Is 'signed size_t' different from 'ssize_t'?

Issue

When I compile with ndk-build, it shows an error message:

warning: format '%zd' expects argument of type 'signed size_t', but argument 5 has type 'ssize_t'.

I thought ssize_t was the same as signed size_t, am I wrong?

Solution

In Android, size_t is unsigned int, but ssize_t is long int. To print or log ssize_t, simply use "%ld".

See sys/types.h:

Traditionally, bionic's ssize_t was "long int". This caused GCC to emit warnings when you pass a ssize_t to a printf-style function. The correct type is __kernel_ssize_t, which is "int", which isn't an ABI change for C code (because they're the same size) but is an ABI change for C++ because "int" and "long int" mangle to "i" and "l" respectively. So until we can fix the ABI, this change should not be propagated to the NDK. http://b/8253769.

Note that on other platforms, using %zd is prefferable.

Answered By - Alex Cohn