fstat (nr 5)

Linux Signature

int fstat(int fd, struct stat *statbuf);

Description

Returns information about a file referred to by the file descriptor fd, writing it into the stat structure at statbuf.

Current Implementation

• Zero-fills the 144-byte struct stat buffer.
• Sets st_mode at offset 24 to S_IFCHR | 0666 (character device, read/write for all), regardless of which fd is queried.
• Always returns 0 (success).

This is sufficient for musl’s stdio initialisation, which calls fstat on stdout to determine whether it is a terminal.

Source: osl/src/syscalls/fs.rs — sys_fstat

Future Work

• Return different st_mode values depending on the fd (e.g., regular file vs. character device).
• Populate other stat fields (st_size, st_ino, st_dev, timestamps, etc.).
• Return -EBADF for invalid file descriptors.
• Validate that statbuf is a writable user-space address.