Thread: Changing chars in a string

Hey there! I want to cut a string by setting a char of to \0.

The string is an argument of a function.

Code:

void FindDirectory(char *path)
{
...
char *pch;
pch = strrchr(path, '*');
memset(path, 'e', pch-path);
OR
memset(path, 'e', (int)pch-(int)path);
OR
path[pch-path] = 'e';
...
}

pch-path returns a correct value (3 in my case).

My program crashes at the line where I want to change the character and I don't understand why, maybe because there are differences between those 3 methods?

Code:

char *path = "string";
...
char path[] = "string";
...
char path[100];
wsprintf(path, "%s", "string");

Please explain me why or show me a reference
TIA Hawk

In this function:

Code:

void FindDirectory(char *path)
{
...
char *pch;
pch = strrchr(path, '*');
memset(path, 'e', pch-path);
OR
memset(path, 'e', (int)pch-(int)path);
OR
path[pch-path] = 'e';
...
}

You've got three different methods that do three different things.

The third argument to memset() is the number of bytes to write at the location pointed to by the first argument. So you get something like:

Code:

char s[] = "string";
memset(s, 'e', 3);
puts(s); /* will print "eeeing" */

This is what your first memset() call is doing. Your second memset() call is not correct because you're casting pointers to ints. Don't do that. Subtracting pointers is perfectly valid in C (with the caveat that you can't subtract arbitrary pointers, but rather pointers that point inside the same object).

The last example you have will set just one character to 'e'. Namely, it will set the last '*' it finds to 'e'. If that's your goal, there is a clearer way to do it, if you weren't aware:

Code:

char *pch;
pch = strrchr(path, '*');
if(pch == NULL) return; /* your code should be doing something like this */
*pch = 'e';

"Clearer" because it's idiomatic.

Thanks for the answers.
As my function FindDirectory() does stuff which needs a pointer to a const char I stay with

Code:

char *path = "string";

but inside the function I copy the read only buffer to a modifiable buffer (as vart said) so I can change chars inside the string.

Code:

char fullpath[MAX_PATH];
memcpy(fullpath, path, strlen(path));
...
char *pch;
pch = strrchr(fullpath, '*');
if(pch == NULL)
{
	SendMessage(GetDlgItem(hwnd, IDC_LISTBOX1), LB_ADDSTRING, 0, (LPARAM)"Couldn't find *");
	continue;
}
*pch = '\0';

Problem solved.

Oh that is so true but isn't strcpy bad too if there's no NULL at the end of binary data?