Debugging with GDB - Examining Source Files

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# Examining Source Files

GDB can print parts of your program's source, since the debugging information recorded in the 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 (see section Selecting a frame), 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; see section Using GDB under GNU Emacs.

## Printing source lines

To print lines from a source file, use the `list` command (abbreviated `l`). By default, ten lines are printed. 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 (see section Examining the 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` prints.

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 that begins the body of the function function. For example: in C, this is the line with the open brace.
`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.

## 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 the 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`.

## 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 looks in the current directory as a last resort.

Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file.

When you start GDB, its source path is empty. To add other directories, use the `directory` command.

`directory dirname ...`
`dir 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 GDB searches it 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.

## Source and machine code

You can use the command `info line` to map source lines to program addresses (and vice versa), and the command `disassemble` to display a range of addresses as machine instructions. When run under GNU Emacs mode, the `info line` command now causes the arrow to point to the line specified. Also, `info line` prints addresses in symbolic form as well as hex.

`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 (see section Printing source lines).

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 (see section Examining memory). Also, this address is saved as the value of the convenience variable `$_` (see section Convenience variables).

`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; GDB dumps the function surrounding this value. Two arguments specify a range of addresses (first inclusive, second exclusive) to dump.

The following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code:

    (gdb) disas 0x32c4 0x32e4
    Dump of assembler code from 0x32c4 to 0x32e4:
    0x32c4 <main+204>:      addil 0,dp
    0x32c8 <main+208>:      ldw 0x22c(sr0,r1),r26
    0x32cc <main+212>:      ldil 0x3000,r31
    0x32d0 <main+216>:      ble 0x3f8(sr4,r31)
    0x32d4 <main+220>:      ldo 0(r31),rp
    0x32d8 <main+224>:      addil -0x800,dp
    0x32dc <main+228>:      ldo 0x588(r1),r26
    0x32e0 <main+232>:      ldil 0x3000,r31
    End of assembler dump.

Some architectures have more than one commonly-used set of instruction mnemonics or other syntax.

`set assembly-language instruction-set`
  Select the instruction set to use when disassembling the program via the `disassemble` or `x/i` commands. Currently this command is only defined for the Intel x86 family. You can set instruction-set to either `i386` or `i8086`. The default is `i386`.

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