Advanced Programming in the UNIX® Environment

ptg10805159

156 StandardI/O Library Chapter 5

library is not much slower than calling thereadandwritefunctions directly.For

most nontrivial applications, the largest amount of user CPU time is taken by the

application, not by the standardI/O routines.

5.9 BinaryI/O

The functions from Section 5.6 operated with one character at a time, and the functions

from Section 5.7 operated with one line at a time. If we’redoing binary I/O, we often

would like to read or write an entirestructure at a time. To do this usinggetcorputc,

we have to loop through the entirestructure, one byte at a time, reading or writing each

byte. Wecan’t use the line-at-a-time functions, sincefputsstops writing when it hits a

null byte, and theremight be null bytes within the structure. Similarly,fgetswon’t

work correctly on input if any of the data bytes arenulls or newlines. Therefore, the

following two functions areprovided for binary I/O.

#include <stdio.h>

size_t fread(void *restrictptr,size_tsize,size_tnobj,

FILE *restrictfp);

size_t fwrite(const void *restrictptr,size_tsize,size_tnobj,

FILE *restrictfp);

Both return: number of objects read or written

These functions have two common uses:

1. Read or write a binary array.For example, to write elements 2 through 5 of a
   floating-point array, we could write
   float data[10];

if (fwrite(&data[2], sizeof(float), 4, fp) != 4)

err_sys("fwrite error");

Here, we specifysizeas the size of each element of the array andnobjas the

number of elements.

2. Read or write a structure. For example, we could write
   struct {
   short count;
   long total;
   char name[NAMESIZE];
   }item;

if (fwrite(&item, sizeof(item), 1, fp) != 1)

err_sys("fwrite error");

Here, we specifysizeas the size of structureandnobjas 1 (the number of objects

to write).