Print character array to file in GDB

I think you want something like this:

(gdb) dump binary memory ~/file.bin 0x100390f4c (0x100390f4c + 940)

The dump command is a little awkward to use. It takes a start an an end address, and expressions that indicate what to dump (you can use value instead of memory to specify an expression, if that works for you, but sometimes I'd rather be specific.) But it appears (as I've tested above) that you can use expressions anyway, as I've specified the end address above in terms of the start address I wanted to dump, plus the amount of bytes I wanted.

You can also do something like this (pass in the expression that results in the pointer value rather than the value of the pointer itself):

(gdb) dump binary memory ~/file.bin buf (buf + len)

For more information, see the documentation here.

To print unlimited characters to the console use

set print elements 0

(gdb) help x
Examine memory: x/FMT ADDRESS.
ADDRESS is an expression for the memory address to examine.
FMT is a repeat count followed by a format letter and a size letter.
Format letters are o(octal), x(hex), d(decimal), u(unsigned decimal),
  t(binary), f(float), a(address), i(instruction), c(char) and s(string).
Size letters are b(byte), h(halfword), w(word), g(giant, 8 bytes).
The specified number of objects of the specified size are printed
according to the format.

Defaults for format and size letters are those previously used.
Default count is 1.  Default address is following last thing printed
with this command or "print".
(gdb) x/8b array
0xbffd7670:     0       0       0       0       0       0       0       0
(gdb) x/16b array
0xbffd7670:     0       0       0       0       0       0       0       0
0xbffd7678:     0       0       0       0       0       0       0       0
(gdb) x/128b array
0xbffd7670:     0       0       0       0       0       0       0       0
0xbffd7678:     0       0       0       0       0       0       0       0
0xbffd7680:     0       0       0       0       0       0       0       0
0xbffd7688:     0       0       0       0       0       0       0       0
0xbffd7690:     0       0       0       0       0       0       0       0
0xbffd7698:     0       0       0       0       0       0       0       0
0xbffd76a0:     0       0       0       0       0       0       0       0
0xbffd76a8:     0       0       0       0       0       0       0       0
0xbffd76b0:     0       0       0       0       0       0       0       0
0xbffd76b8:     0       0       0       0       0       0       0       0
0xbffd76c0:     0       0       0       0       0       0       0       0
0xbffd76c8:     0       0       0       0       0       0       0       0
0xbffd76d0:     0       0       0       0       0       0       0       0
0xbffd76d8:     0       0       0       0       0       0       0       0
0xbffd76e0:     0       0       0       0       0       0       0       0
0xbffd76e8:     0       0       0       0       0       0       0       0
(gdb)

If you want the ASCII character symbols printed as well, use x/<size>c.

(gdb) set logging file ~/gdb_dump.txt
(gdb) set logging on
Copying output to /home/mminich/gdb_dump.txt.
(gdb) x/26c array
0xbfff4b20:     97 'a'  98 'b'  99 'c'  100 'd' 101 'e' 102 'f' 103 'g' 104 'h'
0xbfff4b28:     105 'i' 106 'j' 107 'k' 108 'l' 109 'm' 110 'n' 111 'o' 112 'p'
0xbfff4b30:     113 'q' 114 'r' 115 's' 116 't' 117 'u' 118 'v' 119 'w' 120 'x'
0xbfff4b38:     121 'y' 122 'z'
(gdb) set logging off
Done logging to /home/mminich/gdb_dump.txt.
(gdb)

BTW, I agree wholeheartedly with William Pursell's comment under your question: "I find it much easier to define functions in the code that perform the data dump, and just call them from gdb. (Wrap the definitions in #ifdef DEBUG)"

I personally use embedded python for dumping data.For example:

(gdb) pi open("output_data.log","w").write(gdb.execute("print myarray@100000",to_string=True))