How can I put a breakpoint on “something is printed to the terminal” in gdb

breakpointsconditional-breakpointgdbprintf

I would like to know from where inside a huge application a certain message is printed. The application is so big and old that it uses all conceivable ways of printing text to the terminal; for example printf(), fprintf(stdout, …) etc.

I write to put a breakpoint on the write() system call but then I'm flooded with too many breakpoint stops because of various file I/O operations that use write() as well.

So basically I want gdb to stop whenever the program prints something to the terminal but at the same time I don't want gdb to stop when the program writes something to a file.

Best Answer

Use a conditional breakpoint that checks the first parameter. On 64-bit x86 systems the condition would be:

(gdb) b write if 1==$rdi

On 32-bit systems, it is more complex because the parameter is on the stack, meaning that you need to cast $esp to an int * and index the fd parameter. The stack at that point has the return address, the length, buffer and finally fd.

This varies greatly between hardware platforms.

Related Solutions

C++ – set a breakpoint on ‘memory access’ in GDB

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.

R – How to use fprintf and write to a pipe

If you have a file descriptor and want a FILE*, you can use fdopen

FILE *fdopen(int fd, const char *mode);

fdopen is a Posix function and documented in man fdopen. To do the reverse you can use fileno

Best Answer

Related Solutions

C++ – set a breakpoint on ‘memory access’ in GDB

R – How to use fprintf and write to a pipe

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