#include <stdio.h>
#include <stdlib.h>
#define TEST 5
int main(void)
{
char *p;
if (TEST == 5)
p = (char *)malloc((80 * sizeof(char)));
p = "This is a test\n";
if (p != 0) {
printf("%s", p);
free(p);
}
return 1;
}
help with free()
Author
Was wondering if any of you could tell me why I get this error in MSVC 6.0 SP 5 on this stupid simple test program.
At the point where I try and free p I get a:
Debug Assertion Failed!
Program: test.exe
File: dbgheap.c
Line: 1011
Expression: _CrtIsValidHeapPointer(pUserData)
Now this makes me think I''m passing an invalid pointer to free. But didn''t I test it both by checking for 0 status and printing the string it points to. Any help in clearing up my ignorance is appreciated!
does it fail on every run of the program. even when TEST == 5?
the first obvious problem is that you are not initializing the pointer to NULL. also, NULL != 0 in all cases. so set it to NULL:
char *p = NULL;
by stating char *p; pointer p is never initialized and, most likely will be junk data. thus you will enter the free statement because p != 0, but also, it has never been malloc'd.
[EDIT: obviously if you change the initialization to char *p = NULL; you also have to change the check before the free() block to:
if (p != NULL) { .... ]
-me
[edited by - Palidine on June 25, 2002 2:23:04 PM]
the first obvious problem is that you are not initializing the pointer to NULL. also, NULL != 0 in all cases. so set it to NULL:
char *p = NULL;
by stating char *p; pointer p is never initialized and, most likely will be junk data. thus you will enter the free statement because p != 0, but also, it has never been malloc'd.
[EDIT: obviously if you change the initialization to char *p = NULL; you also have to change the check before the free() block to:
if (p != NULL) { .... ]
-me
[edited by - Palidine on June 25, 2002 2:23:04 PM]
You''re setting p equal to the address of a temporary string (p = "This is a test\n"). If you''re going to use char arrays for strings, make sure you use the string functions (in this case strcpy). ie:
strcpy ( p, "This is a test\n" );
Otherwise, might I suggest std::string?
strcpy ( p, "This is a test\n" );
Otherwise, might I suggest std::string?
Author
Thanks for the insight guys. Make sense now. I thought NULL was deprecated though and 0 is supposed to be used instead. Am I wrong on this, or is this true?
quote:I thought NULL was deprecated though and 0 is supposed to be used instead. Am I wrong on this, or is this true?
not entirely sure myself. i''m just pulling my suggestion from several other threads in which people have been flamed for setting pointers to zero instead of NULL. it had something to do with zeroth memory space as defined by different machines/compilers....don''t entirely remember.
-me
From windows.h
[edited by - terminate on June 25, 2002 5:13:17 PM]
/* Define NULL pointer value */#ifndef NULL#ifdef __cplusplus#define NULL 0#else#define NULL ((void *)0)#endif#endif [edited by - terminate on June 25, 2002 5:13:17 PM]
Stroustrup recommends using 0 instead of NULL in C++ programs, since 0 is implicitly convertible to NULL.
This topic is closed to new replies.
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