gdb.info: Patching

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Patching programs

By default, GDB opens the file containing your program's executable
code (or the corefile) read-only.  This prevents accidental alterations
to machine code; but it also prevents you from intentionally patching
your program's binary.
   If you'd like to be able to patch the binary, you can specify that
explicitly with the `set write' command.  For example, you might want
to turn on internal debugging flags, or even to make emergency repairs.
`set write on'
`set write off'
     If you specify `set write on', GDB opens executable and core files
     for both reading and writing; if you specify `set write off' (the
     default), GDB opens them read-only.
     If you have already loaded a file, you must load it again (using
     the `exec-file' or `core-file' command) after changing `set
     write', for your new setting to take effect.
`show write'
     Display whether executable files and core files are opened for
     writing as well as reading.