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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.
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
list function
list
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 -
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
list command display count source lines (unless
the list argument explicitly specifies some other number).
show listsize
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
list first,last
list ,last
list first,
list +
list -
list
Here are the ways of specifying a single source line--all the kinds of linespec.
number
list command has two linespecs, this refers to
the same source file as the first linespec.
+offset
list command that has
two, this specifies the line offset lines down from the
first linespec.
-offset
filename:number
function
filename:function
*address
There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
fo.
reverse-search regexp
rev.
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 ...
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
show directories
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:
directory with no argument to reset the source path to empty.
directory with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
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.
info line linespec
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
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.
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