assert() : how to use it correctly

Please read the man page first, don't be impatient.

ASSERT(3)                            Linux Programmer's Manual                           ASSERT(3)

NAME
       assert - abort the program if assertion is false

SYNOPSIS
       #include <assert.h>

       void assert(scalar expression);

DESCRIPTION
       If  the  macro  NDEBUG  was  defined  at the moment <assert.h> was last included, the macro
       assert() generates no code, and hence does nothing at all.  Otherwise, the  macro  assert()
       prints an error message to standard error and terminates the program by calling abort(3) if
       expression is false (i.e., compares equal to zero).

       The purpose of this macro is to help programmers find bugs in their programs.  The  message
       "assertion  failed  in  file foo.c, function do_bar(), line 1287" is of no help at all to a
       user.

RETURN VALUE
       No value is returned.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │assert()  │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, C89, C99.  In C89, expression is required to be of type int and
       undefined behavior results if it is not, but in C99 it may have any scalar type.

BUGS
       assert()  is  implemented  as  a  macro; if the expression tested has side-effects, program
       behavior will be different depending on whether NDEBUG is defined.  This may create Heisen‐
       bugs which go away when debugging is turned on.

SEE ALSO
       abort(3), assert_perror(3), exit(3)

COLOPHON
       This  page  is  part  of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version  of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

GNU                                         2015-08-08                                   ASSERT(3)

If you're impatient, watch it from here, hope you're not.

A few key questions:

When to use assert

Assert is usually used to diagnose potential bugs in the program. By using assert(condition), when the condition is false, the program ends running early, which is beneficial to the location of program bugs. So, when you feel that a condition must be true, but some conditions may be false, you can use assert to strengthen the semantics. (Sometimes the logic of the program is very complicated, and you are not sure when the condition is false, so tell the program if the condition is false, please end the program and print the relevant information to help you sort out the logic and find bugs)
What to pay attention to when using assert

assert itself is also a macro definition, and its macro expansion result NDEBUGis closely related to the macro. If it is defined before NDEBUGthe header file <assert.h>, then the assert macro expands to nothing (ashes).

# define __ASSERT_VOID_CAST (void)

/* void assert (int expression);

   If NDEBUG is defined, do nothing.
   If not, and EXPRESSION is zero, print an error message and abort.  */

#ifdef  NDEBUG

# define assert(expr)       (__ASSERT_VOID_CAST (0))
...
#else
...
# define assert(expr)                           \
  ((expr)                               \
   ? __ASSERT_VOID_CAST (0)                     \
   : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))

#endif

So if you put a function in assert, it will generate a deeply hidden bug, >_<

assert(StartWork() == true);

NDEBUGIt will be called when it is not defined StartWorker, but once it is defined somewhere NDEBUG(who knows where it is), StartWorkit will be hidden and will never play. An improvement would be to just put the return value in assert, the function in a separate statement:

int r = StartWork();
assert(r == true);

assert() : how to use it correctly

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