6

Is the GNU function asprintf (print to allocated string) thread-safe?

(IIC, basically, this boils down to the question whether malloc is thread-safe.)

Consider the example code:

#define _GNU_SOURCE
#include <stdio.h>

#include "getValue.h"

char * getValue(int key) {
  char * value;
  asprintf(&value, "%d", key); // TODO: No error handling!
  // If memory allocation wasn't possible, or some other error occurs,  these  functions  will
  // return -1, and the contents of strp is undefined.
  return value;
}

Here, I do not touch any global variables. What if my getValue gets called in concurrent threads? No bad things will occur, will they?

RzR
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imz -- Ivan Zakharyaschev
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    Users would be doomed if malloc weren't thread-safe: http://stackoverflow.com/questions/855763/is-malloc-thread-safe – stefan Feb 17 '15 at 13:23

2 Answers2

6

Yes, it's thread safe except when it reads the locale.

asprintf

Function: int asprintf (char **ptr, const char *template, …)
Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem

About the 'locale' exception, in particular:

Functions annotated with locale as an MT-Safety issue read from the locale object without any form of synchronization. Functions annotated with locale called concurrently with locale changes may behave in ways that do not correspond to any of the locales active during their execution, but an unpredictable mix thereof.

These kinds of functions are known as "conditionally" multithread safe because, in some contexts, they turn out not to be, so the programmer needs to take care of that.

edmz
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  • I've just looked at the referenced [POSIX Safety Concepts](https://www.gnu.org/software/libc/manual/html_node/POSIX-Safety-Concepts.html#POSIX-Safety-Concepts), and there is no hint about the locale exception. So, for me to able to read these lines like the "Preliminary: ..." above, I need additionally to understand the meaning of the modifiers like "locale", "heap", "mem". Where can I look up those? – imz -- Ivan Zakharyaschev Feb 17 '15 at 13:38
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    Some platforms offer `asprintf_l` which takes a `locale_t` parameter and should be completely thread-safe. – nwellnhof Feb 17 '15 at 14:28
2

glibc is free software and is probably the only (or the most important) library implementing asprintf.

So you can study (and even contribute to improve) its source code. See its stdio-common/asprintf.c & libio/vasprintf.c source files.

It looks like indeed, it is calling in a thread safe manner malloc and related things.

Basile Starynkevitch
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