Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Back: C Endianness
Forward: C Floating Point
FastBack: C Floating Point
Up: C Language Portability
FastForward: Cross-Unix Portability
Top: Autoconf, Automake, and Libtool
Contents: Table of Contents
Index: Index
About: About this document

15.1.4 C Structure Layout

C compilers on different systems lay out structures differently. In some cases there can even be layout differences between different C compilers on the same system. Compilers add gaps between fields, and these gaps have different sizes and are at different locations. You can normally assume that there are no gaps between fields of type char or array of char. However, you can not make any assumptions about gaps between fields of any larger type. You also can not make any assumptions about the layout of bitfield types.

These structure layout issues mean that it is difficult to portably use a C struct to define the format of data which may be read on another type of system, such as data in a file or sent over a network connection. Portable code must read and write such data field by field, rather than trying to read an entire struct at once.

Here is an example of non-portable code when reading data which may have been written to a file or a network connection on another type of system. Don't do this.
  /* Example of non-portable code; don't do this */
  struct {
    short i;
    int j;
  } s;
  read (fd, &s, sizeof s);

Instead, do something like this (the struct s is assumed to be the same as above):
  unsigned char buf[6];
  read (fd, buf, sizeof buf); /* Should check return value */
  s.i = buf[0] | (buf[1] << 8);
  s.j = buf[2] | (buf[3] << 8) | (buf[4] << 16) | (buf[5] << 24);
Naturally the code to write out the structure should be similar.

This document was generated by Gary V. Vaughan on February, 8 2006 using texi2html

  Published under the terms of the Open Publication License Design by Interspire