C language printf family

Indefinite parameter series printf

printf

This is the most commonly used , The function is to print the formatted string on the screen

int printf (const char *__format, ...)

The first parameter is the string , The second parameter is …, Is an indefinite parameter , The return value is the formatted string length , among \n\t This escape character is a , Chinese is two lengths

#include <stdio.h>

int main(void)
{
    
    char* a=" Hello ";
    int num;
    num = printf(a); //  If it is a string, you can directly put the header pointer on the first parameter 
    printf(" Last one printf The length is ：%d\t",num);
    return 0;
}

sprintf

This is used to format the string , The formatted string is returned to the first pointer

int sprintf (char *__stream, const char *__format, ...)

The first parameter is the pointer that returns the string , The second parameter is formatted , The third is an indefinite parameter for formatting , The return value is the length of the string

#include <stdio.h>

int main(void)
{
    
    char a[20] = {
    0}; //  Give too much space to hold strings 
    int num = 10;
    num = sprintf(a, "num Size : %d", num); //  If it is a string, you can directly put the header pointer on the first parameter 
    printf(" Last one printf The length is ：%d\n The formatted string is ：%s",num,a);
    return 0;
}

fprintf

This is used to format the string output to a file

int fprintf (FILE *__stream, const char *__format, ...)

The first parameter is the pointer to the return file , The second parameter is formatted , The third is an indefinite parameter for formatting , The return value is the length of the formatted string

#include <stdio.h>

int main()
{
    
   FILE * fp;

   fp = fopen ("file.txt", "w+");
   fprintf(fp, " It's today %d", 20200510);
   fclose(fp);
   return(0);
}

snprintf

This is also used to format strings , contrast sprintf Just one more length parameter , It's safer to use

int snprintf (char *__buf, size_t __len, const char *__format, ...)

The first parameter is the pointer that returns the string , The second is the length , The third parameter is formatted , The fourth is an indefinite parameter for formatting , The return value is the length of the string

#include <stdio.h>

int main()
{
    
    char name1[6] = {
    '\0'};
    char name2[6] = {
    '\0'};
    int size1 = sprintf(name1, "%s", "Arthur");
    printf("name1:%s, size1:%d\n", name1, size1); //  result ：name1:Arthur, size1:6
    int size2 = snprintf(name2, sizeof(name2)/sizeof(char), "%s", "Arthur");
    printf("name2:%s, size2:%d\n", name2, size2); //  result ：name2:Arthu, size2:6
    return 0;
}

Why is it safe , Here we will say , In theory, many college students or college teachers tend to have a misunderstanding when teaching . Many people think that setting a character array to store “Arthur” It should be set to num[6], In fact, this is a wrong idea . As we all know, there is usually a at the end of a string ‘\0’ As the end , When you set it up 6 An array store actually holds seven , So after the continuous memory, the memory has been modified , If your position happens to be a parameter somewhere else , Then the whole program will lead to uncontrollable results at one time . So when we want to save a string, we must prepare an array that is one larger than the length of the string to store it , Will not lead to cross-border .
The above example illustrates why snprintf Than sprintf Security .

va_list series printf

vprintf

This series of functions are generally parameters for functions with indefinite parameters , Maybe it's not very practical, but it can be used for all purposes printf It is necessary to take it out and say

int vprintf(const char *format, va_list arg)

This series of functions should be used with #include <stdarg.h> This library file , This library file defines va_list Structure ,
There are also a series of macro definition functions

#include <stdio.h>
#include <stdarg.h>

void WriteFrmtd(char *format, ...)
{
    
   va_list args;
   
   va_start(args, format);
   vprintf(format, args);
   va_end(args);
}

int main ()
{
    
   WriteFrmtd("%d variable argument\n", 1);
   WriteFrmtd("%d variable %s\n", 2, "arguments");
   
   return(0);
}

vsprintf

and sprintf By comparison, it is also the difference of the last parameter

int vsprintf(char *str, const char *format, va_list arg)

#include <stdio.h>
#include <stdarg.h>

char buffer[80];
int vspfunc(char *format, ...)
{
    
   va_list aptr;
   int ret;

   va_start(aptr, format);
   ret = vsprintf(buffer, format, aptr);
   va_end(aptr);

   return(ret);
}

int main()
{
    
   int i = 5;
   float f = 27.0;
   char str[50] = "hello world";

   vspfunc("%d %f %s", i, f, str);
   printf("%s\n", buffer);
   
   return(0);
}

vfprintf

int vfprintf(FILE *stream, const char *format, va_list arg)

#include <stdio.h>
#include <stdarg.h>

void WriteFrmtd(FILE *stream, char *format, ...)
{
    
   va_list args;

   va_start(args, format);
   vfprintf(stream, format, args);
   va_end(args);
}

int main ()
{
    
   FILE *fp;

   fp = fopen("file.txt","w");

   WriteFrmtd(fp, "This is just one argument %d \n", 10);

   fclose(fp);
   
   return(0);
}

vsnprintf

int vsnprintf(char *str, size_t size, const char *format, va_list ap);

#include <stdio.h>
#include <stdarg.h>
 
#define MAXLEN 10
 
int mon_log(char* format, ...)
{
    
    char str_tmp[MAXLEN];
    int i=0,j=0;
    va_list vArgList;                           
    va_start (vArgList, format);                 
    i=vsnprintf(str_tmp, MAXLEN, format, vArgList);  
    va_end(vArgList);                            
 
    printf("%s", str_tmp); 
 
    for(j=0;j<MAXLEN;j++)
    {
    
        printf("%d ", str_tmp[j]);
    }
    printf("\n");
    return i;
}
 
void main()
{
       
    int i=mon_log("%s,%d,%d,%c","abc",2,3,'\n');
    printf("%d\n",i);
}