CONTEXT(3PVM)

💖 CONTEXT 💖 — the 3PVM page, but make it make sense

okay bestie, we're doing PVM contexts today and honestly? this one's a vibe. it's from PVM Version 3.4, it lives in section 3PVM of the manual, and it's about four little functions that let your tasks keep their conversations straight. 🎀

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✨ NAME ✨

pvm_newcontext, pvm_setcontext, pvm_freecontext, and pvm_getcontext — these four do exactly what they sound like, in order: request a brand new context, switch to a different context, free up an existing context, and ask "hey what context am I in right now". 🦋

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✨ SYNOPSIS ✨

so here's the C situation:

int ctx = pvm_newcontext( void )
int old_ctx = pvm_setcontext( int new_ctx )
int info = pvm_freecontext( ctx )
int ctx = pvm_getcontext( void )

and over in Fortran land:

call pvmfnewcontext( ctx )
call pvmfsetcontext( new_ctx, old_ctx )
call pvmffreecontext( ctx, info )
call pvmfgetcontext( ctx )

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✨ PARAMETERS ✨

• ctx — a context value. 💅
• new_ctx — the new context value you're switching to.
• old_ctx — the context value you were in before. the before-times, if you will.
• info — a result code. it's giving "here's what happened".

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✨ DESCRIPTION ✨

okay so the whole point of these functions is that they hand you a context that's unique across the entire system, plus the tools to mess with it. 🌸

Here's why you'd care: contexts let tasks that are talking to each other automatically sort their messages by the context they were sent in. no cap. so if a message goes out in context A, it has to come in on the receiving end in context A too. the sender can send in literally any context they want — go off — but the recipient will not accept a message that arrived in a context different from its own. nope. blocked. 🚫

One super common use for this? library routines. with contexts, a library routine's internal chatter can be logically separated from your application's chatter, which means neither one accidentally picks up the other's mail. the way that saves you from the most confusing bug of your life 😭

Spawned tasks inherit the spawn-time context of their parent. and existing PVM applications? they just keep working, unchanged, on the default context. iconic behaviour honestly. 👑

Now the four functions, one by one:

• pvm_newcontext returns a newly allocated context. heads up though — this new context is not yet active. it exists, it's just not the one you're currently in. 🙈
• pvm_setcontext changes the current context from old_ctx to new_ctx. that's the switch-up. ✨
• pvm_freecontext frees ctx so it can be reused. and this one matters, bestie — contexts are a system resource that will be exhausted if not recycled. like, actually. recycle your contexts. 🌍
• pvm_getcontext returns the current context of the task that's asking. simple. clean. 🫶

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✨ EXAMPLES ✨

first up, the parent task with context — look at her go:

/* parent task with context */
     int cc, context0, context1;
     char buf[25];

     context0 = pvm_getcontext();       /*  get my current context */
     context1 = pvm_newcontext();       /*  get a new context */
     pvm_setcontext(context1);          /*  set my context to new context */
     printf("My context is: %d", context1);
     pvm_spawn("child", (char**)0, PvmTaskDefault, "", 1, &tid);
     cc = pvm_recv(-1, -1);             /*  receive message from child - in context1 */
     pvm_upkstr(buf);
     printf("%s", buf);
     pvm_setcontext(context0);          /*  reset my context to my original context0 */

and now the child task with context — child inherits the parent's context as default, main character energy fr fr:

/* child task with context - child inherits parent's context as default */
     int context;
     int ptid;
     char buf[25];

     ptid = pvm_parent();
     context = pvm_getcontext();        /*  get my current context */
     sprintf(buf, "Greetings from child who's context is: %d.", context);
     pvm_initsend(PvmDataDefault);
     pvm_pkstr(buf);
     pvm_send(ptid, 1);

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✨ ERRORS ✨

okay good news: only system resource errors will be returned, because the context programs themselves do not generate errors. they're chill like that. 💜

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✨ SEE ALSO ✨

(that's it, bestie — this page keeps its own company 🦋)

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lowkey the takeaway is: get a context, set a context, and for the love of everything, free your context. 💅

and that's the whole page! this one's from PVM Version 3.4, dated 8 April, 1997 — so yeah, she's vintage, but she still ate. 🎀✨ bye bestie, go recycle a context for me 💖