Note "input output"

Output

You can output data from your C-program by writing to standard streams—stdout and stderr—or by writing to file streams.

Standard streams

Streams are objects of the type FILE* (a pointer to a file) which are connected to devices where you can send data. For every C-program there are always two output streams open: the standard output stream, stdout, and the standard error stream, stderr. By default both of these streams are connected to the terminal from which the program is run. Anything you send to these streams are by default shown on the terminal (however, these streams can be easily redirected by your shell to other destinations, see "note-redirect").

You send data to stdout stream with the printf function,

double x = 1.23;
printf("%g\n",x);

or, equivalently, with the fprintf function,

fprintf(stdout,"%g\n",x);

You send data to stderr stream with the fprintf function, for example,

fprintf(stderr,"%g\n",x);

File streams

Usually the two standard streams are enough for a program to output its data via redirection. However sometimes you need to wrie specifically to a file with a certain name. You do this by creating a stream connected with the given file,

FILE* mystream = fopen("out.txt","w");

where "w" means "writing" (read man fopen for details).

Now you can send data directly into file out.txt by writing into stream mystream, for example,

fprintf(mystream,"%g\n",x);

When you are done writing, you close the stream,

fclose(mystream);

Input

You can input data into your C-program from its command-line arguments; by reading from standard input; or by reading from file streams.

Standard input stream and file streams

Every C-program has the standard input stream, stdin, autmatically opened and connected to the terminal from which the program is run (but can be conveniently redirected). Apart from this you can also open your own file streams for reading, for example,

FILE* mystream = fopen("input.txt","r");

You can read from the streams using the scanf function, which reads from stdin, and fscanf function which reads from the specified stream, for example,

double x;
int items;
items = scanf("%g",&x);
items = fscanf(stdin,"%g",&x);
items = fscanf(mystream,"%g",&x);

Command-line arguments

A small amount of data can be passed to a C-program via command-line arguments by declaring the main function as

int main(int argc, char** argv){}

where argc, argument count, is the number of command-line arguments (where the name of the program is also counted), and argv is the array of strings of the size argc that holds the command-line arguments (where the first element, argv[0] is the name of the program and argv[1] is the first actual argument).

When the program is run the operating system calls the main function and provides its arguments as the array of blank-separated strings taken from the command that ran the program.

You can convert a string representation of a double number into the very double number using the function atof from stdlib.h, "ascii to double", for example,

int main(int argc, char** argv){
	if(argc<2) printf("there was no argument\n");
	else {
		double x = atof(argv[1]);
		printf("x = %g\n",x);
	}
return 0;
}