What is vfscanf_s in C?

The vfscanf_s function

The function vfscanf_s is defined in the stdio.h standard library and is used to read data from a file stream. The structure of the vfscanf_s function is as follows:

int vfscanf_s(FILE * stream, const char * format, va_list arglist);

Parameters

  • stream is a pointer to a FILE structure.
  • format is a pointer to a character array which represents a format string.
  • arglist is a list of arguments required by the format string.

Return value

  • A positive number specifying the number of fields successfully read and assigned.
  • EOF if an error occurs or the end of the stream is reached.

Example usage of the vfscanf_s function

The following code demonstrates the use of the vfscanf_s function:

#include<stdio.h>
/*
  call_vfscanf_s is a wrapper function which converts "..." to "arglist" and calls vfscanf_s function
*/
int call_vfscanf_s(FILE * stream, char * format, ...) {
  int result;
  va_list arglist;
  va_start(arglist, format);
  result = vfscanf_s(stream, format, arglist);
  va_end(arglist);
  return result;
}
int main() {
  char text[10];
  int score_obtained;
  int total_score;
  /*
    Contents of file "temp.txt":
      Score: 8 10
  */
  FILE * stream = fopen("temp.txt", "r");
  call_vfscanf_s(stream, "%s %d %d", &text, &score_obtained, &total_score);
  fclose(stream);
  printf("Score Obtained: %d\n", score_obtained);
  printf("Total Score: %d\n", total_score);
  return 0;
}

Output:

Score Obtained: 8
Total Score: 10

In the example above, a file temp.txt is read using the function vfscanf_s by opening a file stream in line 23. The result of the file is stored in 3 variables: text, score_obtained, and total_score, as specified by the format string %s %d %d and the order of the said 3 variables passed to the call_vfscanf_s function. The variables text, score_obtained, and total_score stored Score:, 8, and 10 respectively. Finally, the score_obtained and the total_score were printed onto the standard output.

Note: The function above is not supported by the GCC compiler, so you will get an implicit declaration of function… error.

Use the following function vfscanf to get the job done:

int vfscanf(FILE * stream, const char * format, va_list arglist);

All parameters and return values are the same as the vfscanf_s function.

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