lang/C/the.ansi.c.programming.language/c.programming.notes/sx12b.html

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   9 <title>12.2 I/O with File Pointers</title>
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  15 <H2>12.2 I/O with File Pointers</H2>
  16
  17 <p>For each of the I/O library functions we've been using so far,
  18 there's a companion function
  19 which accepts an additional file pointer argument
  20 telling it where to read from or write to.
  21 The companion function to <TT>printf</TT> is <TT>fprintf</TT>,
  22 and the file pointer argument comes first.
  23 To print a string to the <TT>output.dat</TT> file
  24 we opened in the previous section,
  25 we might call
  26 <pre>
  27         fprintf(ofp, "Hello, world!\n");
  28 </pre>
  29 </p><p>The companion function to <TT>getchar</TT> is <TT>getc</TT>,
  30 and the file pointer is its only argument.
  31 To read a character from the <TT>input.dat</TT> file
  32 we opened in the previous section,
  33 we might call
  34 <pre>
  35         int c;
  36         c = getc(ifp);
  37 </pre>
  38 </p><p>The companion function to <TT>putchar</TT> is <TT>putc</TT>,
  39 and the file pointer argument comes last.
  40 To write a character to <TT>output.dat</TT>,
  41 we could call
  42 <pre>
  43         putc(c, ofp);
  44 </pre>
  45 </p><p>Our own <TT>getline</TT> function calls <TT>getchar</TT>
  46 and so always reads the standard input.
  47 We could write a companion <TT>fgetline</TT> function
  48 which reads from an arbitrary file pointer:
  49 <pre>
  50 #include &lt;stdio.h&gt;
  51
  52 /* Read one line from fp, */
  53 /* copying it to line array (but no more than max chars). */
  54 /* Does not place terminating \n in line array. */
  55 /* Returns line length, or 0 for empty line, or EOF for end-of-file. */
  56
  57 int fgetline(FILE *fp, char line[], int max)
  58 {
  59 int nch = 0;
  60 int c;
  61 max = max - 1;                  /* leave room for '\0' */
  62
  63 while((c = getc(fp)) != EOF)
  64         {
  65         if(c == '\n')
  66                 break;
  67
  68         if(nch &lt; max)
  69                 {
  70                 line[nch] = c;
  71                 nch = nch + 1;
  72                 }
  73         }
  74
  75 if(c == EOF &amp;&amp; nch == 0)
  76         return EOF;
  77
  78 line[nch] = '\0';
  79 return nch;
  80 }
  81 </pre>
  82 </p><p>Now we could read one line from <TT>ifp</TT> by calling
  83 <pre>
  84         char line[MAXLINE];
  85         ...
  86         fgetline(ifp, line, MAXLINE);
  87 </pre>
  88
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