This is Info file ./gdb.info, produced by Makeinfo-1.47 from the 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 4.06, October 1992, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB Version 4.7. 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: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack Selecting a Frame ================= Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected. `frame N' `f N' Select frame number N. Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on. The highest-numbered frame is `main''s frame. `frame ADDR' `f ADDR' Select the frame at address ADDR. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them. On the SPARC architecture, `frame' needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer. `up N' Move N frames up the stack. For positive numbers N, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer. N defaults to one. `down N' Move N frames down the stack. For positive numbers N, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently. N defaults to one. You may abbreviate `down' as `do'. All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line. For example: (gdb) up #1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10 10 read_input_file (argv[i]); After such a printout, the `list' command with no arguments will print ten lines centered on the point of execution in the frame. *Note Printing Source Lines: List. `up-silently N' `down-silently N' These two commands are variants of `up' and `down', respectively; they differ in that they do their work silently, without causing display of the new frame. They are intended primarily for use in GDB command scripts, where the output might be unnecessary and distracting. File: gdb.info, Node: Frame Info, Prev: Selection, Up: Stack Information About a Frame ========================= There are several other commands to print information about the selected stack frame. `frame' `f' When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame. It can be abbreviated `f'. With an argument, this command is used to select a stack frame (*note Selecting a Frame: Selection.). `info frame' `info f' This command prints a verbose description of the selected stack frame, including the address of the frame, the addresses of the next frame down (called by this frame) and the next frame up (caller of this frame), the language that the source code corresponding to this frame was written in, the address of the frame's arguments, the program counter saved in it (the address of execution in the caller frame), and which registers were saved in the frame. The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions. `info frame ADDR' `info f ADDR' Print a verbose description of the frame at address ADDR, without selecting that frame. The selected frame remains unchanged by this command. `info args' Print the arguments of the selected frame, each on a separate line. `info locals' Print the local variables of the selected frame, each on a separate line. These are all variables declared static or automatic within all program blocks that execution in this frame is currently inside of. `info catch' Print a list of all the exception handlers that are active in the current stack frame at the current point of execution. To see other exception handlers, visit the associated frame (using the `up', `down', or `frame' commands); then type `info catch'. *Note Breakpoints and Exceptions: Exception Handling. File: gdb.info, Node: Source, Next: Data, Prev: Stack, Up: Top Examining Source Files ********************** GDB can print parts of your program's source, since the debugging information recorded in your program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (*note Selecting a Frame: Selection.), GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command. If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; *note Using GDB under GNU Emacs: Emacs.. * Menu: * List:: Printing Source Lines * Search:: Searching Source Files * Source Path:: Specifying Source Directories * Machine Code:: Source and Machine Code File: gdb.info, Node: List, Next: Search, Up: Source Printing Source Lines ===================== To print lines from a source file, use the `list' command (abbreviated `l'). There are several ways to specify what part of the file you want to print. Here are the forms of the `list' command most commonly used: `list LINENUM' Print lines centered around line number LINENUM in the current source file. `list FUNCTION' Print lines centered around the beginning of function FUNCTION. `list' Print more lines. If the last lines printed were printed with a `list' command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (*note Examining the Stack: Stack.), this prints lines centered around that line. `list -' Print lines just before the lines last printed. By default, GDB prints ten source lines with any of these forms of the `list' command. You can change this using `set listsize': `set listsize COUNT' Make the `list' command display COUNT source lines (unless the `list' argument explicitly specifies some other number). `show listsize' Display the number of lines that `list' will currently display by default. Repeating a `list' command with RET discards the argument, so it is equivalent to typing just `list'. This is more useful than listing the same lines again. An exception is made for an argument of `-'; that argument is preserved in repetition so that each repetition moves up in the source file. In general, the `list' command expects you to supply zero, one or two "linespecs". Linespecs specify source lines; there are several ways of writing them but the effect is always to specify some source line. Here is a complete description of the possible arguments for `list': `list LINESPEC' Print lines centered around the line specified by LINESPEC. `list FIRST,LAST' Print lines from FIRST to LAST. Both arguments are linespecs. `list ,LAST' Print lines ending with LAST. `list FIRST,' Print lines starting with FIRST. `list +' Print lines just after the lines last printed. `list -' Print lines just before the lines last printed. `list' As described in the preceding table. Here are the ways of specifying a single source line--all the kinds of linespec. `NUMBER' Specifies line NUMBER of the current source file. When a `list' command has two linespecs, this refers to the same source file as the first linespec. `+OFFSET' Specifies the line OFFSET lines after the last line printed. When used as the second linespec in a `list' command that has two, this specifies the line OFFSET lines down from the first linespec. `-OFFSET' Specifies the line OFFSET lines before the last line printed. `FILENAME:NUMBER' Specifies line NUMBER in the source file FILENAME. `FUNCTION' Specifies the line of the open-brace that begins the body of the function FUNCTION. `FILENAME:FUNCTION' Specifies the line of the open-brace that begins the body of the function FUNCTION in the file FILENAME. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files. `*ADDRESS' Specifies the line containing the program address ADDRESS. ADDRESS may be any expression. File: gdb.info, Node: Search, Next: Source Path, Prev: List, Up: Source Searching Source Files ====================== There are two commands for searching through the current source file for a regular expression. `forward-search REGEXP' `search REGEXP' The command `forward-search REGEXP' checks each line, starting with the one following the last line listed, for a match for REGEXP. It lists the line that is found. You can use synonym `search REGEXP' or abbreviate the command name as `fo'. `reverse-search REGEXP' The command `reverse-search REGEXP' checks each line, starting with the one before the last line listed and going backward, for a match for REGEXP. It lists the line that is found. You can abbreviate this command as `rev'. File: gdb.info, Node: Source Path, Next: Machine Code, Prev: Search, Up: Source Specifying Source Directories ============================= Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the "source path". Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name. Note that the executable search path is *not* used for this purpose. Neither is the current working directory, unless it happens to be in the source path. If GDB cannot find a source file in the source path, and the object program records a directory, GDB tries that directory too. If the source path is empty, and there is no record of the compilation directory, GDB will, as a last resort, look in the current directory. Whenever you reset or rearrange the source path, GDB will clear out any information it has cached about where source files are found, where each line is in the file, etc. When you start GDB, its source path is empty. To add other directories, use the `directory' command. `directory DIRNAME ...' Add directory DIRNAME to the front of the source path. Several directory names may be given to this command, separated by `:' or whitespace. You may specify a directory that is already in the source path; this moves it forward, so it will be searched sooner. You can use the string `$cdir' to refer to the compilation directory (if one is recorded), and `$cwd' to refer to the current working directory. `$cwd' is not the same as `.'--the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path. `directory' Reset the source path to empty again. This requires confirmation. `show directories' Print the source path: show which directories it contains. If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows: 1. Use `directory' with no argument to reset the source path to empty. 2. Use `directory' with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command. File: gdb.info, Node: Machine Code, Prev: Source Path, Up: Source Source and Machine Code ======================= You can use the command `info line' to map source lines to program addresses (and viceversa), and the command `disassemble' to display a range of addresses as machine instructions. `info line LINESPEC' Print the starting and ending addresses of the compiled code for source line LINESPEC. You can specify source lines in any of the ways understood by the `list' command (*note Printing Source Lines: List.). For example, we can use `info line' to discover the location of the object code for the first line of function `m4_changequote': (gdb) info line m4_changecom Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350. We can also inquire (using `*ADDR' as the form for LINESPEC) what source line covers a particular address: (gdb) info line *0x63ff Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404. After `info line', the default address for the `x' command is changed to the starting address of the line, so that `x/i' is sufficient to begin examining the machine code (*note Examining Memory: Memory.). Also, this address is saved as the value of the convenience variable `$_' (*note Convenience Variables: Convenience Vars.). `disassemble' This specialized command dumps a range of memory as machine instructions. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; the function surrounding this value will be dumped. Two arguments specify a range of addresses (first inclusive, second exclusive) to dump. We can use `disassemble' to inspect the object code range shown in the last `info line' example (the example shows SPARC machine instructions): (gdb) disas 0x63e4 0x6404 Dump of assembler code from 0x63e4 to 0x6404: 0x63e4 <builtin_init+5340>: ble 0x63f8 <builtin_init+5360> 0x63e8 <builtin_init+5344>: sethi %hi(0x4c00), %o0 0x63ec <builtin_init+5348>: ld [%i1+4], %o0 0x63f0 <builtin_init+5352>: b 0x63fc <builtin_init+5364> 0x63f4 <builtin_init+5356>: ld [%o0+4], %o0 0x63f8 <builtin_init+5360>: or %o0, 0x1a4, %o0 0x63fc <builtin_init+5364>: call 0x9288 <path_search> 0x6400 <builtin_init+5368>: nop End of assembler dump. For example, here is the beginning of the output for the disassembly of a function `fact': (gdb) disas fact Dump of assembler code for function fact: to 0x808c: 0x802c <fact>: 6d f2 mov.w r2,@-r7 0x802e <fact+2>: 6d f3 mov.w r3,@-r7 0x8030 <fact+4>: 6d f6 mov.w r6,@-r7 0x8032 <fact+6>: 0d 76 mov.w r7,r6 0x8034 <fact+8>: 6f 70 00 08 mov.w @(0x8,r7),r0 0x8038 <fact+12> 19 11 sub.w r1,r1 . . . File: gdb.info, Node: Data, Next: Languages, Prev: Source, Up: Top Examining Data ************** The usual way to examine data in your program is with the `print' command (abbreviated `p'), or its synonym `inspect'. It evaluates and prints the value of an expression of the language your program is written in (*note Using GDB with Different Languages: Languages.). `print EXP' `print /F EXP' EXP is an expression (in the source language). By default the value of EXP is printed in a format appropriate to its data type; you can choose a different format by specifying `/F', where F is a letter specifying the format; *note Output formats::.. `print' `print /F' If you omit EXP, GDB displays the last value again (from the "value history"; *note Value History: Value History.). This allows you to conveniently inspect the same value in an alternative format. A more low-level way of examining data is with the `x' command. It examines data in memory at a specified address and prints it in a specified format. *Note Examining Memory: Memory. If you are interested in information about types, or about how the fields of a struct or class are declared, use the `ptype EXP' command rather than `print'. *Note Examining the Symbol Table: Symbols. * Menu: * Expressions:: Expressions * Variables:: Program Variables * Arrays:: Artificial Arrays * Output formats:: Output formats * Memory:: Examining Memory * Auto Display:: Automatic Display * Print Settings:: Print Settings * Value History:: Value History * Convenience Vars:: Convenience Variables * Registers:: Registers * Floating Point Hardware:: Floating Point Hardware File: gdb.info, Node: Expressions, Next: Variables, Up: Data Expressions =========== `print' and many other GDB commands accept an expression and compute its value. Any kind of constant, variable or operator defined by the programming language you are using is legal in an expression in GDB. This includes conditional expressions, function calls, casts and string constants. It unfortunately does not include symbols defined by preprocessor `#define' commands. Because C is so widespread, most of the expressions shown in examples in this manual are in C. *Note Using GDB with Different Languages: Languages, for information on how to use expressions in other languages. In this section, we discuss operators that you can use in GDB expressions regardless of your programming language. Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer so as to examine a structure at that address in memory. GDB supports these operators in addition to those of programming languages: `@' `@' is a binary operator for treating parts of memory as arrays. *Note Artificial Arrays: Arrays, for more information. `::' `::' allows you to specify a variable in terms of the file or function where it is defined. *Note Program Variables: Variables. `{TYPE} ADDR' Refers to an object of type TYPE stored at address ADDR in memory. ADDR may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at ADDR. File: gdb.info, Node: Variables, Next: Arrays, Prev: Expressions, Up: Data Program Variables ================= The most common kind of expression to use is the name of a variable in your program. Variables in expressions are understood in the selected stack frame (*note Selecting a Frame: Selection.); they must either be global (or static) or be visible according to the scope rules of the programming language from the point of execution in that frame. This means that in the function foo (a) int a; { bar (a); { int b = test (); bar (b); } } the variable `a' is usable whenever your program is executing within the function `foo', but the variable `b' is visible only while your program is executing inside the block in which `b' is declared. There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a static variable in a particular function or file, using the colon-colon notation: FILE::VARIABLE FUNCTION::VARIABLE Here FILE or FUNCTION is the name of the context for the static VARIABLE. In the case of file names, you can use quotes to make sure GDB parses the file name as a single word--for example, to print a global value of `x' defined in `f2.c': (gdb) p 'f2.c'::x This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. GDB also supports use of the C++ scope resolution operator in GDB expressions. *Warning:* Occasionally, a local variable may appear to have the wrong value at certain points in a function--just after entry to the function, and just before exit. You may see this problem when you are stepping by machine instructions. This is because on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On function exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone. File: gdb.info, Node: Arrays, Next: Output formats, Prev: Variables, Up: Data Artificial Arrays ================= It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program. This can be done by constructing an "artificial array" with the binary operator `@'. The left operand of `@' should be the first element of the desired array, as an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says int *array = (int *) malloc (len * sizeof (int)); you can print the contents of `array' with p *array@len The left operand of `@' must reside in memory. Array values made with `@' in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (*note Value History: Value History.), after printing one out.) Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent--for example, if you are interested in the values of pointers in an array. One useful work-around in this situation is to use a convenience variable (*note Convenience Variables: Convenience Vars.) as a counter in an expression that prints the first interesting value, and then repeat that expression via RET. For instance, suppose you have an array `dtab' of pointers to structures, and you are interested in the values of a field `fv' in each structure. Here is an example of what you might type: set $i = 0 p dtab[$i++]->fv RET RET ... File: gdb.info, Node: Output formats, Next: Memory, Prev: Arrays, Up: Data Output formats ============== By default, GDB prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an "output format" when you print a value. The simplest use of output formats is to say how to print a value already computed. This is done by starting the arguments of the `print' command with a slash and a format letter. The format letters supported are: `x' Regard the bits of the value as an integer, and print the integer in hexadecimal. `d' Print as integer in signed decimal. `u' Print as integer in unsigned decimal. `o' Print as integer in octal. `t' Print as integer in binary. The letter `t' stands for "two". `a' Print as an address, both absolute in hex and as an offset from the nearest preceding symbol. This format can be used to discover where (in what function) an unknown address is located: (gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396> `c' Regard as an integer and print it as a character constant. `f' Regard the bits of the value as a floating point number and print using typical floating point syntax. For example, to print the program counter in hex (*note Registers::.), type p/x $pc Note that no space is required before the slash; this is because command names in GDB cannot contain a slash. To reprint the last value in the value history with a different format, you can use the `print' command with just a format and no expression. For example, `p/x' reprints the last value in hex. File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output formats, Up: Data Examining Memory ================ You can use the command `x' (for "examine") to examine memory in any of several formats, independently of your program's data types. `x/NFU ADDR' `x ADDR' `x' Use the command `x' to examine memory. N, F, and U are all optional parameters that specify how much memory to display and how to format it; ADDR is an expression giving the address where you want to start displaying memory. If you use defaults for NFU, you need not type the slash `/'. Several commands set convenient defaults for ADDR. N, the repeat count The repeat count is a decimal integer; the default is 1. It specifies how much memory (counting by units U) to display. F, the display format The display format is one of the formats used by `print', or `s' (null-terminated string) or `i' (machine instruction). The default is `x' (hexadecimal) initially, or the format from the last time you used either `x' or `print'. U, the unit size The unit size is any of `b' Bytes. `h' Halfwords (two bytes). `w' Words (four bytes). This is the initial default. `g' Giant words (eight bytes). Each time you specify a unit size with `x', that size becomes the default unit the next time you use `x'. (For the `s' and `i' formats, the unit size is ignored and is normally not written.) ADDR, starting display address ADDR is the address where you want GDB to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. *Note Expressions: Expressions, for more information on expressions. The default for ADDR is usually just after the last address examined--but several other commands also set the default address: `info breakpoints' (to the address of the last breakpoint listed), `info line' (to the starting address of a line), and `print' (if you use it to display a value from memory). For example, `x/3uh 0x54320' is a request to display three halfwords (`h') of memory, formatted as unsigned decimal integers (`u'), starting at address `0x54320'. `x/4xw $sp' prints the four words (`w') of memory above the stack pointer (here, `$sp'; *note Registers::.) in hexadecimal (`x'). Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order will work. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count N must come first; `wx4' will not work.) Even though the unit size U is ignored for the formats `s' and `i', you might still want to use a count N; for example, `3i' specifies that you want to see three machine instructions, including any operands. The command `disassemble' gives an alternative way of inspecting machine instructions; *note Machine Code::.. All the defaults for the arguments to `x' are designed to make it easy to continue scanning memory with minimal specifications each time you use `x'. For example, after you have inspected three machine instructions with `x/3i ADDR', you can inspect the next seven with just `x/7'. If you use RET to repeat the `x' command, the repeat count N is used again; the other arguments default as for successive uses of `x'. The addresses and contents printed by the `x' command are not saved in the value history because there is often too much of them and they would get in the way. Instead, GDB makes these values available for subsequent use in expressions as values of the convenience variables `$_' and `$__'. After an `x' command, the last address examined is available for use in expressions in the convenience variable `$_'. The contents of that address, as examined, are available in the convenience variable `$__'. If the `x' command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output. File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data Automatic Display ================= If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the "automatic display list" so that GDB will print its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this: 2: foo = 38 3: bar[5] = (struct hack *) 0x3804 showing item numbers, expressions and their current values. As with displays you request manually using `x' or `print', you can specify the output format you prefer; in fact, `display' decides whether to use `print' or `x' depending on how elaborate your format specification is--it uses `x' if you specify a unit size, or one of the two formats (`i' and `s') that are only supported by `x'; otherwise it uses `print'. `display EXP' Add the expression EXP to the list of expressions to display each time your program stops. *Note Expressions: Expressions. `display' will not repeat if you press RET again after using it. `display/FMT EXP' For FMT specifying only a display format and not a size or count, add the expression EXP to the auto-display list but arranges to display it each time in the specified format FMT. *Note Output formats::. `display/FMT ADDR' For FMT `i' or `s', or including a unit-size or a number of units, add the expression ADDR as a memory address to be examined each time your program stops. Examining means in effect doing `x/FMT ADDR'. *Note Examining Memory: Memory. For example, `display/i $pc' can be helpful, to see the machine instruction about to be executed each time execution stops (`$pc' is a common name for the program counter; *note Registers::.). `undisplay DNUMS...' `delete display DNUMS...' Remove item numbers DNUMS from the list of expressions to display. `undisplay' will not repeat if you press RET after using it. (Otherwise you would just get the error `No display number ...'.) `disable display DNUMS...' Disable the display of item numbers DNUMS. A disabled display item is not printed automatically, but is not forgotten. It may be enabled again later. `enable display DNUMS...' Enable display of item numbers DNUMS. It becomes effective once again in auto display of its expression, until you specify otherwise. `display' Display the current values of the expressions on the list, just as is done when your program stops. `info display' Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available. If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up. Such an expression is disabled when execution enters a context where one of its variables is not defined. For example, if you give the command `display last_char' while inside a function with an argument `last_char', then this argument will be displayed while your program continues to stop inside that function. When it stops elsewhere--where there is no variable `last_char'--display is disabled. The next time your program stops where `last_char' is meaningful, you can enable the display expression once again. 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 will print 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) `show print address' Show whether or not addresses are to be printed. `set print array' `set print array on' GDB will 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 will stop 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. `show print elements' Display the number of elements of a large array that GDB will print 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 will use to print structures. `set print sevenbit-strings on' Print using only seven-bit characters; if this option is set, GDB will display any eight-bit characters (in strings or character values) using the notation `\'NNN. For example, `M-a' is displayed as `\341'. `set print sevenbit-strings off' Print using either seven-bit or eight-bit characters, as required. This is the default. `show print sevenbit-strings' Show whether or not GDB will print 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 mangled form in which they are passed to the assembler and linker for type-safe linkage. The default is on. `show print demangle' Show whether C++ names will be 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 will be printed in mangled or demangled form. `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 will be 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 GDB's "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" for you to refer to them by. 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. 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 will make 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 don't 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 will 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' If available, provides hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip.