assert() with message - c

Question

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?

Answer 1

Use -Wno-unused-value to stop the warning; (the option -Wall includes -Wunused-value).
I think even better is to use another method, like
assert(condition && "message");

Answer 2

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!)

Answer 3

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

Answer 4

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!");

Answer 5

For unexpected default case of a switch, an options is
assert(!"message");

Answer 6

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))

Answer 7

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__);

Answer 8

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.

Answer 9

According to following link
http://www.cplusplus.com/reference/clibrary/cassert/assert/
assert is expecting only expression. May be you are using some overloaded function.
According to this, only expression is allowed and thus you are getting this warning.