1 .\" Copyright 2014 Garrett D'Amore <garrett@damore.org>
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  10 .Dd "Jul 14, 2014"
  11 .Dt MAKECONTEXT 3C
  12 .Os
  13 .Sh NAME
  14 .Nm makecontext, swapcontext
  15 .Nd manipulate user contexts
  16 .Sh SYNOPSIS
  17 .In ucontext.h
  18 .Ft void
  19 .Fn makecontext "ucontext_t *ucp" "void \*(lp*func\*(rp\*(lp\*(rp, int argc, ..."
  20 .Ft int
  21 .Fn swapcontext "ucontext_t *restrict oucp" "const ucontext_t *restrict ucp"
  22 .Sh DESCRIPTION
  23 The
  24 .Fn makecontext
  25 function modifies the context specified by
  26 .Fa ucp ,
  27 which has been initialized using
  28 .Xr getcontext 2 .
  29 When this context is
  30 resumed using
  31 .Fn swapcontext
  32 or
  33 .Xr setcontext 2 ,
  34 execution continues by calling the function
  35 .Fa func ,
  36 passing it the arguments that follow
  37 .Fa argc
  38 in the
  39 .Fn makecontext
  40 call. The value of
  41 .Fa argc
  42 must match the number of pointer-sized integer arguments passed to
  43 .Fn func ,
  44 otherwise the behavior is undefined.
  45 .Lp
  46 Before a call is made to
  47 .Fn makecontext ,
  48 the context being modified should
  49 have a stack allocated for it. The stack is assigned to the context by
  50 initializing the
  51 .Fa uc_stack
  52 member.
  53 .Lp
  54 The
  55 .Fa uc_link
  56 member is used to determine the context that will be resumed
  57 when the context being modified by
  58 .Fn makecontext
  59 returns.  The
  60 .Fa uc_link
  61 member should be initialized prior to the call to
  62 .Fn makecontext .
  63 If the
  64 .Fa uc_link
  65 member is initialized to
  66 .Dv NULL ,
  67 the thread executing
  68 .Fa func
  69 will exit when
  70 .Fa func
  71 returns. See
  72 .Xr pthread_exit 3C .
  73 .Lp
  74 The
  75 .Fn swapcontext
  76 function saves the current context in the context
  77 structure pointed to by
  78 .Fa oucp
  79 and sets the context to the context
  80 structure pointed to by
  81 .Fa ucp .
  82 .Lp
  83 If the
  84 .Fa ucp
  85 or
  86 .Fa oucp
  87 argument points to an invalid address, the
  88 behavior is undefined and
  89 .Va errno
  90 may be set to
  91 .Er EFAULT .
  92 .Sh RETURN VALUES
  93 .Rv -std swapcontext
  94 .Sh EXAMPLES
  95 .Ss Example 1
  96 The following example illustrates execution context on a stack whose memory was
  97 allocated using
  98 .Xr  mmap 2 :
  99 .Bd -literal -offset indent
 100 #include <stdio.h>
 101 #include <ucontext.h>
 102 #include <sys/mman.h>
 103 
 104 void
 105 assign(long a, int *b)
 106 {
 107         *b = (int)a;
 108 }
 109 
 110 int
 111 main(int argc, char **argv)
 112 {
 113         ucontext_t uc, back;
 114         size_t sz = 0x10000;
 115         int value = 0;
 116 
 117         getcontext(&uc);
 118 
 119         uc.uc_stack.ss_sp = mmap(0, sz,
 120             PROT_READ | PROT_WRITE | PROT_EXEC,
 121             MAP_PRIVATE | MAP_ANON, -1, 0);
 122         uc.uc_stack.ss_size = sz;
 123         uc.uc_stack.ss_flags = 0;
 124 
 125         uc.uc_link = &back;
 126 
 127         makecontext(&uc, assign, 2, 100L, &value);
 128         swapcontext(&back, &uc);
 129 
 130         printf("done %d\en", value);
 131 
 132         return (0);
 133 }
 134 .Ed
 135 .Sh ERRORS
 136 The
 137 .Fn swapcontext
 138 function will fail if:
 139 .Bl -tag -width Er
 140 .It Er ENOMEM
 141 The
 142 .Fa ucp
 143 argument does not have enough stack left to complete the operation.
 144 .El
 145 .Lp
 146 The
 147 .Fn swapcontext
 148 function may fail if:
 149 .Bl -tag -width Er
 150 .It Er EFAULT
 151 The
 152 .Fa ucp
 153 or
 154 .Fa oucp
 155 argument points to an invalid address.
 156 .El
 157 .Sh USAGE
 158 These functions are useful for implementing user-level context switching
 159 between multiple threads of control within a process (co-processing). More
 160 effective multiple threads of control can be obtained by using native support
 161 for multithreading. See
 162 .Xr pthreads 5 .
 163 .Sh INTERFACE STABILITY
 164 .Sy Obsolete Standard .
 165 .Sh MT-LEVEL
 166 .Sy MT-Safe .
 167 .Sh SEE ALSO
 168 .Xr mmap 2 ,
 169 .Xr getcontext 2 ,
 170 .Xr sigaction 2 ,
 171 .Xr sigprocmask 2 ,
 172 .Xr pthread_exit 3C ,
 173 .Xr ucontext.h 3HEAD ,
 174 .Xr standards 5 ,
 175 .Xr pthreads 5
 176 .Sh NOTES
 177 The semantics of the
 178 .Fa uc_stack
 179 member of the
 180 .Ft ucontext_t
 181 structure have changed as they apply to inputs to
 182 .Fn makecontex .
 183 Prior to Solaris 10, the
 184 .Fa ss_sp
 185 member of the
 186 .Fa uc_stack
 187 structure represented the high
 188 memory address of the area reserved for the stack. The \fBss_sp\fR member now
 189 represents the base (low memory address), in keeping with other uses of
 190 .Fa ss_sp .
 191 .Lp
 192 This change in the meaning of
 193 .Fa ss_sp
 194 is now the default behavior. The
 195 .Dv -D__MAKECONTEXT_V2_SOURCE
 196 compilation flag used in Solaris 9 update
 197 releases to access this behavior is obsolete.
 198 .Lp
 199 Binary compatibility has been preserved with releases prior to Solaris 10.
 200 Before recompiling, applications that use
 201 .Fn makecontext
 202 must be updated
 203 to reflect this behavior change.
 204 .Lp
 205 Portable applications should not use this function. Instead, applications
 206 should use
 207 .Xr pthreads 5
 208 routines.
 209 .Lp
 210 Note that the definition of
 211 .Fn makecontext
 212 violates
 213 .St -isoC .
 214 There is no way to declare this function that does not violate that
 215 standard.
 216 .Sh STANDARDS
 217 This function  was introduced in
 218 .St -xpg4.2 ,
 219 and subsequently removed from
 220 .St -p1003.1-2008 .