printed and saved in the value history, if it is not void. File: gdb.info, Node: Patching, Prev: Calling, Up: Altering Patching your Program ===================== By default, GDB opens the file containing your program's executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary. If you'd like to be able to patch the binary, you can specify that explicitly with the `set write' command. For example, you might want to turn on internal debugging flags, or even to make emergency repairs. `set write on' `set write off' If you specify `set write on', GDB will open executable and core files for both reading and writing; if you specify `set write off' (the default), GDB will open them read-only. If you have already loaded a file, you must load it again (using the `exec-file' or `core-file' command) after changing `set write', for your new setting to take effect. `show write' Display whether executable files and core files will be opened for writing as well as reading. File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top GDB's Files *********** GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, GDB must be told the file name of the core dump. * Menu: * Files:: Commands to Specify Files * Symbol Errors:: Errors Reading Symbol Files