misc.c : Introduce ssh_writen()

A call to write() may perform a short write on a local file.
To avoid short writes, ssh_writen() can be used.

Signed-off-by: Eshan Kelkar <eshankelkar@galorithm.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Norbert Pocs <npocs@redhat.com>

include/libssh/misc.h

@@ -120,6 +120,7 @@ int ssh_tmpname(char *name);
 char *ssh_strreplace(const char *src, const char *pattern, const char *repl);
 
 ssize_t ssh_readn(int fd, void *buf, size_t nbytes);
+ssize_t ssh_writen(int fd, const void *buf, size_t nbytes);
 
 #ifdef __cplusplus
 }

src/misc.c

@@ -2036,4 +2036,55 @@ ssize_t ssh_readn(int fd, void *buf, size_t nbytes)
     return total_bytes_read;
 }
 
+/**
+ * @brief Write the requested number of bytes to a local file.
+ *
+ * A call to write() may perform a short write on a local file. This function
+ * can be used to avoid short writes.
+ *
+ * This function tries to write the requested number of bytes until those many
+ * bytes are written or some error occurs.
+ *
+ * On encountering an error due to an interrupt, this function ignores that
+ * error and continues trying to write the data.
+ *
+ * @param[in] fd          The file descriptor of the local file to write to.
+ *
+ * @param[in] buf         Pointer to a buffer in which data to write is stored.
+ *
+ * @param[in] nbytes      Number of bytes to write.
+ *
+ * @returns               Number of bytes written on success,
+ *                        SSH_ERROR on error with errno set to indicate the
+ *                        error.
+ */
+ssize_t ssh_writen(int fd, const void *buf, size_t nbytes)
+{
+    size_t total_bytes_written = 0;
+    ssize_t bytes_written;
+
+    if (fd < 0 || buf == NULL || nbytes == 0) {
+        errno = EINVAL;
+        return SSH_ERROR;
+    }
+
+    do {
+        bytes_written = write(fd,
+                              ((const char *)buf) + total_bytes_written,
+                              nbytes - total_bytes_written);
+        if (bytes_written == -1) {
+            if(errno == EINTR) {
+                /* Ignoring errors due to signal interrupts */
+                continue;
+            }
+
+            return SSH_ERROR;
+        }
+
+        total_bytes_written += (size_t)bytes_written;
+    } while (total_bytes_written < nbytes);
+
+    return total_bytes_written;
+}
+
 /** @} */