CODEWITHSUNDEEP
csegmentation-fault
Mikey Chen
Mikey Chen

Reputation: 2420

setcontext() segmentation fault

I'm trying to create a custom pthread library using context switching in C. I've run into this problem of getting segmentation faults when I call setcontext() - there seems to be limited documentation on this function so I can't really figure out what's going on, and it's already cost me a lot of downtime. My code:

#include <signal.h>
#include <ucontext.h>

struct mypthread_t{
   int id;
   int pid;
   int isRunning;
   mypthread_t *next;
   ucontext_t* context;
}; 

int mypthread_create(mypthread_t *thread, const mypthread_attr_t *attr, void *(*start_routine)(void *), void *arg)
{
   printf("Hello\n");
   thread->id = threadID++;
   thread->context = malloc(sizeof(ucontext_t));
   thread->context->uc_stack.ss_sp = malloc(16384);
   thread->context->uc_stack.ss_size = 16384;
   thread->context->uc_link = 0;

   makecontext(thread->context, (void *) start_routine, 1, arg);
   setcontext(thread->context);
}

It seems like I've malloc'd everything that needs to be malloc'd, so I don't understand why setcontext(thread->context) is giving me a segfault. Any help would be appreciated - the context family of functions is really stumping me. Here's the line it fails at from gdb:

fldenv  (%rcx)

Upvotes: 3

Views: 2127

Answers (1)

apriori
apriori

Reputation: 1262

Initialize your ucontext_t object with a call to getcontext(3) before calling makecontext(3):

if (getcontext(thread->context) == -1) {
    // ... error handling
    // ... errno is set appropriately to indicate the error that occurred
}
makecontext(...);

Upvotes: 3

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