musl: stdio

Implementation notes

The FILE object is struct _IO_FILE in src/internal/stdio_impl.h: read/write cursor pairs (rpos/rend, wpos/wbase, wend), mode flags (F_NORD, F_NOWR, F_SVB, F_APP), a per-stream lock word, and function pointers for read/write/seek/close. feof, ferror, getc_unlocked, putc_unlocked are macros over those fields. Streams live on the ofl list; __stdio_exit() (src/stdio/__stdio_exit.c) walks it at exit, flushing pending output and repositioning read streams. stderr is constructed with buf_size = 0 and lbf = -1, i.e. unbuffered by construction (src/stdio/stderr.c); stdout keeps lbf = '\n' and __stdout_write demotes it to full buffering when the fd is not a tty (src/stdio/__stdout_write.c). Flushes go out as a single writev of buffer plus pending argument (src/stdio/__stdio_write.c).

Deltas vs glibc (verified)

• Private struct: musl’s _IO_FILE is its own ~30-field layout; glibc’s is the libio _IO_FILE chain with exported _IO_2_1_std* objects (nm on libc.so.6).
• %as scanf: musl parses a as the C float conversion and implements only the C11 m allocation modifier (src/stdio/vfscanf.c); glibc treats %as as allocating. Code using %as misparses under musl.
• setvbuf reserves UNGET (8) bytes of a caller buffer and silently ignores undersized buffers (src/stdio/setvbuf.c).
• fmemopen/open_memstream/open_wmemstream/fopencookie all exist (include/stdio.h, src/stdio/fmemopen.c); no auto-wide behavior beyond what fwide/wide ops provide (no source for a wider claim; unverified).
• 1.2.5 WHATSNEW: snprintf no longer fails on n > INT_MAX.

Porting notes

Take the address of stdout/stderr, rely on glibc’s _IO_ struct fields, or use %as scanf and musl breaks. Buffers passed to setvbuf must leave UNGET headroom. Symbol-versioned stdio references have no musl counterpart; see the dynamic-loader page.