good day to all of you,
Im trying to do a C program that gets information from another program (TCPDUMP) and what i`m trying to do is run tcpdump from the terminal and get the information that I need from the output of tcpdump, and use it in my C program. My question is:
In a C program I can run a command from the shell, and use the output of that command in the C program?
Or in a Shell script I can call a C program? and pass the output of some functions in the script to the C program?
If there is a another way to do what I explained above,tell me, please.
Thanks for your time.

You could pipe the output of tcpdump into your C program, and then have the C program read stdin.

You could also have tcpdump capture to a file (see -w) and then just read the file back with your C program.

You could also just use libpcap directly in your C program to monitor the stream directly, but without more info on your purpose, that's probably "reinventing the wheel" ...

you could also do both. having your c program read from stdin is extremely handy for using it in a pipe line. you could have your c program read from stdin if there are no command line arguments, and otherwise use the filename passed to it throught the command line as input.

thanks for the advice.
If i decide to read the file, saved with the option -w of tcpdump, in the C program, how can read and save only some fields of the file, for expample grab only the src/dst port and the bytes sent?
because i only need these fields to work with them.

I'd use popen().

Have a look at libpcap.