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--- informatica:sol:laboratorio20:esercitazionib:readnwriten [03/05/2020 alle 07:40 (5 anni fa)] – creata Massimo Torquati
+++ informatica:sol:laboratorio20:esercitazionib:readnwriten [03/05/2020 alle 10:44 (5 anni fa)] (versione attuale) – Massimo Torquati
@@ Linea 1: / Linea 1: @@
 ===== Implementazione delle funzioni "readn" e "writen" ====
-(tratto da "Advanced Programming In the UNIX Environment" 3rd Edition by W. Richard Stevens and Stephen A. Rago, 2013, Addison-Wesley)
+(tratto da "Advanced Programming In the UNIX Environment" by W. Richard Stevens and Stephen A. Rago, 2013, 3rd Edition, Addison-Wesley)
@@ Linea 18: / Linea 18: @@
 We’ll never see this happen when reading or writing a disk file, except when the file
 system runs out of space or we hit our quota limit and we can’t write all that we
-requested.
+requested.\\
-<code c>
-ssize_t /* Read "n" bytes from a descriptor */
-readn(int fd, void *ptr, size_t n) {
-}
-<\code>
 Generally, when we read from or write to a pipe, network device, or terminal, we
 need to take these characteristics into consideration. We can use the readn and
@@ Linea 32: / Linea 24: @@
 handle a return value that’s possibly less than requested. These two functions simply
 call read or write as many times as required to read or write the entire N bytes of
-data.
+data.\\
+We call writen whenever we’re writing to one of the file types that we mentioned,
+but we call readn only when we know ahead of time that we will be receiving a certain
+number of bytes. Figure 14.24 shows implementations of readn and writen that we
+will use in later examples.
+Note that if we encounter an error and have previously read or written any data, we
+return the amount of data transferred instead of the error. Similarly, if we reach the end
+of file while reading, we return the number of bytes copied to the caller’s buffer if we
+already read some data successfully and have not yet satisfied the amount requested.
+<code c>
+ssize_t  /* Read "n" bytes from a descriptor */
+readn(int fd, void *ptr, size_t n) {
+   size_t   nleft;
+   ssize_t  nread;
+   nleft = n;
+   while (nleft > 0) {
+     if((nread = read(fd, ptr, nleft)) < 0) {
+        if (nleft == n) return -1; /* error, return -1 */
+        else break; /* error, return amount read so far */
+     } else if (nread == 0) break; /* EOF */
+     nleft -= nread;
+     ptr   += nread;
+   }
+   return(n - nleft); /* return >= 0 */
+}
+ssize_t  /* Write "n" bytes to a descriptor */
+writen(int fd, void *ptr, size_t n) {
+   size_t   nleft;
+   ssize_t  nwritten;
+   nleft = n;
+   while (nleft > 0) {
+     if((nwritten = write(fd, ptr, nleft)) < 0) {
+        if (nleft == n) return -1; /* error, return -1 */
+        else break; /* error, return amount written so far */
+     } else if (nwritten == 0) break;
+     nleft -= nwritten;
+     ptr   += nwritten;
+   }
+   return(n - nleft); /* return >= 0 */
+}
+</code>