Reputation: 25810

assert() with message

I saw somewhere assert used with a message in the following way:

assert(("message", condition));

This seems to work great, except that gcc throws the following warning:

warning: left-hand operand of comma expression has no effect

How can I stop the warning?

Upvotes: 63

Answers (10)

Bjørn

Reputation: 31

I like to do it this way in C. This allows calling assert the following ways:

assert(expression);
assert(expression, "reason");
assert(file, "cannot open %s", filename);

Here is the code:

#define assert(cond, ...) \
  if (!(cond)) \
    _assert(#cond, __FILE__, __LINE__, #__VA_ARGS__ __VA_OPT__(,) ##__VA_ARGS__)

void _assert (const char* snippet, const char* file, int line, const char* message, ...)
{
  print("assert failed %s:%d %s\n", file, line, snippet);
 
  if (*message)
  {
    va_list arg;
    va_start(arg, message);
    char* data = va_arg(arg, char*);
    vprintf(data, arg);
  }
}

Upvotes: 3

user9869932

Reputation: 7337

In my case, I changed @pmg's answer to be able to control the output. The (... && "message") didn't work for me.

#include <assert.h>
#include <stdio.h>

#define __DEBUG__ 1

assert ((1 == 1) && 
       (__DEBUG__ && printf("  - debug: check, ok.\n")) || !__DEBUG__);

Upvotes: 0

Flaviu

Reputation: 1039

For unexpected default case of a switch, an options is

assert(!"message");

Upvotes: 1

JiaHao Xu

Reputation: 2748

A function that takes const char* and returns true would probably save you from all sorts of warnings:

#include <assert.h>

int always_true(const char *msg) {
    return 1;
}

#define assert_msg(expr, msg) assert((expr) && always_true(msg))

Upvotes: 0

Mingye Wang

Reputation: 1374

By tradition, (void) communicates to the compiler that you are knowingly ignoring an expression:

/* picard.c, TNG S6E11. */
#define assertmsg(x, msg) assert(((void) msg, x))
assertmsg(2+2==5, "There! are! four! lights!");

Upvotes: 5

alx - recommends codidact

Reputation: 4527

You could write your own macro that provides the same usage of _Static_assert(expr, msg):

#include <assert.h>
#include <stdbool.h>
#include <stdio.h>


/*
 * void assert_msg(bool expr, const char *msg);
 */
#if !defined(NDEBUG)
#define assert_msg(expr, msg)   do                  \
{                                                   \
        const bool  e_ = expr;                      \
                                                    \
        if (!e_) {                                  \
                fputs(msg, stderr);                 \
                fputc('\n', stderr);                \
                assert(e_);                         \
        }                                           \
} while (0)
#else
#define assert_msg(expr, msg)   do                  \
{                                                   \
                                                    \
        if (!(expr))                                \
                warn_bug(msg);                      \
} while (0)
#endif

I also have a macro warn_bug() that prints the name of the program, the file, the line, the function, the errno value and string, and a user message, even if asserts are disabled. The reason behind it is that it won't break the program, but it will warn that a bug will probably be present. You could just define assert_msg to be empty if defined(NDEBUG), though.

Upvotes: -1

bugfeeder

Reputation: 347

Try:

#define assert__(x) for ( ; !(x) ; assert(x) )

use as such:

assert__(x) {
    printf("assertion will fail\n"); 
}

Will execute the block only when assert fails.

IMPORTANT NOTE: This method will evaluate expression x twice, in case x evaluates to false! (First time, when the for loop is checking its condition; second time, when the assert is evaluating the passed expression!)

Upvotes: 23

frmdstryr

Reputation: 21362

If you want to pass a formatted message, you could use the following macros:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <assert.h>

#define clean_errno() (errno == 0 ? "None" : strerror(errno))
#define log_error(M, ...) fprintf(stderr, "[ERROR] (%s:%d: errno: %s) " M "\n", __FILE__, __LINE__, clean_errno(), ##__VA_ARGS__)
#define assertf(A, M, ...) if(!(A)) {log_error(M, ##__VA_ARGS__); assert(A); }

Then use it like printf:

// With no args
assertf(self != NULL,"[Server] Failed to create server.");

// With formatting args
assertf((self->socket = u_open(self->port)) != -1,"[Server] Failed to bind to port %i:",self->port);
// etc...

Output:

[ERROR] (../src/webserver.c:180: errno: Address already in use) [Server] Failed to bind to port 8080: webserver: ../src/webserver.c:180: server_run: Assertion `(self->socket = u_open(self->port)) != -1' failed.

Based on http://c.learncodethehardway.org/book/ex20.html

Upvotes: 13