BIO_READ(3)

💅 BIO_READ(3) — the BIO I/O functions, bestie edition

okay so this is a page from the Library Functions Manual for BIO_READ(3) and honestly it's a whole vibe — it's all about reading and writing through BIOs. grab a strawberry drink 🍓 and let's go.

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✨ NAME ✨

BIO_read, BIO_number_read, BIO_gets, BIO_write, BIO_puts, BIO_indent, BIO_number_written — the BIO I/O functions, aka the whole squad 🫶

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🌸 SYNOPSIS 🌸

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

int
BIO_read(BIO *b, void *buf, int len);

unsigned long
BIO_number_read(BIO *b);

int
BIO_gets(BIO *b, char *buf, int size);

int
BIO_write(BIO *b, const void *buf, int len);

int
BIO_puts(BIO *b, const char *string);

int
BIO_indent(BIO *b, int indent, int max);

unsigned long
BIO_number_written(BIO *b);

like, that's the whole cast. no cap. 👑

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🦋 DESCRIPTION 🦋

BIO_read() attempts to read len bytes from b and places the data in buf. that's it, that's the function 💅

BIO_number_read() returns the grand total of bytes read from b using BIO_read() so far. and bestie, listen — bytes read with BIO_gets() do not count. sorry, them's the rules. BIO_new(3) and BIO_set(3) initialize the counter to 0. and when you're reading very large amounts of data, the counter will eventually wrap around from ULONG_MAX to 0. it's giving "the odometer rolled over" 😭

BIO_gets() performs the BIOs "gets" operation and places the data in buf. usually this operation will attempt to read a line of data from the BIO of maximum length size - 1. but there are exceptions to this, however — for example BIO_gets() on a digest BIO will calculate and return the digest, and other BIOs may not support BIO_gets() at all. the returned string is always NUL-terminated, which is the one thing you can count on 🎀

BIO_write() attempts to write len bytes from buf to b.

BIO_puts() attempts to write the NUL-terminated string to b.

BIO_indent() attempts to write indent space characters to b, but not more than max characters. lowkey the politest function here.

BIO_number_written() returns the grand total of bytes written to b using BIO_write(), BIO_puts(), and BIO_indent() so far. BIO_new(3) and BIO_set(3) initialize the counter to 0. and yeah, same deal — when writing very large amounts of data, the counter will eventually wrap around from ULONG_MAX to 0. ⭐

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⚠️ okay this next part is the important lore, don't skip it ⚠️

One technique sometimes used with blocking sockets is to use a system call (such as select(2), poll(2) or equivalent) to determine when data is available and then call read(2) to read the data.

The equivalent with BIOs — that is, calling select(2) on the underlying I/O structure and then calling BIO_read() to read the data — should not be used 🚩 because a single call to BIO_read() can cause several reads (and writes in the case of SSL BIOs) on the underlying I/O structure and may block as a result.

Instead, select(2) (or equivalent) should be combined with non-blocking I/O so successive reads will request a retry instead of blocking. that's the move, trust 🙈

See BIO_should_retry(3) for details of how to determine the cause of a retry and other I/O issues.

If the BIO_gets() function is not supported by a BIO then it is possible to work around this by adding a buffering BIO BIO_f_buffer(3) to the chain. little hack, iconic behavior 🧚

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💖 RETURN VALUES 💖

• BIO_indent() returns 1 if successful, even if nothing was written, or 0 if writing fails.
• BIO_number_read() and BIO_number_written() return a number of bytes, or 0 if b is a NULL pointer.
• The other functions return either the amount of data successfully read or written (if the return value is positive) or that no data was successfully read or written if the result is 0 or -1. If the return value is -2, then the operation is not implemented in the specific BIO type. and the trailing NUL is not included in the length returned by BIO_gets().

😭 big caveat incoming: a 0 or -1 return is not necessarily an indication of an error. In particular when the source/sink is non-blocking or of a certain type it may merely be an indication that no data is currently available and that the application should retry the operation later. so like... breathe. it's not always bad news, bestie.

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🔥 SEE ALSO 🔥

BIO_meth_new(3), BIO_new(3), BIO_should_retry(3)

go read those too, iykyk 💜

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🌈 HISTORY 🌈

BIO_read(), BIO_gets(), BIO_write(), and BIO_puts() first appeared in SSLeay 0.6.0. BIO_number_read() and BIO_number_written() first appeared in SSLeay 0.6.5. These functions have been available since OpenBSD 2.4.

BIO_indent() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.4. look at the baby of the group growing up 🥹

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and that's the page! mwah 💋 this darling was served up by Debian, dated June 8, 2025 — periodt. go forth and read your BIOs responsibly, bestie 🎀✨