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  10 .TH MAKECONTEXT 3C "Mar 8, 2004"
  11 .SH NAME
  12 makecontext, swapcontext \- manipulate user contexts
  13 .SH SYNOPSIS
  14 .LP
  15 .nf
  16 #include <ucontext.h>
  17 
  18 \fBvoid\fR \fBmakecontext\fR(\fBucontext_t *\fR\fIucp\fR, \fBvoid (*\fR\fIfunc\fR)(), \fBint\fR \fIargc\fR...);
  19 .fi
  20 
  21 .LP
  22 .nf
  23 \fBint\fR \fBswapcontext\fR(\fBucontext_t *restrict\fR \fIoucp\fR,
  24      \fBconst ucontext_t *restrict\fR \fIucp\fR);
  25 .fi
  26 
  27 .SH DESCRIPTION
  28 .sp
  29 .LP
  30 The \fBmakecontext()\fR function modifies the context specified by \fIucp\fR,
  31 which has been initialized using \fBgetcontext\fR(2). When this context is
  32 resumed using \fBswapcontext()\fR or \fBsetcontext\fR(2), execution continues
  33 by calling the function \fIfunc\fR, passing it the arguments that follow
  34 \fIargc\fR in the \fBmakecontext()\fR call. The value of \fIargc\fR must match
  35 the number of pointer-sized integer arguments passed to \fIfunc\fR, otherwise
  36 the behavior is undefined.
  37 .sp
  38 .LP
  39 Before a call is made to \fBmakecontext()\fR, the context being modified should
  40 have a stack allocated for it. The stack is assigned to the context by
  41 initializing the \fBuc_stack\fR member.
  42 .sp
  43 .LP
  44 The \fBuc_link\fR member is used to determine the context that will be resumed
  45 when the context being modified by \fBmakecontext()\fR returns.  The
  46 \fBuc_link\fR member should be initialized prior to the call to
  47 \fBmakecontext()\fR. If the \fBuc_link\fR member is initialized to \fINULL\fR,
  48 the thread executing \fIfunc\fR will exit when \fIfunc\fR returns. See
  49 \fBpthread_exit\fR(3C).
  50 .sp
  51 .LP
  52 The \fBswapcontext()\fR function saves the current context in the context
  53 structure pointed to by \fIoucp\fR and sets the context to the context
  54 structure pointed to by \fIucp\fR.
  55 .sp
  56 .LP
  57 If the \fIucp\fR or \fIoucp\fR argument points to an invalid address, the
  58 behavior is undefined and \fBerrno\fR may be set to \fBEFAULT\fR.
  59 .SH RETURN VALUES
  60 .sp
  61 .LP
  62 On successful completion, \fBswapcontext()\fR returns \fB0\fR. Otherwise,
  63 \fB\(mi1\fR is returned and \fBerrno\fR is set to indicate the error.
  64 .SH ERRORS
  65 .sp
  66 .LP
  67 The \fBswapcontext()\fR function will fail if:
  68 .sp
  69 .ne 2
  70 .na
  71 \fB\fBENOMEM\fR\fR
  72 .ad
  73 .RS 10n
  74 The \fIucp\fR argument does not have enough stack left to complete the
  75 operation.
  76 .RE
  77 
  78 .sp
  79 .LP
  80 The \fBswapcontext()\fR function may fail if:
  81 .sp
  82 .ne 2
  83 .na
  84 \fB\fBEFAULT\fR\fR
  85 .ad
  86 .RS 10n
  87 The \fIucp\fR or \fIoucp\fR argument points to an invalid address.
  88 .RE
  89 
  90 .SH EXAMPLES
  91 .LP
  92 \fBExample 1 \fRAlternate execution context on a stack whose memory was
  93 allocated using \fBmmap()\fR.
  94 .sp
  95 .in +2
  96 .nf
  97 #include <stdio.h>
  98 #include <ucontext.h>
  99 #include <sys/mman.h>
 100 
 101 void
 102 assign(long a, int *b)
 103 {
 104         *b = (int)a;
 105 }
 106 
 107 int
 108 main(int argc, char **argv)
 109 {
 110         ucontext_t uc, back;
 111         size_t sz = 0x10000;
 112         int value = 0;
 113 
 114         getcontext(&uc);
 115 
 116         uc.uc_stack.ss_sp = mmap(0, sz,
 117             PROT_READ | PROT_WRITE | PROT_EXEC,
 118             MAP_PRIVATE | MAP_ANON, -1, 0);
 119         uc.uc_stack.ss_size = sz;
 120         uc.uc_stack.ss_flags = 0;
 121 
 122         uc.uc_link = &back;
 123 
 124         makecontext(&uc, assign, 2, 100L, &value);
 125         swapcontext(&back, &uc);
 126 
 127         printf("done %d\en", value);
 128 
 129         return (0);
 130 }
 131 .fi
 132 .in -2
 133 
 134 .SH USAGE
 135 .sp
 136 .LP
 137 These functions are useful for implementing user-level context switching
 138 between multiple threads of control within a process (co-processing). More
 139 effective multiple threads of control can be obtained by using native support
 140 for multithreading. See \fBthreads\fR(5).
 141 .SH ATTRIBUTES
 142 .sp
 143 .LP
 144 See \fBattributes\fR(5) for descriptions of the following attributes:
 145 .sp
 146 
 147 .sp
 148 .TS
 149 box;
 150 c | c
 151 l | l .
 152 ATTRIBUTE TYPE  ATTRIBUTE VALUE
 153 _
 154 Interface Stability     Standard
 155 _
 156 MT-Level        MT-Safe
 157 .TE
 158 
 159 .SH SEE ALSO
 160 .sp
 161 .LP
 162 \fBmmap\fR(2), \fBgetcontext\fR(2), \fBsigaction\fR(2), \fBsigprocmask\fR(2),
 163 \fBpthread_exit\fR(3C), \fBucontext.h\fR(3HEAD), \fBattributes\fR(5),
 164 \fBstandards\fR(5), \fBthreads\fR(5)
 165 .SH NOTES
 166 .sp
 167 .LP
 168 The semantics of the \fBuc_stack\fR member of the \fBucontext_t\fR structure
 169 have changed as they apply to inputs to \fBmakecontext()\fR. Prior to Solaris
 170 10, the \fBss_sp\fR member of the \fBuc_stack\fR structure represented the high
 171 memory address of the area reserved for the stack. The \fBss_sp\fR member now
 172 represents the base (low memory address), in keeping with other uses of
 173 \fBss_sp\fR.
 174 .sp
 175 .LP
 176 This change in the meaning of \fBss_sp\fR is now the default behavior. The
 177 \fB-D__MAKECONTEXT_V2_SOURCE\fR compilation flag used in Solaris 9 update
 178 releases to access this behavior is obsolete.
 179 .sp
 180 .LP
 181 Binary compatibility has been preserved with releases prior to Solaris 10.
 182 Before recompiling, applications that use \fBmakecontext()\fR must be updated
 183 to reflect this behavior change. The example below demonstates a typical change
 184 that must be applied:
 185 .sp
 186 .in +2
 187 .nf
 188 --- example1_s9.c       Thu Oct  3 11:58:17 2002
 189 +++ example1.c  Thu Jun 27 13:28:16 2002
 190 @@ -27,12 +27,9 @@
 191         uc.uc_stack.ss_sp = mmap(0, sz,
 192             PROT_READ | PROT_WRITE | PROT_EXEC,
 193             MAP_PRIVATE | MAP_ANON, -1, 0);
 194 -       uc.uc_stack.ss_sp = (char *)uc.uc_stack.ss_sp + sz - 8;
 195         uc.uc_stack.ss_size = sz;
 196         uc.uc_stack.ss_flags = 0;
 197 
 198         uc.uc_link = &back
 199 
 200         makecontext(&uc, assign, 2, 100L, &value);
 201 .fi
 202 .in -2
 203