<begin quote>
7.20.4.6 The system function

  Synopsis

1     #include <stdlib.h>
      int system(const char *string);

  Description

2 If 'string' is a null pointer, the system function determines
  whether the host environment has a command processor. If 'string'
  is not a null pointer, the 'system' function passes the string
  pointed to by 'string' to that command processor to be executed
  in a manner which the implementation shall document; this might
  then cause the program calling 'system' to behave in a non-conforming
  manner or to terminate.

  Returns

3 If the argument is a null pointer, the 'system' function returns
  nonzero only if a command processor is available. If the argument
  is not a null pointer, and the 'system' function does return, it
  returns an implementation-defined value.
<end quote>

IOW you'll need to consult the documentation for the
C implementation you're using.