Valgrind Invalid read size of 1

Question

For the life of me i cant work out why I am getting an invalid read size of 1 for this code snippet, I'm pretty sure its got something to do with me abusing the char *url pointer...

char *extractURL(char request[])
{
char *space = malloc(sizeof(char *));
space = strchr(request, ' ')+1;
char *hostend = malloc(sizeof(char *));
hostend = strchr(request, '\r');
int length = hostend - space;
if (length > 0)
{
    printf("Mallocing %d bytes for url\n.", length+1);
    char *url = (char *)malloc((length+1)*sizeof(char));
    url = '\0';
    strncat(url, space, length);
    return url;
}
//else we have hit an error so return NULL
return NULL;    
}

The valgrind error I am getting is :

==4156== Invalid read of size 1
==4156==    at 0x4007518: strncat (mc_replace_strmem.c:206)
==4156==    by 0x8048D25: extractURL ()
==4156==    by 0x8048E59: processRequest ()
==4156==    by 0x8049881: main ()
==4156==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Can someone 'point' me in the right direction?

Solution

Here

char *url = malloc((length+1)*sizeof(char));
url = '\0';
strncat(url, space, length);

you immediately lose the malloced memory by setting url to NULL. Note that '\0' is 0, which is a null pointer constant. And then you try to strncat something to an invalid memory location.

You probably meant to set

*url = '\0';

there.

Comments

[DMcB1888]

That then leads to strncat doing a conditional jump on an unintialized value though does it not?

[Daniel Fischer]

Well, I would actually expect a segfault when calling strncat(NULL,something,n), but valgrind might report that, if it prefers.