12.2 I/O with File Pointers

For each of the I/O library functions we've been using so far, there's a companion function which accepts an additional file pointer argument telling it where to read from or write to. The companion function to printf is fprintf, and the file pointer argument comes first. To print a string to the output.dat file we opened in the previous section, we might call

	fprintf(ofp, "Hello, world!\n");

The companion function to getchar is getc, and the file pointer is its only argument. To read a character from the input.dat file we opened in the previous section, we might call

	int c;
	c = getc(ifp);

The companion function to putchar is putc, and the file pointer argument comes last. To write a character to output.dat, we could call

	putc(c, ofp);

Our own getline function calls getchar and so always reads the standard input. We could write a companion fgetline function which reads from an arbitrary file pointer:

#include <stdio.h>

/* Read one line from fp, */
/* copying it to line array (but no more than max chars). */
/* Does not place terminating \n in line array. */
/* Returns line length, or 0 for empty line, or EOF for end-of-file. */

int fgetline(FILE *fp, char line[], int max)
{
int nch = 0;
int c;
max = max - 1;			/* leave room for '\0' */

while((c = getc(fp)) != EOF)
	{
	if(c == '\n')
		break;

	if(nch < max)
		{
		line[nch] = c;
		nch = nch + 1;
		}
	}

if(c == EOF && nch == 0)
	return EOF;

line[nch] = '\0';
return nch;
}

Now we could read one line from ifp by calling

	char line[MAXLINE];
	...
	fgetline(ifp, line, MAXLINE);

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