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466
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1 <refentry id="longjmp">
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2 <refnamediv>
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568
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3 <refname>Longjmp</refname>
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4 <refname>Setjmp</refname>
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5 <refpurpose>jump to another function</refpurpose>
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466
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6 </refnamediv>
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7
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8 <refsynopsisdiv>
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9 <funcsynopsis>
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468
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10 <funcsynopsisinfo>
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11 #include <setjmp.h>
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12 </funcsynopsisinfo>
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13 <funcprototype>
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14 <funcdef><function>setjmp</function></funcdef>
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15 <paramdef>jmp_buf <parameter>env</parameter></paramdef>
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16 </funcprototype>
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17
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18 <funcprototype>
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19 <funcdef><function>longjmp</function></funcdef>
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20 <paramdef>jmp_buf <parameter>env</parameter></paramdef>
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21 <paramdef>int <parameter>val</parameter></paramdef>
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466
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22 </funcprototype>
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23 </funcsynopsis>
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24
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25 </refsynopsisdiv>
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26
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27 <refsect1><title>Description</title>
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28 <para>
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29 These functions allow the return of program control directly to
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30 a higher level function. They are most useful when dealing
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31 with errors and interrupts encountered in a low level routine.
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32 </para>
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33 <para>
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34 "Goto" in C has scope only in the function in which it is
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35 used; i.e. the label which is the object of a "goto" may only
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36 be in the same function. Control can only be transferred
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37 elsewhere by means of the function call, which, of course
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38 returns to the caller. In certain abnormal situations a
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39 programmer would prefer to be able to start some section of
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40 code again, but this would mean returning up a ladder of
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41 function calls with error indications all the way.
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42 </para>
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43 <para>
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44 Setjmp is used to "mark" a point in the program where a
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45 subsequent longjmp can reach. It places in the buffer, defined
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46 in the header file, enough information for longjmp to restore
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47 the environment to that existing at the relevant call to
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48 setjmp.
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49 </para>
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50 <para>
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51 Longjmp is called with the environment buffer as an argument
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52 and also, a value which can be used by the caller of setjmp as,
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53 perhaps, an error status.
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54 </para>
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55 <para>
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56 To set the system up, a function will call setjmp to set up the
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57 buffer, and if the returned value is zero, the program will
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58 know that the call was the "first time through". If, however,
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59 the returned value is non-zero, it must be a longjmp returning
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60 from some deeper level of the program.
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61 </para>
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62 <para>
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63 NOTE that the function calling setjmp must <emphasis>not have returned</emphasis>
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64 at the time of calling longjmp, and the environment buffer must
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65 be declared <emphasis>globally</emphasis>.
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66 </para>
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67 </refsect1>
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68 </refentry>
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