A C string (char *) is an array of char, terminated with a #0 char.
C library functions require C, not Pascal style string arguments. However, Pascal style strings are automatically converted to C style strings when passed to a routine that expects C style strings. This works only if the routine reads from the string, not if it modifies it.
E.g., this is how you could access the system() call in your C library (which is not necessary anymore, since Execute is already built-in):
program SysCall; function System (CmdLine: CString): Integer; external name 'system'; var Result: Integer; begin Result := System ('ls -l'); WriteLn ('system() call returned: ', Result) end.
You could use the type PChar instead of CString. Both CString and PChar are predefined as ^Char – though we recommend CString because it makes it clearer that we're talking about some kind of string rather than a single character.
A lot of library routines in Pascal for many applications exist in the GPC unit and some other units. Where available, they should be preferred (e.g. Execute rather than system(), and then you won't have to worry about CStrings.)
Do not pass a C style string as a const or var argument if the C prototype says const char * or you will probably get a segfault.