BIO_S_FD(3)

okay so bestie, welcome to BIO_S_FD(3) from the Library Functions Manual — this is the file descriptor BIO page, and honestly it's giving “wrap your fd in a BIO and go” energy 💅✨

🌸 NAME 🌸

BIO_s_fd, BIO_set_fd, BIO_get_fd, BIO_new_fd, BIO_fd_non_fatal_error, BIO_fd_should_retry — file descriptor BIO. That's the whole squad, no cap 🫶

✨ SYNOPSIS ✨

First, you're rolling with the libcrypto library and this include:

library “libcrypto” #include <openssl/bio.h>

and these prototypes, exactly as they live:

const BIO_METHOD *
BIO_s_fd(void);

long
BIO_set_fd(BIO *b, int fd, long close_flag);

long
BIO_get_fd(BIO *b, int *c);

BIO *
BIO_new_fd(int fd, int close_flag);

int
BIO_fd_non_fatal_error(int errnum);

int
BIO_fd_should_retry(int retval);

💖 DESCRIPTION 💖

• BIO_s_fd() returns the file descriptor BIO method. This is a wrapper around the platform's file descriptor routines such as read(2) and write(2).
• BIO_read(3) and BIO_write(3) read or write the underlying descriptor. BIO_puts(3) is supported but BIO_gets(3) is not — so don't come for BIO_gets(3), it's not invited to this party 🙈
• If the close flag is set, close(2) is called on the underlying file descriptor when the BIO is freed.
• BIO_reset(3) attempts to set the file pointer to the start of the file using lseek(fd, 0, 0).
• BIO_seek(3) sets the file pointer to position ofs from start of file using lseek(fd, ofs, 0).
• BIO_tell(3) returns the current file position by calling lseek(fd, 0, 1).
• BIO_set_fd() sets the file descriptor of BIO b to fd and the close flag to close_flag.
• BIO_get_fd() places the file descriptor in c if it is not NULL and also returns the file descriptor.
• BIO_new_fd() returns a file descriptor BIO using fd and close_flag.
• BIO_fd_non_fatal_error() determines whether the error status code errnum represents a recoverable error. BIO_fd_should_retry() determines whether a recoverable error occurred by inspecting both errno(2) and retval, which is supposed to usually be the return value of a previously called function like BIO_read(3) or BIO_write(3). These two functions are mostly used internally; in application code, it is usually easier and more robust to use BIO_should_retry(3), which works for any BIO type. iykyk 💅
• The behaviour of BIO_read(3) and BIO_write(3) depends on the behavior of the platform's read(2) and write(2) calls on the descriptor. If the underlying file descriptor is in a non-blocking mode, then the BIO will behave in the manner described in the BIO_read(3) and BIO_should_retry(3) manual pages.

Warning bestie: File descriptor BIOs should not be used for socket I/O. Use socket BIOs instead. This is not the vibe you're looking for for sockets, fr fr 🫶

BIO_ctrl(3) cmd arguments correspond to macros as follows:

• BIO_C_FILE_SEEK → BIO_seek(3)
• BIO_C_FILE_TELL → BIO_tell(3)
• BIO_C_GET_FD → BIO_get_fd()
• BIO_C_SET_FD → BIO_set_fd()
• BIO_CTRL_GET_CLOSE → BIO_get_close(3)
• BIO_CTRL_RESET → BIO_reset(3)
• BIO_CTRL_SET_CLOSE → BIO_set_close(3)

💖 RETURN VALUES 💖

• BIO_s_fd() returns the file descriptor BIO method.
• When called on a file descriptor BIO object, BIO_method_type(3) returns the constant BIO_TYPE_FD and BIO_method_name(3) returns a pointer to the static string "file descriptor".
• BIO_set_fd() always returns 1. Iconic consistency, honestly 💅
• BIO_get_fd() returns the file descriptor or -1 if the BIO has not been initialized.
• BIO_new_fd() returns the newly allocated BIO or NULL if an error occurred.
• BIO_fd_non_fatal_error() returns 1 if errnum is EAGAIN, EALREADY, EINPROGRESS, EINTR, or ENOTCONN and 0 otherwise, even if errnum is 0.
• BIO_fd_should_retry() returns 1 if BIO_fd_non_fatal_error(errno) is 1 and retval is either 0 or -1, or 0 otherwise.

🦋 EXAMPLES 🦋

This is a file descriptor BIO version of "Hello World":

BIO *out;
out = BIO_new_fd(fileno(stdout), BIO_NOCLOSE);
BIO_printf(out, "Hello World\n");
BIO_free(out);

🌈 SEE ALSO 🌈

• BIO_new(3)
• BIO_read(3)
• BIO_s_socket(3)
• BIO_seek(3)
• BIO_should_retry(3)

👑 HISTORY 👑

• BIO_s_fd(), BIO_set_fd(), and BIO_get_fd() first appeared in SSLeay 0.6.0.
• BIO_fd_should_retry() first appeared in SSLeay 0.6.5.
• BIO_new_fd() and BIO_fd_non_fatal_error() first appeared in SSLeay 0.8.0.
• All these functions have been available since OpenBSD 2.4. Vintage but still main character energy 👑

---

mwah 💋 this page is from Debian, dated September 10, 2026 — periodt, go wrap a file descriptor, bestie 💅🎀