{"id":1942,"date":"2025-01-26T20:26:52","date_gmt":"2025-01-26T20:26:52","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=1942"},"modified":"2025-01-27T18:32:22","modified_gmt":"2025-01-27T18:32:22","slug":"a-c-not-c-exception-mechanism","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/exceptions-2\/a-c-not-c-exception-mechanism\/","title":{"rendered":"A C (not C++) Exception Mechanism"},"content":{"rendered":"\n

<\/span>A C (not C++) Exception Mechanism<\/span><\/h2>\n\n\n\n

While C does not have exceptions built-in, it is possible to add a reasonably efficient and portable (albeit somewhat crude) exception mechanism using macros and the setjmp<\/code><\/strong>\/longjmp<\/code><\/strong> C Run-Time-Library functions (see here<\/a>).<\/p>\n\n\n\n

<\/span>The Roberts Proposal<\/span><\/h2>\n\n\n\n
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A very useful C exception mechanism was proposed by Eric S. Roberts in 1989:
Implementing Exceptions in C<\/em>, by Eric S. Roberts<\/a>, Digital Systems Research Center Report #40, 1989<\/em><\/p>\n<\/blockquote>\n\n\n\n

Roberts’ approach allows the following (example in C<\/em>):<\/p>\n\n\n\n

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#include<\/span> <\/span>"exception.h"<\/span><\/span>\n  ...<\/span><\/span>\nexception OpenError;<\/span> \/\/ exception is a type<\/span><\/span>\n  ...<\/span><\/span>\nTRY<\/span>                 \/\/ TRY is a C macro<\/span><\/span>\n  ...<\/span><\/span>\n  file = <\/span>OpenFile<\/span>(<\/span>"test.dat"<\/span>, <\/span>"r"<\/span>);<\/span><\/span>\n  ...<\/span><\/span>\nEXCEPT<\/span>(OpenError)<\/span>  \/\/ EXCEPT and ENDTRY are C macros<\/span><\/span>\n  <\/span>fprintf<\/span>(stderr, <\/span>"Cannot open file 'test.dat'<\/span>\\n<\/span>"<\/span>);<\/span><\/span>\nENDTRY<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
<\/div>\n<\/div>\n\n\n\n
The general form of a TRY<\/code>–ENDTRY<\/code><\/strong> block is:<\/p>\n\n\n\n
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TRY<\/span><\/span>\n  statements<\/span><\/span>\nEXCEPT<\/span>(exception-name-<\/span>1<\/span>)<\/span><\/span>\n  statements<\/span><\/span>\nEXCEPT<\/span>(exception-name-<\/span>2<\/span>)<\/span><\/span>\n  statements<\/span><\/span>\n...<\/span><\/span>\nENDTRY<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
<\/div>\n<\/div>\n\n\n\n
An exception is raised by:<\/p>\n\n\n\n
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RAISE<\/span>(exception-name, value);<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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where the value<\/code><\/strong> is an int<\/code><\/strong> to be communicated back to the handler, to provide additional information about the exception.<\/p>\n<\/div>\n<\/div>\n\n\n\n
<\/span>The TRY-FINALLY Block, and Resource Acquisition<\/span><\/h3>\n\n\n\n
This approach also included the option of a finally block<\/strong><\/em> (again, in C<\/em>):<\/p>\n\n\n\n
Roberts also specified a TRY<\/code>–FINALLY<\/code><\/strong> block, which is of the form:<\/p>\n\n\n\n
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TRY<\/span><\/span>\n  statements<\/span><\/span>\nFINALLY<\/span><\/span>\n  statements<\/span><\/span>\nENDTRY<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
<\/div>\n<\/div>\n\n\n\n
The major point of a FINALLY<\/strong><\/code> block is to allow for convenient cleanup in all cases:<\/p>\n\n\n\n
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#include<\/span> <\/span>"exception.h"<\/span><\/span>\n  ...<\/span><\/span>\nAcquire<\/span>(Resource);<\/span><\/span>\nTRY<\/span><\/span>\n  \/* code for accessing the resource *\/<\/span><\/span>\nFINALLY<\/span> \/\/ FINALLY is a C macro<\/span><\/span>\n  \/* this is executed whether or not an exception occurs *\/<\/span><\/span>\n  <\/span>Release<\/span>(Resource);<\/span><\/span>\nENDTRY<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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This allows for acquired resources to be released, whether the access is successful or not<\/p>\n<\/div>\n<\/div>\n\n\n\n
<\/span>setjmp<\/strong>, longjmp<\/strong> and jmp_buf<\/strong><\/span><\/h3>\n\n\n\n
Roberts’ exception handling mechanism made use of the setjmp<\/strong><\/code> and longjmp<\/strong><\/code> functions, which use a buffer called jmp_buf<\/strong><\/code>.<\/p>\n\n\n\n
<\/span>Function setjmp<\/code><\/strong><\/span><\/h4>\n\n\n\n
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#include<\/span> <\/span><setjmp.h><\/span><\/span>\n...<\/span><\/span>\nint<\/span> <\/span>setjmp<\/span>(jmp_buf <\/span>env<\/span>);<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
<\/div>\n<\/div>\n\n\n\n
saves the current ‘environment’ in its jmp_buf<\/code><\/strong> argument, and returns 0<\/code><\/strong>.<\/p>\n\n\n\n
<\/span>Function longjmp<\/code><\/strong><\/span><\/h4>\n\n\n\n
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#include<\/span> <\/span><setjmp.h><\/span><\/span>\n...<\/span><\/span>\nvoid<\/span> <\/span>longjmp<\/span>(jmp_buf <\/span>env<\/span>, <\/span>int<\/span> <\/span>val<\/span>);<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
<\/div>\n<\/div>\n\n\n\n
restores the environment saved by setjmp<\/code><\/strong>, which causes a return from the setjmp<\/code><\/strong> call with a value of val<\/code><\/strong>.  longjmp<\/code><\/strong> never itself returns!<\/p>\n\n\n\n
<\/span>jmp_buf<\/span><\/h4>\n\n\n\n
Type jmp_buf<\/code><\/strong> is a system-specific type that is of sufficient size to save the ‘environment’ (such as machine registers, etc.) so that the machine state may be saved, and later restored.  (It must be an array type, so that a pointer to it is always passed to a function.)<\/p>\n\n\n\n
<\/span>An Example:<\/span><\/h3>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  setjmpTest.c<\/span><\/span>\n\/\/  setjmp & longjmp<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 9\/23\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><stdio.h><\/span><\/span>\n#include<\/span> <\/span><setjmp.h><\/span><\/span>\n<\/span>\njmp_buf main_env;<\/span><\/span>\n<\/span>\nint<\/span> <\/span>InputNumber<\/span>(<\/span>void<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>int<\/span> n, ret;<\/span><\/span>\n  <\/span>fflush<\/span>(stdin);<\/span>  \/\/ ensure there's nothing left<\/span><\/span>\n                  <\/span>\/\/ from the last scanf<\/span><\/span>\n  ret = <\/span>scanf<\/span>(<\/span>"<\/span>%d<\/span>"<\/span>, &n);<\/span><\/span>\n                  <\/span>\/\/ read an integer from stdin<\/span><\/span>\n  <\/span>if<\/span> (ret <= <\/span>0<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>\/\/ something went wrong during the read...<\/span><\/span>\n    <\/span>printf<\/span>(<\/span>"Return value from scanf = <\/span>%d<\/span> "<\/span>, ret);<\/span><\/span>\n    <\/span>longjmp<\/span>(main_env, <\/span>1<\/span>);<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>return<\/span> n;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>int<\/span> n;<\/span><\/span>\n  <\/span>if<\/span> (<\/span>setjmp<\/span>(main_env) != <\/span>0<\/span>)<\/span><\/span>\n    <\/span>printf<\/span>(<\/span>"<\/span>\\n<\/span>Invalid, try again...<\/span>\\n<\/span>"<\/span>);<\/span><\/span>\n  <\/span>printf<\/span>(<\/span>"Enter a valid number:"<\/span>);<\/span><\/span>\n  n = <\/span>InputNumber<\/span>();<\/span><\/span>\n  <\/span>printf<\/span>(<\/span>"You entered the number: <\/span>%d<\/span>\\n<\/span>"<\/span>, n);<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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Here’s the user’s (my) interaction with the program:<\/p>\n\n\n\n
Enter a valid number:x<\/kbd>\nReturn value from scanf = 0 <\/kbd>\nInvalid, try again...\nEnter a valid number:52<\/kbd>\nYou entered the number: 52\nProgram ended with exit code: 0<\/code><\/pre>\n\n\n\n
See if you can figure out how the program works!<\/em><\/p>\n\n\n\n
Note that longjmp<\/code><\/strong> does not clean up any resources that may have been allocated since the setjmp<\/code><\/strong>.  That is the programmer’s responsibility!<\/em><\/p>\n<\/div>\n<\/div>\n\n\n\n
Well, enough already of Ada and C implementations. <\/p>\n\n\n\n
Let’s see what C++ implements…<\/p>\n\n\n\n
<\/div>\n\n\n\n
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Next: C++ Exceptions<\/a><\/div>\n<\/div>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
A C (not C++) Exception Mechanism While C does not have exceptions built-in, it is possible to add a reasonably efficient and portable (albeit somewhat crude) exception mechanism using macros and the setjmp\/longjmp C Run-Time-Library functions (see here). The Roberts Proposal A very useful C exception mechanism was proposed by Eric S. Roberts in 1989:Implementing […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1928,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-1942","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/1942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/comments?post=1942"}],"version-history":[{"count":7,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/1942\/revisions"}],"predecessor-version":[{"id":2014,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/1942\/revisions\/2014"}],"up":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/1928"}],"wp:attachment":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/media?parent=1942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}