asprintf() usage

[keshavp]

Hi, can someone please help me in understanding the argument 'char **ret' in the prototype of asprintf() function?
It is written in the man pages that "asprintf() return a pointer to a buffer sufficiently large to hold the string in the 'ret' argument".
I have a declaration in my program
char **buf;

it gives me a bus error when i run the program.
Will someone Please help me?
regards,
keshav

 

[rbobbitt]

You're probably not initializing buf properly before passing it to asprintf().

For your purposes, you most likely don't need to declare buf as a double pointer. Try something like this:

char *buf;
const char foo[]="hello";

asprintf(&buf,"!!%s!!",foo);

if (buf!=NULL) {
/* asprintf() succeeded and buf points to "!!hello!!" */

/* Sometime later ... */
free(buf);
}

By passing the address of buf, you give asprintf() the actual pointer rather than just a copy of it (C has no pass by reference), which is what asprintf() needs.

If you really want to use a double pointer for some reason, you'll have to initialize and pass it like this:

char **buf;
const char foo[]="hello";

buf=malloc(sizeof *buf); /* Allocate space for 1 pointer to char */

if (buf!=NULL) {
asprintf(buf,"!!%s!!",foo);
if (*buf!=NULL) {
/* asprintf() succeeded and *buf points to "!!hello!!" */

/* Sometime later ... */
free(*buf);
}
free(buf);
}

This is a silly way to do it, obviously the 1st example using a single pointer is more sensible for this sort of thing.

It should be pointed out that asprintf() isn't part of C -- it's a compiler-specific extension that's not available on all C implementations (for those that might try using it and wonder why their compiler says that it doesn't exist).

Russ
bobbitts@hotmail.com

http://home.earthlink.net/~bobbitts