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C language function printf has a very different parameter which is ..., something like this below:
int printf(const char *format, ...);

It's something about var_list dealing with var_args. I have a similar function for recording log informations.
void LOG(const char* format,...) {
    va_list list;
    time_t t=time(NULL);
    tm* tp=localtime(&t);
    char log_spin[256]="";
    char log_buffer[1024*4]="";
    if(format==NULL||!LOGFILE) return;
    sprintf(log_spin,"[%04d-%02d-%02d %02d:%02d:%02d]",
    size_t occuppied=strlen(log_buffer);
    size_t remain=sizeof(log_buffer)-occuppied;
    int count=0;
    if(count>=remain) sprintf(&log_buffer[sizeof(log_buffer)-5],"...\n\0");
    else log_buffer[strlen(log_buffer)]='\n';

This codes use the simiar function vsnprintf dealing with format and recording parameters. It worked well utill recording parameters contains some characters like %, which when I tried to log some url with muti-bytes characters, as you know not supported in URL, there would be some conversion by using % together with hex value like %2d%4f...
My URL may like this:
see, there is a lot characters including %.
But, when you pass the url to this LOG function , sh*t happened. When I try to debug it, exception occurried (MSVC). OK, I believe there is something wrong with my LOG codes. Did the standard C function "printf" support it? I try to print the same url on console, all OK! That means there is a proper way to deal with % character which I just don't know. There must be a standard answer for it.
So, This is my problem , how to deal with character % in va_list?

What I have tried:

To simplify my code , I paste the code and the test input here.
int vsnprintf_test(char* format,...) {
    char buffer[256]={0};
    va_list list;
    int count=0;
    return count;

the caller:
char msg[256] = { 0 };//"";
msg[0] = '%'; //the character which caused the exception 
msg[1] = 'A';
printf("%s\n",msg); //all ok
vsnprintf_test(msg); // crashed.

I don't know how to fix this code. How did function printf do with character %?
Updated 23-Jan-23 23:35pm

Look at the documentation:[^] and you will see that the format is not the first parameter to vnsprintf - it's the third.
So the string you pass to printf in your tiny example is the data to print, but the format to print it with in your vnsprintf call: "%A" which is an unrecognised print format. Pass it "%s" instead, and it'll work.

So go back to your LOG function, and work out what you should be passing, and what you should be doing with it - because it's very unlikely that you want to pass a format to a "Log this" function - instead, you want to pass it data to actually log ...
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Yount_0701 24-Jan-23 3:50am    
quote "%A" which is an unrecognised print format. Pass it "%s" instead, and it'll work.
you mean vsnprintf_test("%s",msg)? Do you ever try? Does it work ?
As far as I have done, apparently the output was not what we expected.
Hold on before go back to LOG function , because here on my msvc , the output was not "%A" even if I adjust the caller like "vsnprintf_test("%s",msg)",it's still something like "0X0.0000000000002P-1022" , nonsense as adjust before.
OriginalGriff 24-Jan-23 4:08am    
No, read what I said ...
Yount_0701 24-Jan-23 4:15am
the pics show the result. forget about LOG function before the vsnprintf_test really works.
You said no confused me, could you plz show me your exact way by code?
OriginalGriff 24-Jan-23 4:49am    
Read what I said. Read the documentation.
Think about the code *you* wrote. What do you pass to the function, and why?
Yount_0701 24-Jan-23 5:09am    
I read your comment again, right now I'm not sure what you are talking about. I believe this is not a very deep or hard question. I try my best to show what am doing and what am expected. At last I show my code together with result pictures. I think you have already be clear about my stuck and my perpose. I believe there is an certain answer about it. Also I'm sure we could both make a progress by present coding on this topic.Otherwise, repeat reading don't make any sense. Thank you@OriginalGriff

Because you are missing the format string parameter in your call to vsnprintf_test. It should be:
vsnprintf_test("%s\n", msg);
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Yount_0701 24-Jan-23 4:10am    
As the comment before this, it's not work,that's my test on my MSVC environment. In case of cheap talk , I provide the pics here
Richard MacCutchan 24-Jan-23 4:25am    
Yes, because the first call to vsnprintf_test is still incorrect:

In this case vsnprintf_test will try to use msg as a format string. And when it sees the % character it will try to extract a parameter from the call stack. But since there are no parameters it will crash the program.
Yount_0701 24-Jan-23 4:43am
The same story.
Yount_0701 24-Jan-23 4:31am
Now what ? guys?
Richard MacCutchan 24-Jan-23 5:58am    
You still have the same issue in your vsnprintf_test function:
    printf(buffer); // buffer contains the string "%A", so printf needs a format string

So that printf statement should always be:
    printf("%s\n", buffer); // ensure that printf does not crash

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