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Subject[RFC PATCH v1 2/3] Add %v support to vsnprintf()
Date
Add %v support to vsnprintf()

vsnprintf() is extended to support a '%v' in the format string. '%v'
takes two arguments, the first is a format string and the second is a
va_list. vsnprintf() is recursively called to process the specified
format string and arguments, before returning to the original format
string.

This allows functions like the following, which prepend a
subsystem-specific string to an error message:

int subsystem_error(char *fmt, ...) {
va_list ap;
int rc;
va_start(ap, fmt);
rc = printk("%s: %v\n", "SUBSYSTEM", fmt, ap);
va_end(ap);
return rc;
}
Functions printing errors inside the subsystem would do like:

subsystem_error("Failed to get widget: %d", 3);

The resulting kernel message would be:

SUBSYSTEM: Failed to get widget: 3\n

The advantage over using say 2 printk() calls is that message output
will be contiguous, whereas separate printk() calls might be broken
by an intervening message from another kernel thread. Also a single
message buffer is used, which avoids double-handling and excessive
memory use.

Signed-off-by: Nick Andrew <nick@nick-andrew.net>
---
lib/vsprintf.c | 14 ++++++++++++++
1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 1e2dcaf..e779f79 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -632,6 +632,7 @@ static char *pointer(const char *fmt, char *buf, char *end, void *ptr, int field
* %pS output the name of a text symbol
* %pF output the name of a function pointer
* %pR output the address range in a struct resource
+ * %v processes a supplied format string and args, recursively
*
*/

@@ -787,6 +788,19 @@ static char *__vsnprintfr(char *buf, char *str, char *end,
case 'u':
break;

+ case 'v': {
+ char *fmt2;
+ va_list args2;
+
+ fmt2 = va_arg(args, char *);
+ args2 = va_arg(args, va_list);
+ /* recurse to format the (fmt,args) pair */
+ str = __vsnprintfr(buf, str, end, fmt2, args2);
+ va_end(args2);
+
+ continue;
+ }
+
default:
if (str < end)
*str = '%';


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