How to set a conditional breakpoint in gdb, when char* x points to a string whose value equals “hello”

watch only breaks on write, rwatch let you break on read, and awatch let you break on read/write.

You can set read watchpoints on memory locations:

gdb$ rwatch *0xfeedface
Hardware read watchpoint 2: *0xfeedface

but one limitation applies to the rwatch and awatch commands; you can't use gdb variables in expressions:

gdb$ rwatch $ebx+0xec1a04f
Expression cannot be implemented with read/access watchpoint.

So you have to expand them yourself:

gdb$ print $ebx 
$13 = 0x135700
gdb$ rwatch *0x135700+0xec1a04f
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
gdb$ c
Hardware read watchpoint 3: *0x135700 + 0xec1a04f

Value = 0xec34daf
0x9527d6e7 in objc_msgSend ()

Edit: Oh, and by the way. You need either hardware or software support. Software is obviously much slower. To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpoints environment setting.

gdb$ show can-use-hw-watchpoints
Debugger's willingness to use watchpoint hardware is 1.

set print elements 0

From the GDB manual:

set print elements number-of-elements

Set a limit on how many elements of an array GDB will print. If GDB is printing a large array, it stops printing after it has printed the number of elements set by the set print elements command. This limit also applies to the display of strings. When GDB starts, this limit is set to 200. Setting number-of-elements to zero means that the printing is unlimited.