Failure in Pointer Dereferencing in C while using same function and variable

Question

I am trying to using C to work with files with different extensions. So this is the code that I have written.

#include <stdio.h>
#include <windows.h>
#include <unistd.h>
#include <conio.h>

void mvjpg(char *arg);
void mvpng(char *arg);
void wincmd(const char *c,...);

int main(int argc,char **argv){
   char *ext;

   for(int i=1;i<argc;i++){
       ext=strrchr(argv[i],'.');
       if(ext == NULL)
           continue;

       if(strcmp(ext,".jpg")==0)
            mvjpg(argv[i]);

       if(strcmp(ext,".png")==0)
            mvpng(argv[i]);
   }

   return 0;
}

void mvjpg(char *arg){

//Do tasks such as ...
    wincmd("move %s Done\\ ",arg);  //Here this runs properly
    printf("%s\n\n",arg);           //This also outputs the file name
    wincmd("copy %s JPGs\\ ",arg);  //Here the value is gibberish
    printf("%s\n\n",arg);           //This too outputs the file name
   

}

void mvpng(char *arg){
     //Do tasks such as ...
}


void wincmd(const char *c,...){
    char cmd[50];
    sprintf(cmd,c);
    printf("%s\n",cmd);
    system(cmd);
}

The Output is:

D:\Folder1>mv new.jpg
move new.jpg Done\
        1 file(s) moved.
new.jpg

copy 6Q½6ⁿ JPGs\
The system cannot find the file specified.
new.jpg

Why is one pointer working in first and in other it is not. The value of pointer is not altered between those commands.

Answer 1

It is unclear why you are using a variable argument list.

It would be much simpler to declare the function like

void wincmd( const char *fmt, const char *s );

and within the function to write

sprintf( cmd, fmt, s);

Nevertheless this call

sprintf(cmd,c);

invokes undefined behavior because there is not specified the third argument that corresponds to the conversion specification &s that is present in the string c.

You need to include header <stdarg.h> that is designed to deal with variable argument lists and process the unnamed argument using macros defined in the header.

Here is a demonstration program.

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

void wincmd( const char *c, ... ) {
    char cmd[50];

    va_list arg;

    va_start( arg, c );

    const char *p = va_arg( arg, const char *);

    sprintf( cmd, c, p );

    va_end( arg );

    puts( cmd );
}

int main( void )
{
    const char *s = "move %s Done\\ ";
    const char *t = "new.jpg";

    wincmd( s, t );

    s = "copy %s JPGs\\ ";

    wincmd( s, t );
}

The program output is

move new.jpg Done\
copy new.jpg JPGs\