Can't execlp netcat

Question

I'm trying to exec the following command from C:

netcat 127.0.0.1 4444

This is my code:

#include <unistd.h>

int main() {
    execlp("/usr/bin/netcat", "127.0.0.1", "4444", 0);
}

However, I keep getting the error: Error: no ports specified for connection .

To answer a few questions I feel may come up:

• which netcat gives /usr/bin/netcat
• I've tried using execvp and got the same results
• Running the command normally, ie via a terminal, is successful (Usually i also give the -e /bin/bash argument but I've omitted it for simplicity. Adding it doesn't change the results.)

Answer 1

The problem is, that your first argument is "4444". Why?

Look at this line:

execlp("/usr/bin/netcat", "127.0.0.1", "4444", 0);

The first argument to execlp() is the binary to execute. The rest of the arguments will form the argv[] -vector passed to the process.

By convention, argv[0] should contain the name of the executable and the first "real" argument is argv[1] , which is "4444" in your case. So what you do is the equivalent on the shell of

netcat 4444

and the correct call would be:

execlp("/usr/bin/netcat", "netcat", "127.0.0.1", "4444", 0);

Btw. the use of execlp() makes only sense when calling a binary without a full path, since it looks for the correct path itself (using the PATH environment variable). Otherwise, execl() should be used. But it will work anyway.