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Excellent stuff. My couple of notes.. 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely

Excellent stuff. My couple of notes.. 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. 2. printf of floats used to require linking to the math library on some platforms (-lm) 3. I am pathetically grateful for nice diagnostics you get when you use the wrong % formatter in more recent C compilers. This used to be a rich source of errors and non-portability.

lelf 7y ago
4. On GNU/Linux you can add your own %-conversions (e. g. %Y) with register_printf_function.
- saagarjha 7y ago
  register_printf_function is deprecated; you should use register_printf_specifier instead.
  - eqvinox 7y ago
    I strongly advise against using either since both are not only non-portable, but also break any compiler support for getting you warnings when you're using it wrong. The Linux kernel style suffix extensions (cf. https://www.kernel.org/doc/Documentation/printk-formats.txt https://www.kernel.org/doc/Documentation/printk-formats.txt) are much better on the latter front.
s_m_t 7y ago
>1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. I feel like you've just saved my future self from a terrible bug in the future. Thanks
brandmeyer 7y ago
You can also use __attribute__((format(printf, ...))) for functions of yours that ultimately pass their varargs to a member of the printf family. It is exceedingly rare for malloc() to give you a recovery chance on failure. Folks are so used to the Linux model of overcommit+OOM killer that almost nobody's code can gracefully deal with memory exhaustion.
saagarjha 7y ago
> printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. Alternatively: don’t call printf from your interposed malloc, bad things will happen ;)
luke0016 7y ago
> 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. You can avoid this by doing something like: setvbuf(stdout, NULL, _IONBF, 0);
- saagarjha 7y ago
  You sure about that? This seems to suggest otherwise: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf-internal.c;h=547a3a868b4668bf615cf3f39a92e3c11cbb98ad;hb=HEAD#l1343 https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-commo...
  - crististm 7y ago
    It will allocate a buffer on the stack: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf-internal.c;h=547a3a868b4668bf615cf3f39a92e3c11cbb98ad;hb=HEAD#l2346 https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-commo...
kstenerud 7y ago
Also, don't use printf in a signal handler for the same reason. Any attempt to malloc or free within a signal handler context runs the risk of a deadlock.
- deleted 7y ago
  [deleted]
eqvinox 7y ago
> 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. I'd like to point out that FreeBSD's printf only does this if: 1. you use a lot of arguments (i.e. a compile-time decision, and quite rare) 2. use wchar_t support (also a compile-time decision and quite rare) 3. the output is to something that buffers/mallocs (i.e. stdio) 4. locale support does it for some reason (I simply haven't checked this) Which makes it great to use in e.g. crash handlers. It's actually Async-Signal-Safe if you cut out the stdio bits. And it's available under BSD license... and actually a nice read (more on that in a separate post.)

lelf 7y ago

4. On GNU/Linux you can add your own %-conversions (e. g. %Y) with register_printf_function.

saagarjha 7y ago
register_printf_function is deprecated; you should use register_printf_specifier instead.
- eqvinox 7y ago
  I strongly advise against using either since both are not only non-portable, but also break any compiler support for getting you warnings when you're using it wrong. The Linux kernel style suffix extensions (cf. https://www.kernel.org/doc/Documentation/printk-formats.txt https://www.kernel.org/doc/Documentation/printk-formats.txt) are much better on the latter front.

s_m_t 7y ago

>1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. I feel like you've just saved my future self from a terrible bug in the future. Thanks

brandmeyer 7y ago

You can also use __attribute__((format(printf, ...))) for functions of yours that ultimately pass their varargs to a member of the printf family. It is exceedingly rare for malloc() to give you a recovery chance on failure. Folks are so used to the Linux model of overcommit+OOM killer that almost nobody's code can gracefully deal with memory exhaustion.

saagarjha 7y ago

> printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. Alternatively: don’t call printf from your interposed malloc, bad things will happen ;)

luke0016 7y ago

> 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. You can avoid this by doing something like: setvbuf(stdout, NULL, _IONBF, 0);

saagarjha 7y ago
You sure about that? This seems to suggest otherwise: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf-internal.c;h=547a3a868b4668bf615cf3f39a92e3c11cbb98ad;hb=HEAD#l1343 https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-commo...
- crististm 7y ago
  It will allocate a buffer on the stack: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf-internal.c;h=547a3a868b4668bf615cf3f39a92e3c11cbb98ad;hb=HEAD#l2346 https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-commo...

kstenerud 7y ago

Also, don't use printf in a signal handler for the same reason. Any attempt to malloc or free within a signal handler context runs the risk of a deadlock.

deleted 7y ago
[deleted]

eqvinox 7y ago

> 1. printf may malloc so don't use it in an out of memory situation. Though I think this problem is vanishingly unlikely these days. I'd like to point out that FreeBSD's printf only does this if: 1. you use a lot of arguments (i.e. a compile-time decision, and quite rare) 2. use wchar_t support (also a compile-time decision and quite rare) 3. the output is to something that buffers/mallocs (i.e. stdio) 4. locale support does it for some reason (I simply haven't checked this) Which makes it great to use in e.g. crash handlers. It's actually Async-Signal-Safe if you cut out the stdio bits. And it's available under BSD license... and actually a nice read (more on that in a separate post.)