Dynamic Output (The GNU C Library)

#### 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, …)` ¶

  Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

  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. Current and future versions of the GNU C Library write a null pointer to ‘`*ptr`’ upon failure. To achieve similar behavior with previous versions, initialize ‘`*ptr`’ to a null pointer before calling `asprintf`. (Specifications for `asprintf` only require a valid pointer value in ‘`*ptr`’ if `asprintf` succeeds, but no implementations are known which overwrite a null pointer with a pointer that cannot be freed on failure.)

  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, …)` ¶

  Preliminary: | MT-Safe race:obstack locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

  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).