misc.c : Introduce ssh_readn()

A call to read() may peform a short read from a local file even when
sufficient data is present in the file. ssh_readn() can be used
instead of read() to avoid such short reads.

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

@@ -21,6 +21,21 @@
 #ifndef MISC_H_
 #define MISC_H_
 
+#ifdef _WIN32
+
+# ifdef _MSC_VER
+#  ifndef _SSIZE_T_DEFINED
+#   undef ssize_t
+#   include <BaseTsd.h>
+    typedef _W64 SSIZE_T ssize_t;
+#   define _SSIZE_T_DEFINED
+#  endif /* _SSIZE_T_DEFINED */
+# endif /* _MSC_VER */
+
+#else
+# include <sys/types.h>
+#endif /* _WIN32 */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -103,6 +118,9 @@ int ssh_newline_vis(const char *string, char *buf, size_t buf_len);
 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);
+
 #ifdef __cplusplus
 }
 #endif

src/misc.c

@@ -1976,4 +1976,64 @@ char *ssh_strerror(int err_num, char *buf, size_t buflen)
 #endif /* defined(__linux__) && defined(__GLIBC__) && defined(_GNU_SOURCE) */
 }
 
+/**
+ * @brief Read the requested number of bytes from a local file.
+ *
+ * A call to read() may perform a short read even when sufficient data is
+ * present in the file. This function can be used to avoid such short reads.
+ *
+ * This function tries to read the requested number of bytes from the file
+ * until one of the following occurs :
+ *     - Requested number of bytes are read.
+ *     - EOF is encountered before reading the requested number of bytes.
+ *     - An error occurs.
+ *
+ * On encountering an error due to an interrupt, this function ignores that
+ * error and continues trying to read the data.
+ *
+ * @param[in] fd          The file descriptor of the local file to read from.
+ *
+ * @param[out] buf        Pointer to a buffer in which read data will be
+ *                        stored.
+ *
+ * @param[in] nbytes      Number of bytes to read.
+ *
+ * @returns               Number of bytes read on success,
+ *                        SSH_ERROR on error with errno set to indicate the
+ *                        error.
+ */
+ssize_t ssh_readn(int fd, void *buf, size_t nbytes)
+{
+    size_t total_bytes_read = 0;
+    ssize_t bytes_read;
+
+    if (fd < 0 || buf == NULL || nbytes == 0) {
+        errno = EINVAL;
+        return SSH_ERROR;
+    }
+
+    do {
+        bytes_read = read(fd,
+                          ((char *)buf) + total_bytes_read,
+                          nbytes - total_bytes_read);
+        if (bytes_read == -1) {
+            if (errno == EINTR) {
+                /* Ignoring errors due to signal interrupts */
+                continue;
+            }
+
+            return SSH_ERROR;
+        }
+
+        if (bytes_read == 0) {
+            /* EOF encountered on the local file before reading nbytes */
+            break;
+        }
+
+        total_bytes_read += (size_t)bytes_read;
+    } while (total_bytes_read < nbytes);
+
+    return total_bytes_read;
+}
+
 /** @} */