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12.12.8 Dynamically Allocating Formatted Output

The functions in this section do formatted output and place the results in dynamically allocated memory.

— Function: int asprintf (char **ptr, const char *template, ...)

This function is similar to sprintf, except that it dynamically allocates a string (as with malloc; see Unconstrained Allocation) to hold the output, instead of putting the output in a buffer you allocate in advance. The ptr argument should be the address of a char * object, and a successful call to asprintf stores a pointer to the newly allocated string at that location.

The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated.

Here is how to use asprintf to get the same result as the snprintf example, but more easily:

          /* Construct a message describing the value of a variable
             whose name is name and whose value is value. */
          char *
          make_message (char *name, char *value)
            char *result;
            if (asprintf (&result, "value of %s is %s", name, value) < 0)
              return NULL;
            return result;
— Function: int obstack_printf (struct obstack *obstack, const char *template, ...)

This function is similar to asprintf, except that it uses the obstack obstack to allocate the space. See Obstacks.

The characters are written onto the end of the current object. To get at them, you must finish the object with obstack_finish (see Growing Objects).