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Info file gdb.info, produced by Makeinfo, -*- Text -*- from input file gdb-all.texi. This file documents the GNU debugger GDB. Copyright (C) 1988, 1989, 1990, 1991 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: Breakpoints, Next: Continuing and Stepping, Prev: Stopping, Up: Stopping Breakpoints, Watchpoints, and Exceptions ======================================== A "breakpoint" makes your program stop whenever a certain point in the program is reached. For each breakpoint, you can add various conditions to control in finer detail whether the program will stop. You can set breakpoints with the `break' command and its variants (*note Set Breaks::.), to specify the place where the program should stop by line number, function name or exact address in the program. In languages with exception handling (such as GNU C++), you can also set breakpoints where an exception is raised (*note Exception Handling::.). A "watchpoint" is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (*note Set Watchpoints::.), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands. Each breakpoint or watchpoint is assigned a number when it is created; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be "enabled" or "disabled"; if disabled, it has no effect on the program until you enable it again. * Menu: * Set Breaks:: Setting Breakpoints * Set Watchpoints:: Setting Watchpoints * Exception Handling:: Breakpoints and Exceptions * Delete Breaks:: Deleting Breakpoints * Disabling:: Disabling Breakpoints * Conditions:: Break Conditions * Break Commands:: Breakpoint Command Lists * Breakpoint Menus:: Breakpoint Menus * Error in Breakpoints:: File: gdb.info, Node: Set Breaks, Next: Set Watchpoints, Prev: Breakpoints, Up: Breakpoints Setting Breakpoints ------------------- Breakpoints are set with the `break' command (abbreviated `b'). You have several ways to say where the breakpoint should go. `break FUNCTION' Set a breakpoint at entry to function FUNCTION. When using source languages that permit overloading of symbols, such as C++, FUNCTION may refer to more than one possible place to break. *Note Breakpoint Menus::, for a discussion of that situation. `break +OFFSET' `break -OFFSET' Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected frame. `break LINENUM' Set a breakpoint at line LINENUM in the current source file. That file is the last file whose source text was printed. This breakpoint will stop the program just before it executes any of the code on that line. `break FILENAME:LINENUM' Set a breakpoint at line LINENUM in source file FILENAME. `break FILENAME:FUNCTION' Set a breakpoint at entry to function FUNCTION found in file FILENAME. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions. `break *ADDRESS' Set a breakpoint at address ADDRESS. You can use this to set breakpoints in parts of the program which do not have debugging information or source files. `break' When called without any arguments, `break' sets a breakpoint at the next instruction to be executed in the selected stack frame (*note Stack::.). In any selected frame but the innermost, this will cause the program to stop as soon as control returns to that frame. This is similar to the effect of a `finish' command in the frame inside the selected frame--except that `finish' doesn't leave an active breakpoint. If you use `break' without an argument in the innermost frame, GDB will stop the next time it reaches the current location; this may be useful inside loops. GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when the program stopped. `break ... if COND' Set a breakpoint with condition COND; evaluate the expression COND each time the breakpoint is reached, and stop only if the value is nonzero--that is, if COND evaluates as true. `...' stands for one of the possible arguments described above (or no argument) specifying where to break. *Note Conditions::, for more information on breakpoint conditions. `tbreak ARGS' Set a breakpoint enabled only for one stop. ARGS are the same as for the `break' command, and the breakpoint is set in the same way, but the breakpoint is automatically disabled the first time it is hit. *Note Disabling::. `rbreak REGEX' Set breakpoints on all functions matching the regular expression REGEX. This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set. Once these breakpoints are set, they are treated just like the breakpoints set with the `break' command. They can be deleted, disabled, made conditional, etc., in the standard ways. When debugging C++ programs, `rbreak' is useful for setting breakpoints on overloaded functions that are not members of any special classes. `info breakpoints [N]' `info break [N]' Print a list of all breakpoints (but not watchpoints) set and not deleted, showing their numbers, where in the program they are, and any special features in use for them. Disabled breakpoints are included in the list, but marked as disabled. `info break' with a breakpoint number N as argument lists only that breakpoint. The convenience variable `$_' and the default examining-address for the `x' command are set to the address of the last breakpoint listed (*note Memory::.). The equivalent command for watchpoints is `info watch'. GDB allows you to set any number of breakpoints at the same place in the program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (*note Conditions::.). File: gdb.info, Node: Set Watchpoints, Next: Exception Handling, Prev: Set Breaks, Up: Breakpoints Setting Watchpoints ------------------- You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen. Watchpoints currently execute two orders of magnitude more slowly than other breakpoints, but this can well be worth it to catch errors where you have no clue what part of your program is the culprit. Some processors provide special hardware to support watchpoint evaluation; future releases of GDB will use such hardware if it is available. `watch EXPR' Set a watchpoint for an expression. `info watchpoints' This command prints a list of watchpoints; it is otherwise similar to `info break'. File: gdb.info, Node: Exception Handling, Next: Delete Breaks, Prev: Set Watchpoints, Up: Breakpoints Breakpoints and Exceptions -------------------------- Some languages, such as GNU C++, implement exception handling. You can use GDB to examine what caused the program to raise an exception, and to list the exceptions the program is prepared to handle at a given point in time. `catch EXCEPTIONS' You can set breakpoints at active exception handlers by using the `catch' command. EXCEPTIONS is a list of names of exceptions to catch. You can use `info catch' to list active exception handlers; *note Frame Info::.. There are currently some limitations to exception handling in GDB. These will be corrected in a future release. * If you call a function interactively, GDB normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to the user and cause the program to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits. * You cannot raise an exception interactively. * You cannot interactively install an exception handler. Sometimes `catch' is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it's better to stop *before* the exception handler is called, since that way you can see the stack before any unwinding takes place. If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised. To stop just before an exception handler is called, you need some knowledge of the implementation. In the case of GNU C++, exceptions are raised by calling a library function named `__raise_exception' which has the following ANSI C interface: /* ADDR is where the exception identifier is stored. ID is the exception identifier. */ void __raise_exception (void **ADDR, void *ID); To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on `__raise_exception' (*note Breakpoints::.). With a conditional breakpoint (*Note Conditions::) that depends on the value of ID, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop the program when any of a number of exceptions are raised. File: gdb.info, Node: Delete Breaks, Next: Disabling, Prev: Exception Handling, Up: Breakpoints Deleting Breakpoints -------------------- It is often necessary to eliminate a breakpoint or watchpoint once it has done its job and you no longer want the program to stop there. This is called "deleting" the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten. With the `clear' command you can delete breakpoints according to where they are in the program. With the `delete' command you can delete individual breakpoints or watchpoints by specifying their breakpoint numbers. It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address. `clear' Delete any breakpoints at the next instruction to be executed in the selected stack frame (*note Selection::.). When the innermost frame is selected, this is a good way to delete a breakpoint that the program just stopped at. `clear FUNCTION' `clear FILENAME:FUNCTION' Delete any breakpoints set at entry to the function FUNCTION. `clear LINENUM' `clear FILENAME:LINENUM' Delete any breakpoints set at or within the code of the specified line. `delete [breakpoints] [BNUMS...]' Delete the breakpoints or watchpoints of the numbers specified as arguments. If no argument is specified, delete all breakpoints (GDB asks confirmation, unless you've `set confirm off'). You can abbreviate this command as `d'. File: gdb.info, Node: Disabling, Next: Conditions, Prev: Delete Breaks, Up: Breakpoints Disabling Breakpoints --------------------- Rather than deleting a breakpoint or watchpoint, you might prefer to "disable" it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can "enable" it again later. You disable and enable breakpoints and watchpoints with the `enable' and `disable' commands, optionally specifying one or more breakpoint numbers as arguments. Use `info break' or `info watch' to print a list of breakpoints or watchpoints if you don't know which numbers to use. A breakpoint or watchpoint can have any of four different states of enablement: * Enabled. The breakpoint will stop the program. A breakpoint set with the `break' command starts out in this state. * Disabled. The breakpoint has no effect on the program. * Enabled once. The breakpoint will stop the program, but when it does so it will become disabled. A breakpoint set with the `tbreak' command starts out in this state. * Enabled for deletion. The breakpoint will stop the program, but immediately after it does so it will be deleted permanently. You can use the following commands to enable or disable breakpoints and watchpoints: `disable [breakpoints] [BNUMS...]' Disable the specified breakpoints--or all breakpoints, if none are listed. A disabled breakpoint has no effect but is not forgotten. All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later. You may abbreviate `disable' as `dis'. `enable [breakpoints] [BNUMS...]' Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping the program. `enable [breakpoints] once BNUMS...' Enable the specified breakpoints temporarily. Each will be disabled again the next time it stops the program. `enable [breakpoints] delete BNUMS...' Enable the specified breakpoints to work once and then die. Each of the breakpoints will be deleted the next time it stops the program. Save for a breakpoint set with `tbreak' (*note Set Breaks::.), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above. (The command `until' can set and delete a breakpoint of its own, but it will not change the state of your other breakpoints; *note Continuing and Stepping::..) File: gdb.info, Node: Conditions, Next: Break Commands, Prev: Disabling, Up: Breakpoints Break Conditions ---------------- The simplest sort of breakpoint breaks every time the program reaches a specified place. You can also specify a "condition" for a breakpoint. A condition is just a Boolean expression in your programming language. (*Note Expressions::). A breakpoint with a condition evaluates the expression each time the program reaches it, and the program stops only if the condition is *true*. This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated--that is, when the condition is false. In C, if you want to test an assertion expressed by the condition ASSERT, you should set the condition `! ASSERT' on the appropriate breakpoint. Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow--but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one. Break conditions ca have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint first and stop the program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible for the purpose of performing side effects when a breakpoint is reached (*note Break Commands::.). Break conditions can be specified when a breakpoint is set, by using `if' in the arguments to the `break' command. *Note Set Breaks::. They can also be changed at any time with the `condition' command. The `watch' command doesn't recognize the `if' keyword; `condition' is the only way to impose a further condition on a watchpoint. `condition BNUM EXPRESSION' Specify EXPRESSION as the break condition for breakpoint or watchpoint number BNUM. From now on, this breakpoint will stop the program only if the value of EXPRESSION is true (nonzero, in C). When you use `condition', GDB checks EXPRESSION immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint. GDB does not actually evaluate EXPRESSION at the time the `condition' command is given, however. *Note Expressions::. `condition BNUM' Remove the condition from breakpoint number BNUM. It becomes an ordinary unconditional breakpoint. A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the "ignore count" of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if the program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is N, the breakpoint will not stop the next N times it is reached. `ignore BNUM COUNT' Set the ignore count of breakpoint number BNUM to COUNT. The next COUNT times the breakpoint is reached, your program's execution will not stop; other than to decrement the ignore count, GDB takes no action. To make the breakpoint stop the next time it is reached, specify a count of zero. `continue COUNT' `c COUNT' `fg COUNT' Continue execution of the program, setting the ignore count of the breakpoint that the program stopped at to COUNT minus one. Thus, the program will not stop at this breakpoint until the COUNT'th time it is reached. An argument to this command is meaningful only when the program stopped due to a breakpoint. At other times, the argument to `continue' is ignored. The synonym `fg' is provided purely for convenience, and has exactly the same behavior as other forms of the command. If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, the condition will be checked. You could achieve the effect of the ignore count with a condition such as `$foo-- <= 0' using a debugger convenience variable that is decremented each time. *Note Convenience Vars::. File: gdb.info, Node: Break Commands, Next: Breakpoint Menus, Prev: Conditions, Up: Breakpoints Breakpoint Command Lists ------------------------ You can give any breakpoint (or watchpoint) a series of commands to execute when the program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints. `commands [BNUM]' `... COMMAND-LIST ...' `end' Specify a list of commands for breakpoint number BNUM. The commands themselves appear on the following lines. Type a line containing just `end' to terminate the commands. To remove all commands from a breakpoint, type `commands' followed immediately by `end'; that is, give no commands. With no BNUM argument, `commands' refers to the last breakpoint or watchpoint set (not to the breakpoint most recently encountered). Pressing RET as a means of repeating the last GDB command is disabled within a COMMAND-LIST. You can use breakpoint commands to start the program up again. Simply use the `continue' command, or `step', or any other command that resumes execution. Subsequent commands in the command list are ignored. If the first command specified is `silent', the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If the remaining commands too print nothing, you will see no sign that the breakpoint was reached at all. `silent' is meaningful only at the beginning of a breakpoint command list. The commands `echo' and `output' that allow you to print precisely controlled output are often useful in silent breakpoints. *Note Output::. For example, here is how you could use breakpoint commands to print the value of `x' at entry to `foo' whenever `x' is positive. break foo if x>0 commands silent echo x is\040 output x echo \n cont end One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the `continue' command so that the program does not stop, and start with the `silent' command so that no output is produced. Here is an example: break 403 commands silent set x = y + 4 cont end One deficiency in the operation of automatically continuing breakpoints under Unix appears when your program uses raw mode for the terminal. GDB switches back to its own terminal modes (not raw) before executing commands, and then must switch back to raw mode when your program is continued. This causes any pending terminal input to be lost. Under Unix, you can get around this problem by writing actions into the breakpoint condition rather than in commands. For example condition 5 (x = y + 4), 0 specifies a condition expression (*Note Expressions::) that will change `x' as needed, then always have the value zero so the program will not stop. No input is lost here, because GDB evaluates break conditions without changing the terminal modes. When you want to have nontrivial conditions for performing the side effects, the operators `&&', `||' and `?...:' may be useful. File: gdb.info, Node: Breakpoint Menus, Next: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints Breakpoint Menus ---------------- Some programming languages (notably C++) permit a single function name to be defined several times, for application in different contexts. This is called "overloading". When a function name is overloaded, `break FUNCTION' is not enough to tell GDB where you want a breakpoint. GDB offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the prompt `>'. The first two options are always `[0] cancel' and `[1] all'. Typing `1' sets a breakpoint at each definition of FUNCTION, and typing `0' aborts the `break' command without setting any new breakpoints. For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol `String::after'. We choose three particular definitions of that function name: (gdb) b String::after [0] cancel [1] all [2] file:String.cc; line number:867 [3] file:String.cc; line number:860 [4] file:String.cc; line number:875 [5] file:String.cc; line number:853 [6] file:String.cc; line number:846 [7] file:String.cc; line number:735 > 2 4 6 Breakpoint 1 at 0xb26c: file String.cc, line 867. Breakpoint 2 at 0xb344: file String.cc, line 875. Breakpoint 3 at 0xafcc: file String.cc, line 846. Multiple breakpoints were set. Use the "delete" command to delete unwanted breakpoints. (gdb) File: gdb.info, Node: Error in Breakpoints, Prev: Breakpoint Menus, Up: Breakpoints "Cannot Insert Breakpoints" --------------------------- Under some operating systems, breakpoints cannot be used in a program if any other process is running that program. In this situation, attempting to run or continue a program with a breakpoint causes GDB to stop the other process. When this happens, you have three ways to proceed: 1. Remove or disable the breakpoints, then continue. 2. Suspend GDB, and copy the file containing the program to a new name. Resume GDB and use the `exec-file' command to specify that GDB should run the program under that name. Then start the program again. 3. Relink the program so that the text segment is nonsharable, using the linker option `-N'. The operating system limitation may not apply to nonsharable executables. File: gdb.info, Node: Continuing and Stepping, Next: Signals, Prev: Breakpoints, Up: Stopping Continuing and Stepping ======================= "Continuing" means resuming program execution until your program completes normally. In contrast, "stepping" means resuming program execution for a very limited time: one line of source code, or one machine instruction. Either when continuing or when stepping, the program may stop even sooner, due to a breakpoint or to a signal. (If due to a signal, you may want to use `handle', or use `signal 0' to resume execution; *note Signals::..) `continue [IGNORE-COUNT]' Resume program execution, at the address where the program last stopped; any breakpoints set at that address are bypassed. The optional argument IGNORE-COUNT allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of `ignore' (*note Conditions::.). To resume execution at a different place, you can use `return' (*note Returning::.) to go back to the calling function; or `jump' (*note Jumping::.) to go to an arbitrary location in your program. A typical technique for using stepping is to set a breakpoint (*note Breakpoints::.) at the beginning of the function or the section of the program in which a problem is believed to lie, run the program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen. `step' Continue running the program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated `s'. *Warning:* If you use the `step' command while control is within a function that was compiled without debugging information, execution will proceed until control reaches another function. `step COUNT' Continue running as in `step', but do so COUNT times. If a breakpoint is reached or a signal not related to stepping occurs before COUNT steps, stepping stops right away. `next [COUNT]' Continue to the next source line in the current (innermost) stack frame. Similar to `step', but any function calls appearing within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the stack level which was executing when the `next' command was given. This command is abbreviated `n'. An argument COUNT is a repeat count, as for `step'. `next' within a function that lacks debugging information acts like `step', but any function calls appearing within the code of the function are executed without stopping. `finish' Continue running until just after function in the selected stack frame returns. Print the returned value (if any). Contrast this with the `return' command (*note Returning::.). `until' `u' Continue running until a source line past the current line, in the current stack frame, is reached. This command is used to avoid single stepping through a loop more than once. It is like the `next' command, except that when `until' encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump. This means that when you reach the end of a loop after single stepping though it, `until' will cause the program to continue execution until the loop is exited. In contrast, a `next' command at the end of a loop will simply step back to the beginning of the loop, which would force you to step through the next iteration. `until' always stops the program if it attempts to exit the current stack frame. `until' may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines. For example, in the following excerpt from a debugging session, the `f' (`frame') command shows that execution is stopped at line `206'; yet when we use `until', we get to line `195': (gdb) f #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 206 expand_input(); (gdb) until 195 for ( ; argc > 0; NEXTARG) { This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop--even though the test in a C `for'-loop is written before the body of the loop. The `until' command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement--not in terms of the actual machine code. `until' with no argument works by means of single instruction stepping, and hence is slower than `until' with an argument. `until LOCATION' `u LOCATION' Continue running the program until either the specified location is reached, or the current stack frame returns. LOCATION is any of the forms of argument acceptable to `break' (*note Set Breaks::.). This form of the command uses breakpoints, and hence is quicker than `until' without an argument. `stepi' `si' Execute one machine instruction, then stop and return to the debugger. It is often useful to do `display/i $pc' when stepping by machine instructions. This will cause the next instruction to be executed to be displayed automatically at each stop. *Note Auto Display::. An argument is a repeat count, as in `step'. `nexti' `ni' Execute one machine instruction, but if it is a function call, proceed until the function returns. An argument is a repeat count, as in `next'. File: gdb.info, Node: Signals, Prev: Continuing and Stepping, Up: Stopping Signals ======= A signal is an asynchronous event that can happen in a program. The operating system defines the possible kinds of signals, and gives each kind a name and a number. For example, in Unix `SIGINT' is the signal a program gets when you type an interrupt (often `C-c'); `SIGSEGV' is the signal a program gets from referencing a place in memory far away from all the areas in use; `SIGALRM' occurs when the alarm clock timer goes off (which happens only if the program has requested an alarm). Some signals, including `SIGALRM', are a normal part of the functioning of the program. Others, such as `SIGSEGV', indicate errors; these signals are "fatal" (kill the program immediately) if the program has not specified in advance some other way to handle the signal. `SIGINT' does not indicate an error in the program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program. GDB has the ability to detect any occurrence of a signal in the program running under GDB's control. You can tell GDB in advance what to do for each kind of signal. Normally, GDB is set up to ignore non-erroneous signals like `SIGALRM' (so as not to interfere with their role in the functioning of the program) but to stop the program immediately whenever an error signal happens. You can change these settings with the `handle' command. `info signals' Print a table of all the kinds of signals and how GDB has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals. `handle SIGNAL KEYWORDS...' Change the way GDB handles signal SIGNAL. SIGNAL can be the number of a signal or its name (with or without the `SIG' at the beginning). The KEYWORDS say what change to make. The keywords allowed by the `handle' command can be abbreviated. Their full names are: `nostop' GDB should not stop the program when this signal happens. It may still print a message telling you that the signal has come in. `stop' GDB should stop the program when this signal happens. This implies the `print' keyword as well. `print' GDB should print a message when this signal happens. `noprint' GDB should not mention the occurrence of the signal at all. This implies the `nostop' keyword as well. `pass' GDB should allow the program to see this signal; the program will be able to handle the signal, or may be terminated if the signal is fatal and not handled. `nopass' GDB should not allow the program to see this signal. When a signal has been set to stop the program, the program cannot see the signal until you continue. It will see the signal then, if `pass' is in effect for the signal in question at that time. In other words, after GDB reports a signal, you can use the `handle' command with `pass' or `nopass' to control whether that signal will be seen by the program when you later continue it. You can also use the `signal' command to prevent the program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time. For example, if the program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but the program would probably terminate immediately as a result of the fatal signal once it sees the signal. To prevent this, you can continue with `signal 0'. *Note Signaling::. File: gdb.info, Node: Stack, Next: Source, Prev: Stopping, Up: Top Examining the Stack ******************* When your program has stopped, the first thing you need to know is where it stopped and how it got there. Each time your program performs a function call, the information about where in the program the call was made from is saved in a block of data called a "stack frame". The frame also contains the arguments of the call and the local variables of the function that was called. All the stack frames are allocated in a region of memory called the "call stack". When your program stops, the GDB commands for examining the stack allow you to see all of this information. One of the stack frames is "selected" by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in the program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in. When the program stops, GDB automatically selects the currently executing frame and describes it briefly as the `frame' command does (*note Info: Frame Info.). * Menu: * Frames:: Stack Frames * Backtrace:: Backtraces * Selection:: Selecting a Frame * Frame Info:: Information on a Frame File: gdb.info, Node: Frames, Next: Backtrace, Prev: Stack, Up: Stack Stack Frames ============ The call stack is divided up into contiguous pieces called "stack frames", or "frames" for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing. When your program is started, the stack has only one frame, that of the function `main'. This is called the "initial" frame or the "outermost" frame. Each time a function is called, a new frame is made. Each time a function returns, the frame for that function invocation is eliminated. If a function is recursive, there can be many frames for the same function. The frame for the function in which execution is actually occurring is called the "innermost" frame. This is the most recently created of all the stack frames that still exist. Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one of those bytes whose address serves as the address of the frame. Usually this address is kept in a register called the "frame pointer register" while execution is going on in that frame. GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands. Some compilers allow functions to be compiled so that they operate without stack frames. (For example, the `gcc' option `-fomit-frame-pointer' will generate functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB will nevertheless regard it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack. File: gdb.info, Node: Backtrace, Next: Selection, Prev: Frames, Up: Stack Backtraces ========== A backtrace is a summary of how the program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack. `backtrace' `bt' Print a backtrace of the entire stack: one line per frame for all frames in the stack. You can stop the backtrace at any time by typing the system interrupt character, normally `C-c'. `backtrace N' `bt N' Similar, but print only the innermost N frames. `backtrace -N' `bt -N' Similar, but print only the outermost N frames. The names `where' and `info stack' (abbreviated `info s') are additional aliases for `backtrace'. Each line in the backtrace shows the frame number and the function name. The program counter value is also shown--unless you use `set print address off'. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number. Here is an example of a backtrace. It was made with the command `bt 3', so it shows the innermost three frames. #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) at macro.c:71 (More stack frames follow...) The display for frame zero doesn't begin with a program counter value, indicating that the program has stopped at the beginning of the code for line `993' of `builtin.c'. 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 the 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 the program has multiple stacks and switches between them. _if_(1) On the SPARC architecture, `frame' needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer. _fi_(1) `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 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 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 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 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 built it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (*note 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 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, Prev: Source, 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 ten lines centered around line number LINENUM in the current source file. `list FUNCTION' Print ten lines centered around the beginning of function FUNCTION. `list' Print ten more lines. If the last lines printed were printed with a `list' command, this prints ten 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 Stack::.), this prints ten lines centered around that line. `list -' Print ten lines just before the lines last printed. 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 ten lines centered around the line specified by LINESPEC. `list FIRST,LAST' Print lines from FIRST to LAST. Both arguments are linespecs. `list ,LAST' Print ten lines ending with LAST. `list FIRST,' Print ten lines starting with FIRST. `list +' Print ten lines just after the lines last printed. `list -' Print ten 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 abbreviate the command name as `fo'. The synonym `search REGEXP' is also supported. `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'.