Until the early 1980s, large software development projects had a continual problem with the inclusion of headers. One group might have produced a graphics.h, for example, which started by including <stdioio.h>h. Another group might have produced keyboard.h, which also included <stdioio.h>h. If <stdioio.h>h could not safely be included several times, arguments would break out about which header should include it. Sometimes an agreement was reached that each header should include no other headers, and therefore some application programs started with dozens of #include lines, and sometimes they got the ordering wrong or forgot a header that was needed.
...
Consequently, the first time that header.h is #include'd, all of its contents are included. If the header file is subsequently #include'd again, its contents are bypassed.
| Wiki Markup |
|---|
Because solutions such as this one make it possible to create a header file that can be included more than once, the C standard \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] guarantees that the standard headers are safe for multiple inclusion. |
Risk Assessment
Failure to include header files in an inclusion sandwich can result in unexpected behavior.
...
| Wiki Markup |
|---|
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment," and Section 7.1.2, "Standard headers" \[[Plum 85|AA. C References#Plum 85]\] Rule 1-14 |
...