How do I print the full value of a long string in gdb?

Question

I want to print the full length of a C-string in GDB. By default it's being abbreviated, how do I force GDB to print the whole string?

Hooked

Answer 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.

Hooked

Nowadays you might also need "set print repeats 0", otherwise GDB will omit repeated elements of the string/array. – Bendicta

this also applies to array types too – Arterial

You may also need to "set max-value-size unlimited". – Scholasticate

Answer 2

As long as your program's in a sane state, you can also call (void)puts(your_string) to print it to stdout. Same principle applies to all functions available to the debugger, actually.

Argil

This answer is even better than "set print elements 0" (for my purposes) because it respects the newline/carriage return chars instead of escaping them. – Curassow

Good solution, but doesn't work when trying to analyze core dump file – Thiazine

Note: this option only works if you are debugging a live program. You can't use GDB's "call" command when you are debugging a core file. – Sapanwood

also requires gdb to be sane, which increasingly seems to NOT be the case (I get "No symbol "puts" in current context." on my Mac OS X machine) – Presentable

I simply wrtie "puts(your_string)" in the output window of xcode – Leviathan

Also doesn't work when you debugging a program which doesn't have puts() and problematic when puts() uses a UART and the UART of your target requires a running CPU. – Declass

Answer 3

The printf command will print the complete strings:

(gdb) printf "%s\n", string

Craigie

sorry but this is not true – Fighterbomber

This seems to respect the set print elements nnn limit, and will not print the complete string unless you do set print elements 0. – Superabundant

When I try this I only get: "Value can't be converted to integer." – Superable

for std::string you need string.c_str() in order to avoid the "Value can't be converted to integer" error – Pseudonym

Doesn't work if you are debugging a core dump. – Shied

Answer 4

There is a third option: the x command, which allows you to set a different limit for the specific command instead of changing a global setting. To print the first 300 characters of a string you can use x/300s your_string. The output might be a bit harder to read. For example printing a SQL query results in:

(gdb) x/300sb stmt.c_str()
0x9cd948:    "SELECT article.r"...
0x9cd958:    "owid FROM articl"...
..

Sewing

I was wondering what "x/300sb" meant. With the help of this cheat sheet (pdf), I've translated "x/300sb cstr" as "eXamine 300 units (Bytes) of memory at address cstr, interpreted as a NULL-terminated string (S).". If your string has length 100, then you will see lots of garbage, because all 300 bytes are printed, whether they make sense or not. +1 nevertheless for introducing me to x! – Bobker

Answer 5

Just to complete it:

(gdb) p (char[10]) *($ebx)
$87 =   "asdfasdfe\n"

You must give a length, but may change the representation of that string:

(gdb) p/x (char[10]) *($ebx)
$90 =   {0x61,
  0x73,
  0x64,
  0x66,
  0x61,
  0x73,
  0x64,
  0x66,
  0x65,
  0xa}

This may be useful if you want to debug by their values

Radiocarbon

Answer 6

Using set elements ... isn't always the best way. It would be useful if there were a distinct set string-elements ....

So, I use these functions in my .gdbinit:

define pstr
  ptype $arg0._M_dataplus._M_p
  printf "[%d] = %s\n", $arg0._M_string_length, $arg0._M_dataplus._M_p
end

define pcstr
  ptype $arg0
  printf "[%d] = %s\n", strlen($arg0), $arg0
end

Caveats:

• The first is c++ lib dependent as it accesses members of std::string, but is easily adjusted.
• The second can only be used on a running program as it calls strlen.

Psychotherapy