vfprintf Function

Prints formatted data to a stream using a variable length argument list.

Include

<stdio.h>

<stdarg.h>

Prototype

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

Arguments

Return Value

Returns number of characters generated or a negative number if an error occurs.

Remarks

The format argument has the same syntax and use that it has in printf.

To access the variable length argument list, the ap variable must be initialized by the macro va_start and may be reinitialized by additional calls to va_arg. This must be done before the vfprintf function is called. Invoke va_end after the function returns. For more details, see stdarg.h.

This function requires a heap.

Example

#include <stdio.h>  /* for vfprintf, fopen, */
                    /* fclose, printf,      */
                    /* FILE, NULL           */

#include <stdarg.h> /* for va_start,        */
                    /* va_list, va_end      */

FILE *myfile;

void errmsg(const char *fmt, ...)
{
  va_list ap;

  va_start(ap, fmt);
  vfprintf(myfile, fmt, ap);
  va_end(ap);
}
int main(void)
{
  int num = 3;

  if ((myfile = fopen("afile.txt", "w")) == NULL)
    printf("Cannot open afile.txt\n");
  else
  {
    errmsg("Error: The letter '%c' is not %s\n", 'a',
          "an integer value.");
    errmsg("Error: Requires %d%s%c", num, 
           " or more characters.", '\n');
  }
  fclose(myfile);
}

Example Output

Contents of afile.txt:

Error: The letter 'a' is not an integer value.
Error: Requires 3 or more characters.