call printf using va_list
CPrintfVariadic FunctionsC Problem Overview
void TestPrint(char* format, ...)
{
va_list argList;
va_start(argList, format);
printf(format, argList);
va_end(argList);
}
int main()
{
TestPrint("Test print %s %d\n", "string", 55);
return 0;
}
I need to get:
Test print string 55
Actually, I get garbage output. What is wrong in this code?
C Solutions
Solution 1 - C
Use vprintf()
instead.
Solution 2 - C
Instead of printf
, I recommend you try vprintf
instead, which was created for this specific purpose:
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
void errmsg( const char* format, ... )
{
va_list arglist;
printf( "Error: " );
va_start( arglist, format );
vprintf( format, arglist );
va_end( arglist );
}
int main( void )
{
errmsg( "%s %d %s", "Failed", 100, "times" );
return EXIT_SUCCESS;
}
Solution 3 - C
As others have pointed out already: In this case you should use vprintf
instead.
But if you really want to wrap printf
, or want to wrap a function that does not have a v...
version, you can do that in GCC using the non-standard __builtin_apply
feature:
int myfunction(char *fmt, ...)
{
void *arg = __builtin_apply_args();
void *ret = __builtin_apply((void*)printf, arg, 100);
__builtin_return(ret);
}
The last argument to __builtin_apply
is the max. total size of the arguments in bytes. Make sure that you use a value here that is large enough.
Solution 4 - C
This is not how you use printf()
. If you want to use va_lists
, use vprintf()
instead. Look here for reference.