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SIGBLOCKALL(3)        DragonFly Library Functions Manual        SIGBLOCKALL(3)


NAME

     sigblockall, sigunblockall - quickly block and unblock all maskable
     signals


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <signal.h>

     int
     sigblockall(void);

     int
     sigunblockall(void);


DESCRIPTION

     These functions operate on a per-thread basis and temporarily prevent
     signal posts to the calling thread, protecting the function from
     potential reentrancy issues due to signal interruption.  The signal mask
     is not modified in any way and synchronous signals caused by traps (such
     as SIGSEGV) will still be immediately posted.

     These functions operate via the /dev/lpmap per-thread shared page
     facility and, once primed, will operate without having to issue any
     system calls.  They are designed to be very fast, typically sub-10nS.

     The sigblockall() procedure temporarily blocks the posting of any
     maskable signal.  SIGSTOP and SIGKILL are not maskable and will still be
     delivered, as will any signals caused by synchronous traps.  This
     facility does not prevent signals from being delivered to the thread, it
     simply prevents the signals from being posted to (interrupting) the
     thread.

     Callers must pair all calls to sigblockall() with calls to
     sigunblockall().  These functions may be used recursively.

     The sigunblockall() procedure unwinds the operation.  On the last
     transition, any blocked signals not otherwise masked through the normal
     signal mask will be immediately posted prior to the function returning.


RETURN VALUES

     Both procedures return the recursive blocking count's value after the
     operation.  Thus the first call to sigblockall() will generally return
     the value 1 and the last call to sigunblockall() will generally return
     the value 0.

     The returned value may have bit 31 set, resulting in a negative number.
     Bit 31 set indicates that one or more signals may have been blocked from
     being posted and represents cumulative state until the last call to
     sigunblockall().

     If bit 31 is set in the value returned by the last call to
     sigunblockall(), it is an indication that signals were previously blocked
     and have now been posted.  Bit 31 will be cleared in libc's internal
     state for the calling thread on the last call.


WARNINGS

     Any blocking count for the current thread present as-of any fork(2) or
     vfork(2) will be inherited by the child and must be unwound in both
     parent and child.  This also allows programs to conveniently prevent
     signal delivery in a new child process until they are ready to deal with
     it, if desired.

     Care must be taken if coupling the use of these functions with the
     longjmp and ucontext facilities.  The blocking count is associated with
     the thread, NOT the context.


ERRORS

     These functions return -1 on error, which can only occur if the
     /dev/lpmap facility is not available.  However, generally speaking,
     because these procedures are meant to be used in critical code paths,
     callers are not expected to check for this failure condition in critical
     code paths.  Detecting support can be done at the start of the program,
     if desired, simply by issuing the function pair and retaining the result.


SEE ALSO

     sigprocmask(2)


HISTORY

     The sigblockall() familiy of functions first appeared in DragonFly 5.7.

DragonFly 5.7-DEVELOPMENT      November 12, 2019     DragonFly 5.7-DEVELOPMENT

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