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terminal: add comments describing the sync/async writing modes

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Daniel Eklöf 2019-11-04 12:36:43 +01:00
parent a3f79ead94
commit a706c1e804
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GPG key ID: 5BBD4992C116573F
1 changed files with 25 additions and 1 deletions
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26
terminal.c
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@ -29,6 +29,22 @@
enum ptmx_write_status {PTMX_WRITE_DONE, PTMX_WRITE_REMAIN, PTMX_WRITE_ERR};
/*
* Primitive that writes data to the slave/client
*
* _data: points to the beginning of the buffer
* len: total size of the data buffer
* idx: pointer to byte offset into data buffer - writing starts here.
*
* Thus, the total amount of data to write is (len - *idx). *idx is
* updated such that it points to the next unwritten byte in the data
* buffer.
*
* I.e. if the return value is:
* - PTMX_WRITE_DONE, then the *idx == len.
* - PTMX_WRITE_REMAIN, then *idx < len
* - PTMX_WRITE_ERR, there was an error, and no data was written
*/
static enum ptmx_write_status
to_slave(struct terminal *term, const void *_data, size_t len, size_t *idx)
{
@ -68,6 +84,7 @@ term_to_slave(struct terminal *term, const void *_data, size_t len)
switch (to_slave(term, _data, len, &(size_t){0})) {
case PTMX_WRITE_REMAIN:
/* Switch to asynchronous mode; let FDM write the remaining data */
if (!fdm_event_add(term->fdm, term->ptmx, EPOLLOUT))
return false;
goto enqueue_data;
@ -81,6 +98,10 @@ term_to_slave(struct terminal *term, const void *_data, size_t len)
return false;
enqueue_data:
/*
* We're in asynchronous mode - push data to queue and let the FDM
* handler take care of it
*/
{
void *copy = malloc(len);
memcpy(copy, _data, len);
@ -92,7 +113,6 @@ enqueue_data:
};
tll_push_back(term->ptmx_buffer, queued);
}
return true;
}
@ -100,8 +120,11 @@ static bool
fdm_ptmx_out(struct fdm *fdm, int fd, int events, void *data)
{
struct terminal *term = data;
/* If there is no queued data, then we shouldn't be in asynchronous mode */
assert(tll_length(term->ptmx_buffer) > 0);
/* Don't use pop() since we may not be able to write the entire buffer */
tll_foreach(term->ptmx_buffer, it) {
switch (to_slave(term, it->item.data, it->item.len, &it->item.idx)) {
case PTMX_WRITE_DONE:
@ -114,6 +137,7 @@ fdm_ptmx_out(struct fdm *fdm, int fd, int events, void *data)
}
}
/* No more queued data, switch back to synchronous mode */
fdm_event_del(term->fdm, term->ptmx, EPOLLOUT);
return true;
}