When I write a C program and use system command

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http://www.linuxquestions.org – When I write a C program, and use the system command in the middle of the program to take advantage of one of UNIX commands, does the UNIX command become a part of the binary C program, or is the C program going to call the UNIX command whenever necessary? the reason I am asking this is, because I might run the C program under different UNIX systems~ Thanks, experts @ Linux Questions~ (HowTos)