man3/assert.3

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  25 .\" Modified Sat Jul 24 21:42:42 1993 by Rik Faith <faith@cs.unc.edu>
  26 .\" Modified Tue Oct 22 23:44:11 1996 by Eric S. Raymond <esr@thyrsus.com>
  27 .\" Modified Thu Jun  2 23:44:11 2016 by Nikos Mavrogiannopoulos <nmav@redhat.com>
  28 .TH ASSERT 3  2017-09-15 "GNU" "Linux Programmer's Manual"
  29 .SH NAME
  30 assert \- abort the program if assertion is false
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <assert.h>
  34 .PP
  35 .BI "void assert(scalar " expression );
  36 .fi
  37 .SH DESCRIPTION
  38 This macro can help programmers find bugs in their programs,
  39 or handle exceptional cases
  40 via a crash that will produce limited debugging output.
  41 .PP
  42 If
  43 .I expression
  44 is false (i.e., compares equal to zero),
  45 .BR assert ()
  46 prints an error message to standard error
  47 and terminates the program by calling
  48 .BR abort (3).
  49 The error message includes the name of the file and function containing the
  50 .BR assert ()
  51 call, the source code line number of the call, and the text of the argument;
  52 something like:
  53 .PP
  54     prog: some_file.c:16: some_func: Assertion `val == 0' failed.
  55 .PP
  56 If the macro
  57 .B NDEBUG
  58 is defined at the moment
  59 .I <assert.h>
  60 was last included, the macro
  61 .BR assert ()
  62 generates no code, and hence does nothing at all.
  63 It is not recommended to define
  64 .B NDEBUG
  65 if using
  66 .BR assert ()
  67 to detect error conditions since the software
  68 may behave non-deterministically.
  69 .SH RETURN VALUE
  70 No value is returned.
  71 .SH ATTRIBUTES
  72 For an explanation of the terms used in this section, see
  73 .BR attributes (7).
  74 .TS
  75 allbox;
  76 lb lb lb
  77 l l l.
  78 Interface       Attribute       Value
  79 T{
  80 .BR assert ()
  81 T}      Thread safety   MT-Safe
  82 .TE
  83 .sp 1
  84 .SH CONFORMING TO
  85 POSIX.1-2001, POSIX.1-2008, C89, C99.
  86 In C89,
  87 .I expression
  88 is required to be of type
  89 .I int
  90 and undefined behavior results if it is not, but in C99
  91 it may have any scalar type.
  92 .\" See Defect Report 107 for more details.
  93 .SH BUGS
  94 .BR assert ()
  95 is implemented as a macro; if the expression tested has side-effects,
  96 program behavior will be different depending on whether
  97 .B NDEBUG
  98 is defined.
  99 This may create Heisenbugs which go away when debugging
 100 is turned on.
 101 .SH SEE ALSO
 102 .BR abort (3),
 103 .BR assert_perror (3),
 104 .BR exit (3)