From: Rich Felker <dalias@aerifal.cx>
Date: Sat, 24 Feb 2018 16:19:54 +0000 (-0500)
Subject: avoid use of readv syscall in __stdio_read backend when not needed
X-Git-Tag: v1.1.20~150
X-Git-Url: https://git.librecmc.org/?a=commitdiff_plain;h=e7eeeb9f2a4a358fb0bbed81e145ef5538ff60f0;p=oweals%2Fmusl.git

avoid use of readv syscall in __stdio_read backend when not needed

formally, calling readv with a zero-length first iov component should
behave identically to calling read on just the second component, but
presence of a zero-length iov component has triggered bugs in some
kernels and performs significantly worse than a simple read on some
file types.
---

diff --git a/src/stdio/__stdio_read.c b/src/stdio/__stdio_read.c
index 909c36a9..ea675da3 100644
--- a/src/stdio/__stdio_read.c
+++ b/src/stdio/__stdio_read.c
@@ -9,7 +9,8 @@ size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 	};
 	ssize_t cnt;
 
-	cnt = syscall(SYS_readv, f->fd, iov, 2);
+	cnt = iov[0].iov_len ? syscall(SYS_readv, f->fd, iov, 2)
+		: syscall(SYS_read, f->fd, iov[1].iov_base, iov[1].iov_len);
 	if (cnt <= 0) {
 		f->flags |= cnt ? F_ERR : F_EOF;
 		return 0;