man3/pthread_setconcurrency.3

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  25 .TH PTHREAD_SETCONCURRENCY 3 2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 pthread_setconcurrency, pthread_getconcurrency \- set/get
  28 the concurrency level
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <pthread.h>
  32 .PP
  33 .BI "int pthread_setconcurrency(int " new_level );
  34 .BI "int pthread_getconcurrency(void);
  35 .PP
  36 Compile and link with \fI\-pthread\fP.
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR pthread_setconcurrency ()
  41 function informs the implementation of the application's
  42 desired concurrency level, specified in
  43 .IR new_level .
  44 The implementation takes this only as a hint:
  45 POSIX.1 does not specify the level of concurrency that
  46 should be provided as a result of calling
  47 .BR pthread_setconcurrency ().
  48 .PP
  49 Specifying
  50 .I new_level
  51 as 0 instructs the implementation to manage the concurrency level
  52 as it deems appropriate.
  53 .PP
  54 .BR pthread_getconcurrency ()
  55 returns the current value of the concurrency level for this process.
  56 .SH RETURN VALUE
  57 On success,
  58 .BR pthread_setconcurrency ()
  59 returns 0;
  60 on error, it returns a nonzero error number.
  61 .PP
  62 .BR pthread_getconcurrency ()
  63 always succeeds, returning the concurrency level set by a previous call to
  64 .BR pthread_setconcurrency (),
  65 or 0, if
  66 .BR pthread_setconcurrency ()
  67 has not previously been called.
  68 .SH ERRORS
  69 .BR pthread_setconcurrency ()
  70 can fail with the following error:
  71 .TP
  72 .B EINVAL
  73 .I new_level
  74 is negative.
  75 .PP
  76 POSIX.1 also documents an
  77 .BR EAGAIN
  78 error ("the value specified by
  79 .I new_level
  80 would cause a system resource to be exceeded").
  81 .SH VERSIONS
  82 These functions are available in glibc since version 2.1.
  83 .SH ATTRIBUTES
  84 For an explanation of the terms used in this section, see
  85 .BR attributes (7).
  86 .TS
  87 allbox;
  88 lbw25 lb lb
  89 l l l.
  90 Interface       Attribute       Value
  91 T{
  92 .BR pthread_setconcurrency (),
  93 .BR pthread_getconcurrency ()
  94 T}      Thread safety   MT-Safe
  95 .TE
  96 .SH CONFORMING TO
  97 POSIX.1-2001, POSIX.1-2008.
  98 .SH NOTES
  99 The default concurrency level is 0.
 100 .PP
 101 Concurrency levels are meaningful only for M:N threading implementations,
 102 where at any moment a subset of a process's set of user-level threads
 103 may be bound to a smaller number of kernel-scheduling entities.
 104 Setting the concurrency level allows the application to
 105 give the system a hint as to the number of kernel-scheduling entities
 106 that should be provided for efficient execution of the application.
 107 .PP
 108 Both LinuxThreads and NPTL are 1:1 threading implementations,
 109 so setting the concurrency level has no meaning.
 110 In other words,
 111 on Linux these functions merely exist for compatibility with other systems,
 112 and they have no effect on the execution of a program.
 113 .SH SEE ALSO
 114 .BR pthread_attr_setscope (3),
 115 .BR pthreads (7)