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In this section, we saw examples only of the linkage specification for the C language; extern "C" extern "C" is the only linkage specification guaranteed to be supported by all C++ implementations An implementation can provide other linkage specifications for languages commonly used in its environment For example extern "Ada" can be used to declare functions written in the Ada language, extern "FORTRAN" for functions written in the FORTRAN language, and so on Because the additional linkage specifications are implementation-specific, we recommend that you consult your implementation's user's guide for further information on the other linkage specifications it may provide This section introduces a first use of the keyword extern in C++ In Section 82, we will see other uses of extern with declarations of objects and functions Exercise 714 exit(), printf(), malloc(), strcpy(), and strlen() are C language library routines Modify the following C program so that it compiles and links under C++
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const char *str = "hello"; void *malloc( int ); char *strcpy( char *, const char * ); int printf( const char *, ); int exit( int ); int strlen( const char * ); int main() { /* C language program */ char* s = malloc( strlen(str)+1 ); strcpy( s, str ); printf( "%s, world\n", s ); exit( 0 ); }
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main(): Handling Command Line Options
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Often, when we execute our programs we pass command line options For example, we might write
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prog -d -o ofile data0
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In effect, the command line options are arguments to main() and can be accessed within main() through an array of C-style character strings named argv In this section, we illustrate how to support command line options Prior to this section, all our definitions of main() have declared an empty parameter list:
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int main() { }
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An expanded signature for main() allows us access to the options, if any, specified by the user on the command line:
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int main( int argc, char *argv[] ) { }
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argc holds a count of the command line options argv holds the argc count of C-style character strings representing the white space separated command options For example, given the command line
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prog -d -o ofile data0
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argc is set to 5, and argv is set to the following C-style character strings:
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argv[ argv[ argv[ argv[ argv[
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"prog"; "-d"; "-o"; "ofile"; "data0";
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argv[0] is always set to the command being invoked The indexes 1 through argc-1 represent the actual options passed to the command Let's look at how we might extract and evaluate the command line options stored in argv In our example, we'll support the following usage:
program_name [-d] [-h] [-v] [-o output_file] [-l limit_value] file_name [ file_name [file_name [ ]]]
Anything in brackets is optional Thus, for example, a minimal command line simply indicates a file to process:
prog chap1doc
Alternative possible invocations include the following:
prog -l 1024 -o chap1-2out chap1doc chap2doc prog -d chap3doc prog -l 512 -d chap4doc
The basic steps in handling the command line options are the following:
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1 Extract each option in turn from argv We'll use a for loop to do this, beginning our iteration at index 1 (thereby skipping the program name):
for ( int ix = 1; ix < argc; ++ix ) { char *pchar = argv[ ix ]; // }
2 Determine the type of option If it begins with a hyphen, then we know it is one of {h, d, v, l, o } Otherwise, it is either the actual limit value associated with -l, an output file name associated with -o, or the name of a file for the program to process We'll use a switch statement to determine whether a hyphen is present:
switch ( pchar[ 0 ] ) { case '-': { // recognize -h, -d, -v, -l, -o } default: { // handle limit value following -l // output file following -o // file names } }
3 Fill in the handling of the two cases of item 2 If a hyphen is present, we then simply switch on the next character to determine the option specified by the user Here is the general outline of that portion of our implementation:
case '-': { switch( pchar[ 1 ] ) { case 'd': // handle debug break; case 'v': // handle version request break; case 'h': // handle help break; case 'o': // prepare to handle output_file break; case 'l': // prepare to handle limit_value break; default: // unrecognized option:
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