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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. Specify source lines in any of the ways understood by the list command (see Printing source lines).

For instance, we can use info line to discover the location of the object code for the first line of function, m4_changequote, as in the following example.

(gdb) info line m4_changecom
Line 895 of “builtin.c” starts at pc 0x634c and ends at 0x6350.

We can also inquire (using *addras, the form for linespec) what source line covers a particular address, as in the following example.

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

We can use disassemble to inspect the object code range shown in the last info line example (the example shows SPARCmachine 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>: 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.