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7.5 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 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:

 
(No value for GDBP) info line m4_changequote
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:

 
(No value for GDBP) 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; No value for GDBN 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:

 
(No value for GDBP) 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.

For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location—it's actually a location of the relocation table. On some architectures, No value for GDBN might be able to resolve these to actual function names.

set disassembly-flavor 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 intel or att. The default is att, the AT&T flavor used by default by Unix assemblers for x86-based targets.

show disassembly-flavor

Show the current setting of the disassembly flavor.


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