TLS_READ(3) | Library Functions Manual | TLS_READ(3)

# NAME

`tls_read`, `tls_write`, `tls_handshake`, `tls_error`, `tls_close` — use a TLS connection

# SYNOPSIS

library “libtls” libssl libcrypto
`#include <tls.h>`

ssize_t
`tls_read`(struct tls *ctx, void *buf, size_t buflen);

ssize_t
`tls_write`(struct tls *ctx, const void *buf, size_t buflen);

int
`tls_handshake`(struct tls *ctx);

const char *
`tls_error`(struct tls *ctx);

int
`tls_close`(struct tls *ctx);

# DESCRIPTION

`tls_read`() reads buflen bytes of data from the socket into buf. It returns the amount of data read.

`tls_write`() writes buflen bytes of data from buf to the socket. It returns the amount of data written.

`tls_handshake`() explicitly performs the TLS handshake. It is only necessary to call this function if you need to guarantee that the handshake has completed, as both `tls_read`() and `tls_write`() automatically perform the TLS handshake when necessary.

The `tls_error`() function may be used to retrieve a string containing more information about the most recent error relating to a context.

`tls_close`() closes a connection after use. Only the TLS layer will be shut down and the caller is responsible for closing the file descriptors, unless the connection was established using tls_connect(3) or tls_connect_servername(3). After closing the connection, ctx can be passed to tls_free(3).

# RETURN VALUES

`tls_read`() and `tls_write`() return a size on success or -1 on error.

`tls_handshake`() and `tls_close`() return 0 on success or -1 on error.

The `tls_read`(), `tls_write`(), `tls_handshake`(), and `tls_close`() functions also have two special return values:

`TLS_WANT_POLLIN`
  The underlying read file descriptor needs to be readable in order to continue.
`TLS_WANT_POLLOUT`
  The underlying write file descriptor needs to be writeable in order to continue.

In the case of blocking file descriptors, the same function call should be repeated immediately. In the case of non-blocking file descriptors, the same function call should be repeated when the required condition has been met.

Callers of these functions cannot rely on the value of the global errno. To prevent mishandling of error conditions, `tls_read`(), `tls_write`(), `tls_handshake`(), and `tls_close`() all explicitly clear errno.

`tls_error`() returns `NULL` if no error occurred with ctx during or since the last call to `tls_handshake`(), `tls_read`(), `tls_write`(), `tls_close`(), or tls_reset(3) involving ctx, or if memory allocation failed while trying to assemble the string describing the most recent error related to ctx.

# EXAMPLES

The following example demonstrates how to handle TLS writes on a blocking file descriptor:

    ...
    while (len > 0) {
    	ssize_t ret;

    	ret = tls_write(ctx, buf, len);
    	if (ret == TLS_WANT_POLLIN || ret == TLS_WANT_POLLOUT)
    		continue;
    	if (ret == -1)
    		errx(1, "tls_write: %s", tls_error(ctx));
    	buf += ret;
    	len -= ret;
    }
    ...

The following example demonstrates how to handle TLS writes on a non-blocking file descriptor using poll(2):

    ...
    pfd[0].fd = fd;
    pfd[0].events = POLLIN|POLLOUT;
    while (len > 0) {
    	nready = poll(pfd, 1, 0);
    	if (nready == -1)
    		err(1, "poll");
    	if ((pfd[0].revents & (POLLERR|POLLNVAL)))
    		errx(1, "bad fd %d", pfd[0].fd);
    	if ((pfd[0].revents & (pfd[0].events|POLLHUP))) {
    		ssize_t ret;

    		ret = tls_write(ctx, buf, len);
    		if (ret == TLS_WANT_POLLIN)
    			pfd[0].events = POLLIN;
    		else if (ret == TLS_WANT_POLLOUT)
    			pfd[0].events = POLLOUT;
    		else if (ret == -1)
    			errx(1, "tls_write: %s", tls_error(ctx));
    		else {
    			buf += ret;
    			len -= ret;
    		}
    	}
    }
    ...

# SEE ALSO

tls_accept_socket(3), tls_configure(3), tls_conn_version(3), tls_connect(3), tls_init(3), tls_ocsp_process_response(3)

# HISTORY

`tls_read`(), `tls_write`(), `tls_error`(), and `tls_close`() appeared in OpenBSD 5.6 and got their final names in OpenBSD 5.7.

`tls_handshake`() appeared in OpenBSD 5.9.

# AUTHORS

Joel Sing <jsing@openbsd.org> with contributions from
Bob Beck <beck@openbsd.org>

# CAVEATS

The function `tls_error`() returns an internal pointer. It must not be freed by the application, or a double free error will occur. The pointer will become invalid when the next error occurs with ctx. Consequently, if the application may need the message at a later time, it has to copy the string before calling the next libtls function involving ctx, or a segmentation fault or read access to unintended data is the likely result.

July 7, 2025 | Debian