[Libguestfs] [nbdkit PATCH 3/3] log: Guarantee the operation of %m in nbdkit_error()
Eric Blake
eblake at redhat.com
Thu Nov 29 17:21:30 UTC 2018
printf("%m") is a useful glibc extension, if you are sure
that errno is unchanged between the time of the actual problem
and your output message; it beats the longhand of writing
strerror(errno) yourself. However, BSD libc does not support
the extension, and can result in awkward error messages like
pread: m
instead of an intended
pread: Input/output error
Solve the problem by probing at configure time if %m works,
(this is a runtime test, but can be overridden for testing or
cross-compiling by setting the nbdkit_cv_func_printf_percent_m
cache variable), and if not, inserting our own wrapper around
vfprintf to manually expand a single instance of %m. (Thankfully,
it is MUCH easier to do this rewrite for %m, since it does not
consume anything from the va_list arg, than it would be for
any other % sequence where we'd have to write a full printf
parser).
As a caller is unlikely to pass multiple %m in a single format
string, I didn't bother with replacing a second instance; the
documentation updates mention this restriction.
Signed-off-by: Eric Blake <eblake at redhat.com>
---
I have not actually tested on a BSD machine, but did prove to
myself that it works on a glibc system with:
./configure nbdkit_cv_func_printf_percent_m=no
and comparing gdb traces of default and override behaviors.
---
docs/nbdkit-filter.pod | 13 +++++++++----
docs/nbdkit-plugin.pod | 13 +++++++++----
configure.ac | 27 +++++++++++++++++++++++++++
src/internal.h | 6 ++++++
src/log.c | 21 +++++++++++++++++++++
5 files changed, 72 insertions(+), 8 deletions(-)
diff --git a/docs/nbdkit-filter.pod b/docs/nbdkit-filter.pod
index 7d5a549..77ecd7b 100644
--- a/docs/nbdkit-filter.pod
+++ b/docs/nbdkit-filter.pod
@@ -524,7 +524,10 @@ L<printf(3)>:
void nbdkit_error (const char *fs, ...);
void nbdkit_verror (const char *fs, va_list args);
-For convenience, C<nbdkit_error> preserves the value of C<errno>.
+For convenience, C<nbdkit_error> preserves the value of C<errno>, and
+also supports the glibc extension of a single C<%m> in a format string
+expanding to C<strerror(errno)>, even on platforms that don't support
+that natively.
=head1 DEBUGGING
@@ -540,9 +543,11 @@ L<printf(3)>:
void nbdkit_debug (const char *fs, ...);
void nbdkit_vdebug (const char *fs, va_list args);
-For convenience, C<nbdkit_debug> preserves the value of C<errno>.
-Note that C<nbdkit_debug> only prints things when the server is in
-verbose mode (I<-v> option).
+For convenience, C<nbdkit_debug> preserves the value of C<errno>, and
+also supports the glibc extension of a single C<%m> in a format string
+expanding to C<strerror(errno)>, even on platforms that don't support
+that natively. Note that C<nbdkit_debug> only prints things when the
+server is in verbose mode (I<-v> option).
=head2 Debug Flags
diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
index 4754d2c..0f8ef79 100644
--- a/docs/nbdkit-plugin.pod
+++ b/docs/nbdkit-plugin.pod
@@ -254,7 +254,10 @@ L<printf(3)>:
void nbdkit_error (const char *fs, ...);
void nbdkit_verror (const char *fs, va_list args);
-For convenience, C<nbdkit_error> preserves the value of C<errno>.
+For convenience, C<nbdkit_error> preserves the value of C<errno>, and
+also supports the glibc extension of a single C<%m> in a format string
+expanding to C<strerror(errno)>, even on platforms that don't support
+that natively.
C<nbdkit_set_error> can be called at any time, but only has an impact
during callbacks for serving data, and only when the callback returns
@@ -831,9 +834,11 @@ L<printf(3)>:
void nbdkit_debug (const char *fs, ...);
void nbdkit_vdebug (const char *fs, va_list args);
-For convenience, C<nbdkit_debug> preserves the value of C<errno>.
-Note that C<nbdkit_debug> only prints things when the server is in
-verbose mode (I<-v> option).
+For convenience, C<nbdkit_debug> preserves the value of C<errno>, and
+also supports the glibc extension of a single C<%m> in a format string
+expanding to C<strerror(errno)>, even on platforms that don't support
+that natively. Note that C<nbdkit_debug> only prints things when the
+server is in verbose mode (I<-v> option).
=head2 Debug Flags
diff --git a/configure.ac b/configure.ac
index 4b1b004..5735921 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,6 +57,7 @@ dnl Check for basic C environment.
AC_PROG_CC_STDC
AC_PROG_INSTALL
AC_PROG_CPP
+AC_CANONICAL_HOST
AC_C_PROTOTYPES
test "x$U" != "x" && AC_MSG_ERROR([Compiler not ANSI compliant])
@@ -173,6 +174,32 @@ AC_CHECK_FUNCS([\
get_current_dir_name \
mkostemp])
+dnl Check whether printf("%m") works
+AC_CACHE_CHECK([whether the printf family supports %m],
+ [nbdkit_cv_func_printf_percent_m],
+ [AC_RUN_IFELSE(
+ [AC_LANG_PROGRAM([[
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+ ]], [[
+ char buf[200] = "";
+ errno = EINVAL;
+ snprintf(buf, sizeof buf, "%m");
+ return !!strcmp (buf, strerror (EINVAL));
+ ]])],
+ [nbdkit_cv_func_printf_percent_m=yes],
+ [nbdkit_cv_func_printf_percent_m=no],
+ [[
+ case "$host_os" in
+ *-gnu* | gnu*) nbdkit_cv_func_printf_percent_m=yes;;
+ *) nbdkit_cv_func_printf_percent_m="guessing no";;
+ esac
+ ]])])
+AS_IF([test "x$nbdkit_cv_func_printf_percent_m" = xyes],
+ [AC_DEFINE([HAVE_VFPRINTF_PERCENT_M],[1],
+ [Define to 1 if vfprintf supports %m.])])
+
old_LIBS="$LIBS"
AC_SEARCH_LIBS([dlopen], [dl dld], [
AS_IF([test "x$ac_cv_search_dlopen" != "xnone required"],
diff --git a/src/internal.h b/src/internal.h
index 4da8851..29ab843 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -148,6 +148,12 @@ extern int crypto_negotiate_tls (struct connection *conn, int sockin, int sockou
#define debug nbdkit_debug
/* log-*.c */
+#if !HAVE_VFPRINTF_PERCENT_M
+#include <stdio.h>
+#define vfprintf nbdkit_vfprintf
+int __attribute__ ((format (printf, 2, 0)))
+nbdkit_vfprintf(FILE *f, const char *fmt, va_list args);
+#endif
void log_stderr_verror (const char *fs, va_list args);
void log_syslog_verror (const char *fs, va_list args);
diff --git a/src/log.c b/src/log.c
index 148df5f..b314ca0 100644
--- a/src/log.c
+++ b/src/log.c
@@ -75,3 +75,24 @@ nbdkit_error (const char *fs, ...)
nbdkit_verror (fs, args);
va_end (args);
}
+
+#if !HAVE_VFPRINTF_PERCENT_M
+/* Work around lack of %m in BSD */
+#undef vfprintf
+
+/* Call the real vfprintf after first changing %m into strerror(errno). */
+int
+nbdkit_vfprintf(FILE *f, const char *fmt, va_list args)
+{
+ char *repl = NULL;
+ char *p = strstr (fmt, "%m"); /* assume strstr doesn't touch errno */
+ int ret;
+
+ if (p && asprintf(&repl, "%.*s%s%s", (int) (p - fmt), fmt, strerror (errno),
+ p + 2) > 0)
+ fmt = repl;
+ ret = vfprintf (f, fmt, args);
+ free (repl);
+ return ret;
+}
+#endif
--
2.17.2
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