I want to only printf if some condition is true. I know printf is a variadic function but sadly I can't seem to find any thread here explaining I can wrap it.
Basically every in the code where I'd write :
printf(" [text and format] ", ... args ...);
I want to write something like
my_custom_printf(" [text and format] ", ... args ...);
Which then is implemented like this :
int my_custom_printf(const char* text_and_format, ... args ...)
{
if(some_condition)
{
printf(text_and_format, ... args...);
}
}
A first version of the condition would be independent of the args (it would be on some global variable), but it might be in the future that it's a condition on argument that's wanted.
Anyway, right now I just need the syntax for ... args ... in the prototype and the body of my_custom_printf.
I'm using GCC but I don't know which C standard - but we can just try stuff out.
You can use vprintf:
#include <stdio.h>
#include <stdarg.h>
#include <stdbool.h>
static bool canPrint = true;
int myprintf(const char *fmt, ...)
{
va_list ap;
int res = 0;
if (canPrint) {
va_start(ap, fmt);
res = vprintf(fmt, ap);
va_end(ap);
}
return res;
}
int main(void)
{
myprintf("%d %s\n", 1, "Hello");
return 0;
}
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?
Use vprintf() instead.
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;
}
Source
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.
This is not how you use printf(). If you want to use va_lists, use vprintf() instead. Look here for reference.
I have been trying to pass variable arguments to other function in C but it is producing inconsistent result in different runtime environment as well as in different runs in same environment:
int main()
{
int result = myprintf("Something \n %d", 9);
return result;
}
int myprintf(const char *format, ...){
printf("Something \n %d", 9);
printf("\n");
va_list args;
va_start(args, format);
int result = printf(format,args);
printf("\n");
va_end(args);
return result;
}
And the result produced is:
WWW.FIRMCODES.COM
9
WWW.FIRMCODES.COM
438656664
I could not find the reason for "438656664".
You cannot pass the variadic arguments to a variadic function. Instead, you must call a function that takes a va_list as argument. The standard library provides variants of printf and scanf that take a va_list; their names have the prefix v.
Your example should look like:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
int printfln(const char *format, ...)
{
int result;
va_list args;
va_start(args, format);
result = vprintf(format, args);
printf("\n");
va_end(args);
return result;
}
int main()
{
int result = printfln("Something \n %d", 9);
printf("(%d)\n", result);
return 0;
}
There are some gotchas, for example when you want to call two v... function for printing to the screen and a log file: The v... function may exhaust the va_list, so you must pass in a fresh one to each call if your code should be portable.
For the C++ fellow also reading this. You can actually do it using pack expansion without using vprintf.
This trick is quite handy when you need to wrap a method that takes the ellipsis (...)
and not a va_list.
For instance:
template <class ... Args>
void foo(const char *format, Args ... args)
{
printf(format, args...);
}
Here class ... Args is template parameter pack, Args ... args is function parameter pack, and args... is function parameter pack expansion.
Alternatively, you can simply use a wrapper macro:
#include <stdio.h>
#define myprintf(fmt, ...) ( printf("Something \n %d\n", 9), printf(fmt, __VA_ARGS__) )
int main (void)
{
int result = myprintf("Something \n %d\n", 9);
printf("%d\n", result);
}
Note the use of the comma operator to preserve the returned value of the right-hand printf call to the caller.
This isn't any less type safe than the (equally dangerous) stdarg.h variadic functions.
Just a simple demonstration and worked example with "a fresh one va_list" when you need to print/output-as-string a template string like constexpr const char* example = R"(template "%s")"; .
std::string print_templ(const char* format, ...)
{
va_list args1;
va_start(args1, format);
va_list args2;
va_copy(args2, args1);
std::vector<char> str(std::vsnprintf(nullptr, 0, format, args1) + 1);
va_end(args1);
const int ret = std::vsnprintf(str.data(), str.size(), format, args2);
va_end(args2);
return std::string(str.begin(), str.begin()+ret);
}
So, here's a small problem I'm facing right now -> I'm trying to write a function that will accept a char* message and a variable number of arguments. My function will modify the message a little, and then It'll call printf with the message and given parameters. Essentialy, I'm trying to write something like that:
void modifyAndPrintMessage(char* message,...){
char* newMessage; //copy message.
//Here I'm modifying the newMessage to be printed,and then I'd like to print it.
//passed args won't be changed in any way.
printf(newMessage,...); //Of course, this won't work. Any ideas?
fflush(stdout);
}
So, anybody knows what should I do to make it happen? I'd be most grateful for any help :)
You want to use varargs...
void modifyAndPrintMessage( char* message, ... )
{
// do somehthing custom
va_list args;
va_start( args, message );
vprintf( newMessage, args );
va_end( args );
}
void modifyAndPrintMessage(char* message,...)
{ char newMessage[1024]; // **Make sure the buffer is large enough**
va_list args;
va_start(args, message);
vsnprintf(newMessage, message, args);
printf(newMessage);
fflush(stdout);
}
Use varargs to accept variable number of parameters then use sprintf to create the new message
You can use va_list from stdarg.h,
C example: http://www.tutorialspoint.com/cprogramming/c_variable_arguments.htm
C++ example: http://www.cprogramming.com/tutorial/lesson17.html.
An of course, see the man page: http://linux.die.net/man/3/stdarg
Man page example for reference:
#include <stdio.h>
#include <stdarg.h>
void
foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch (*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\n", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
}
There is a library which includes this functionality. Here is some example code from the reference:
#include <stdarg.h> /* va_list, va_start, va_arg, va_end */
int FindMax (int n, ...)
{
int i,val,largest;
va_list vl;
va_start(vl,n);
largest=va_arg(vl,int);
for (i=1;i<n;i++)
{
val=va_arg(vl,int);
largest=(largest>val)?largest:val;
}
va_end(vl);
return largest;
}
The ellipsis is actually valid code, and you can use the va_list object to parse a variable number of parameters.
Say I have a C function which takes a variable number of arguments: How can I call another function which expects a variable number of arguments from inside of it, passing all the arguments that got into the first function?
Example:
void format_string(char *fmt, ...);
void debug_print(int dbg_lvl, char *fmt, ...) {
format_string(fmt, /* how do I pass all the arguments from '...'? */);
fprintf(stdout, fmt);
}
To pass the ellipses on, you initialize a va_list as usual and simply pass it to your second function. You don't use va_arg(). Specifically;
void format_string(char *fmt,va_list argptr, char *formatted_string);
void debug_print(int dbg_lvl, char *fmt, ...)
{
char formatted_string[MAX_FMT_SIZE];
va_list argptr;
va_start(argptr,fmt);
format_string(fmt, argptr, formatted_string);
va_end(argptr);
fprintf(stdout, "%s",formatted_string);
}
There's no way of calling (eg) printf without knowing how many arguments you're passing to it, unless you want to get into naughty and non-portable tricks.
The generally used solution is to always provide an alternate form of vararg functions, so printf has vprintf which takes a va_list in place of the .... The ... versions are just wrappers around the va_list versions.
Variadic Functions can be dangerous. Here's a safer trick:
void func(type* values) {
while(*values) {
x = *values++;
/* do whatever with x */
}
}
func((type[]){val1,val2,val3,val4,0});
In magnificent C++11 you could use variadic templates:
template <typename... Ts>
void format_string(char *fmt, Ts ... ts) {}
template <typename... Ts>
void debug_print(int dbg_lvl, char *fmt, Ts... ts)
{
format_string(fmt, ts...);
}
Though you can solve passing the formatter by storing it in local buffer first, but that needs stack and can sometime be issue to deal with. I tried following and it seems to work fine.
#include <stdarg.h>
#include <stdio.h>
void print(char const* fmt, ...)
{
va_list arg;
va_start(arg, fmt);
vprintf(fmt, arg);
va_end(arg);
}
void printFormatted(char const* fmt, va_list arg)
{
vprintf(fmt, arg);
}
void showLog(int mdl, char const* type, ...)
{
print("\nMDL: %d, TYPE: %s", mdl, type);
va_list arg;
va_start(arg, type);
char const* fmt = va_arg(arg, char const*);
printFormatted(fmt, arg);
va_end(arg);
}
int main()
{
int x = 3, y = 6;
showLog(1, "INF, ", "Value = %d, %d Looks Good! %s", x, y, "Infact Awesome!!");
showLog(1, "ERR");
}
Hope this helps.
You can try macro also.
#define NONE 0x00
#define DBG 0x1F
#define INFO 0x0F
#define ERR 0x07
#define EMR 0x03
#define CRIT 0x01
#define DEBUG_LEVEL ERR
#define WHERESTR "[FILE : %s, FUNC : %s, LINE : %d]: "
#define WHEREARG __FILE__,__func__,__LINE__
#define DEBUG(...) fprintf(stderr, __VA_ARGS__)
#define DEBUG_PRINT(X, _fmt, ...) if((DEBUG_LEVEL & X) == X) \
DEBUG(WHERESTR _fmt, WHEREARG,__VA_ARGS__)
int main()
{
int x=10;
DEBUG_PRINT(DBG, "i am x %d\n", x);
return 0;
}
You can use inline assembly for the function call. (in this code I assume the arguments are characters).
void format_string(char *fmt, ...);
void debug_print(int dbg_level, int numOfArgs, char *fmt, ...)
{
va_list argumentsToPass;
va_start(argumentsToPass, fmt);
char *list = new char[numOfArgs];
for(int n = 0; n < numOfArgs; n++)
list[n] = va_arg(argumentsToPass, char);
va_end(argumentsToPass);
for(int n = numOfArgs - 1; n >= 0; n--)
{
char next;
next = list[n];
__asm push next;
}
__asm push fmt;
__asm call format_string;
fprintf(stdout, fmt);
}
Ross' solution cleaned-up a bit. Only works if all args are pointers. Also language implementation must support eliding of previous comma if __VA_ARGS__ is empty (both Visual Studio C++ and GCC do).
// pass number of arguments version
#define callVardicMethodSafely(...) {value_t *args[] = {NULL, __VA_ARGS__}; _actualFunction(args+1,sizeof(args) / sizeof(*args) - 1);}
// NULL terminated array version
#define callVardicMethodSafely(...) {value_t *args[] = {NULL, __VA_ARGS__, NULL}; _actualFunction(args+1);}
Short answer
/// logs all messages below this level, level 0 turns off LOG
#ifndef LOG_LEVEL
#define LOG_LEVEL 5 // 0:off, 1:error, 2:warning, 3: info, 4: debug, 5:verbose
#endif
#define _LOG_FORMAT_SHORT(letter, format) "[" #letter "]: " format "\n"
/// short log
#define log_s(level, format, ...) \
if (level <= LOG_LEVEL) \
printf(_LOG_FORMAT_SHORT(level, format), ##__VA_ARGS__)
usage
log_s(1, "fatal error occurred");
log_s(3, "x=%d and name=%s",2, "ali");
output
[1]: fatal error occurred
[3]: x=2 and name=ali
log with file and line number
const char* _getFileName(const char* path)
{
size_t i = 0;
size_t pos = 0;
char* p = (char*)path;
while (*p) {
i++;
if (*p == '/' || *p == '\\') {
pos = i;
}
p++;
}
return path + pos;
}
#define _LOG_FORMAT(letter, format) \
"[" #letter "][%s:%u] %s(): " format "\n", _getFileName(__FILE__), __LINE__, __FUNCTION__
#ifndef LOG_LEVEL
#define LOG_LEVEL 5 // 0:off, 1:error, 2:warning, 3: info, 4: debug, 5:verbose
#endif
/// long log
#define log_l(level, format, ...) \
if (level <= LOG_LEVEL) \
printf(_LOG_FORMAT(level, format), ##__VA_ARGS__)
usage
log_s(1, "fatal error occurred");
log_s(3, "x=%d and name=%s",2, "ali");
output
[1][test.cpp:97] main(): fatal error occurred
[3][test.cpp:98] main(): x=2 and name=ali
custom print function
you can write custom print function and pass ... args to it and it is also possible to combine this with methods above. source from here
int print_custom(const char* format, ...)
{
static char loc_buf[64];
char* temp = loc_buf;
int len;
va_list arg;
va_list copy;
va_start(arg, format);
va_copy(copy, arg);
len = vsnprintf(NULL, 0, format, arg);
va_end(copy);
if (len >= sizeof(loc_buf)) {
temp = (char*)malloc(len + 1);
if (temp == NULL) {
return 0;
}
}
vsnprintf(temp, len + 1, format, arg);
printf(temp); // replace with any print function you want
va_end(arg);
if (len >= sizeof(loc_buf)) {
free(temp);
}
return len;
}
Let's say you have a typical variadic function you've written. Because at least one argument is required before the variadic one ..., you have to always write an extra argument in usage.
Or do you?
If you wrap your variadic function in a macro, you need no preceding arg. Consider this example:
#define LOGI(...)
((void)__android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__))
This is obviously far more convenient, since you needn't specify the initial argument every time.
I'm unsure if this works for all compilers, but it has worked so far for me.
void inner_func(int &i)
{
va_list vars;
va_start(vars, i);
int j = va_arg(vars);
va_end(vars); // Generally useless, but should be included.
}
void func(int i, ...)
{
inner_func(i);
}
You can add the ... to inner_func() if you want, but you don't need it. It works because va_start uses the address of the given variable as the start point. In this case, we are giving it a reference to a variable in func(). So it uses that address and reads the variables after that on the stack. The inner_func() function is reading from the stack address of func(). So it only works if both functions use the same stack segment.
The va_start and va_arg macros will generally work if you give them any var as a starting point. So if you want you can pass pointers to other functions and use those too. You can make your own macros easily enough. All the macros do is typecast memory addresses. However making them work for all the compilers and calling conventions is annoying. So it's generally easier to use the ones that come with the compiler.