Note "streams"

Streams

The C language uses streams—sequences of bytes—for all file operations. The operating system connects these streams with physical devices such as keyborads, terminals, files and others. Streams in the C language are objects of the type FILE* (pointer to file).

Standard streams

For a C-program three standard streams are always available: two output streams, the standard output stream, stdout, and the standard error stream, stderr; and one input stream, the standard input stream stdin. By default the standard output and standard error streams are connected to the terminal from which the program is run, while the standard input stream is connected to the terminal's keyboard. (However, these streams can be easily redirected by the operating system to other desinations.)

File streams

The standard streams are often enough for a program to move its data around (using redirections). However sometimes you need to write/read specifically to/from a file with a certain name. You do this by creating streams explicitly connected with the given files, for example,

FILE* my_in_stream = fopen("input.txt","r");
FILE* my_out_stream = fopen("out.txt","w");

where "r" means "reading" and "w" means "writing" (read man fopen for details and other modes of stream operations). When you are done with your file operations, you should not forget to close your file streams,

fclose(my_in_stream);
fclose(my_out_stream);

otherwise bad things might happen.

Output

You can send your data to stdout stream with the printf function,

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

You can send your data to all other streams with the fprintf function,

fprintf(stdout,"x=%g\n",x); // to stdout, same as printf
fprintf(stderr,"x=%g\n",x); // to stderr
fprintf(my_out_stream,"x=%g\n",x); // to my_out_stream

Input

You can input data into your C-program by

reading from standard input stream, stdin;
reading from file streams;
reading command-line arguments.

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

int items;
double x;
items = scanf("%g",&x); // from stdin into "x"
items = fscanf(stdin,"%g",&x); // also from stdin
items = fscanf(my_in_stream,"%g",&x); // from my_in_stream