将终端输出到C程序中的文件

Question

I would like everything that gets output to stdout to also be saved in a file in my C code. I know that I can do this by calling the process on the command line and piping it to a file:

 myprogram.exe 1>logfile.txt

for example. But I am wondering if there is a way to do this from within the C code itself. Is there a function in the printf() family that can output to both the terminal and a specified file with the same arguments?

If not, what would be the syntax required to write my own printf()-style function which calls both printf() and fprintf() within it, using the same argument style as printf()?

Answer 1

You can use the fprintf() function, which is quite similar to printf() in the way it works.

Here is an example:

FILE *fp;
int var = 5;
fp = fopen("file_name.txt", "w");// "w" means that we are going to write on this file
fprintf(fp, "Writting to the file. This is an int variable: %d", var);

The output on your file would be this:

This is being written in the file. This is an int variable: 5

NB: Opening the file using w as parameter will destroy the file's content every time you open it.

For writing to a file you have to use file operation commands, it is not possible to write to a file using printf (it prints only to the stdout). You can use:

sprintf(buf,"%d",var);  //for storing in the buffer
printf(buf);       //output to stdout
fputs(buf, fp);   //output to file

Answer 2

Going off the suggestion to use variadic functions:

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

/*
 * Not all compilers provide va_copy(), but __va_copy() is a
 * relatively common pre-C99 extension.
 */
#ifndef va_copy
#ifdef __va_copy
#define va_copy(dst, src) __va_copy((dst), (src))
#endif
#endif

#ifdef va_copy
#define HAVE_VACOPY 1
#endif

int
ftee(FILE *outfile, const char *format, ...)
{
    int result;
    va_list ap;
#if HAVE_VACOPY
    va_list ap_copy;
#endif

    va_start(ap, format);

#if HAVE_VACOPY
    va_copy(ap_copy, ap);
    result = vfprintf(outfile, format, ap_copy);
    va_end(ap_copy);
    if (result >= 0)
        result = vprintf(format, ap);
#else
    result = vfprintf(outfile, format, ap);
    if (result >= 0) {
        va_end(ap);
        va_start(ap, outfile);
        result = vprintf(format, ap);
    }
#endif
    va_end(ap);
    return result;
}

That can be used like the standard fprintf function in that you specify an output file, except it will also write the normal output to stdout . I attempted to support relatively recent compilers that still didn't have a va_copy() macro (defined in C99) such as that which comes with Visual Studio 2012 (VS2013 finally has one). Some C runtimes also conditionally define va_copy() such that compiling with strict C89/C90 mode enabled will make it undefined while __va_copy() may remain defined.