Why use a different C library?

Question

I was looking into using Apple's Blocks runtime with clang on Linux, and I uncovered the incompatibility issues with the GNU libc's <unistd.h>. Looking around a bit for more information I found one user suggesting:

Oh well time to use a different libc? I hear FreeBSD has a nice one! :-)

As seemingly implied, this is a rather silly reason to use a different libc. But that did make me think:

Why would you ever swap your C library? Are there advantages/disadvantages to using one over the other (e.g. GNU libc vs. FreeBSD libc)? (I suppose this also applies to libstdc++ vs. libc++).

Answer 1

I can think of three reasons off the top of my head.

1. Legacy Reasons: You have code written against the old Rogue Wave,
2. Size: There's musl and dietlibc,
3. To Perform Android Cross Compile using Bionic