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TLS_READ(3)

Library Functions Manual

TLS_READ(3)

NAME

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

SYNOPSIS

#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);

void
tls_reset(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() 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; 
		} 
	} 
} 
...

HISTORY

tls_read(), tls_write(), tls_error(), tls_close(), and tls_reset() 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 9, 2019

Debian