{"id":148,"date":"2008-03-09T17:00:20","date_gmt":"2008-03-10T01:00:20","guid":{"rendered":"http:\/\/www.outflux.net\/blog\/archives\/2008\/03\/09\/using-select-on-a-fifo\/"},"modified":"2008-03-09T22:59:53","modified_gmt":"2008-03-10T06:59:53","slug":"using-select-on-a-fifo","status":"publish","type":"post","link":"https:\/\/outflux.net\/blog\/archives\/2008\/03\/09\/using-select-on-a-fifo\/","title":{"rendered":"using select on a fifo"},"content":{"rendered":"

The right way to handle on-going input from file descriptors is to use select()<\/code>. All readable events are flagged (one such event is “end of file”, which is indicated by a 0-sized read()<\/code>). For example, if we’re reading from file descriptor fd<\/code>:<\/p>\n

\r\n  fd_set rfds;\r\n  int rc;\r\n\r\n  FD_ZERO(&rfds);\r\n  FD_SET(fd, &rfds);\r\n\r\n  tv.tv_sec = 1;\r\n  tv.tv_usec = 0;\r\n\r\n  rc = select(fd + 1, &rfds, NULL, NULL, &tv);\r\n  if (rc > 0) {\r\n    char buf[80];\r\n    ssize_t got = read(fd, buf, sizeof(buf));\r\n    if (got < 0) {\r\n      perror(\"read\");\r\n      return 1;\r\n    }\r\n    else if (got == 0) {\r\n      printf(\"EOF\\\\n\");\r\n      return 1;\r\n    }\r\n    else {\r\n      printf(\"read bytes: %d\\\\n\", got);\r\n    }\r\n  }\r\n<\/pre>\n
When dealing with sockets, the above loop is sane — EOF means the other end hung up and you’ll never get more data from the file descriptor.  In the case of a FIFO, however, “0 length read” means there are no more FIFO writers — but more could attach later and continue feeding in data!  The problem with this is that select misbehaves and marks the file descriptor as “EOF” forever.  Only the initial select()<\/code> call blocks until there is something to read — once it hits EOF, select will always immediately return, defeating the purpose of select()<\/code>.<\/p>\n
One solution is to re-open the FIFO, but you might miss writers<\/a> between your 0-length read()<\/code> and the next open()<\/code>.<\/p>\n
The seemingly correct solution<\/a> is rather simple: the FIFO reader should open the FIFO as a writer also.  In this case, select()<\/code> never thinks all the writers have vanished, so there is never an EOF condition, and the reader never misses any writers.  So, instead of O_RDONLY<\/code>, use:<\/p>\n
\r\nfd = open(FIFO_PATHNAME, O_RDWR<\/b> | O_NONBLOCK);\r\n<\/pre>\n
© 2008, Kees Cook<\/a>. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License<\/a>.
\"CC<\/a> <\/p>\n","protected":false},"excerpt":{"rendered":"
The right way to handle on-going input from file descriptors is to use select(). All readable events are flagged (one such event is “end of file”, which is indicated by a 0-sized read()). For example, if we’re reading from file descriptor fd: fd_set rfds; int rc; FD_ZERO(&rfds); FD_SET(fd, &rfds); tv.tv_sec = 1; tv.tv_usec = 0; […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[14],"tags":[],"_links":{"self":[{"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/posts\/148"}],"collection":[{"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/comments?post=148"}],"version-history":[{"count":0,"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/posts\/148\/revisions"}],"wp:attachment":[{"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/media?parent=148"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/categories?post=148"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/outflux.net\/blog\/wp-json\/wp\/v2\/tags?post=148"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}