User input with FILE I/O

Question

When the user enters the character "Z" into the program, followed by a file name, I want the compiler to understand to open that file.

For example, a user enters: "Z test.txt" I want the compiler to understand to open "test.txt"..

From my understanding of my code below, the Z is ignored, and the string following the "Z" ("%s), is then put into fileName. Which is then used to actually open the file. Any suggestions how I can make this work?

   int main()
   {
      int infiniteLoop = 0;
      char input[MAXC], fileName[MAXC];
      FILE* fp = NULL;
      char buff[255];

        while (infiniteLoop != 1)
        {
          printf("Enter Student's Grade(s) >>> ");
          fgets(input, MAXC, stdin);
          parseUserInput(input);

            if (*input == '\n')
            {
               break;
            }

            if (sscanf(input, "Z %127s", fileName) == 1)
            {
               fp = fopen(fileName, "r");
            }
            if (fp == NULL)
            {
                printf("File I/O Error...\n");
                return -1;
            }

            while (fscanf(fp, "%s", buff) == 1)
            {
                parseUserInput(buff);
            }
            fclose(fp);
        }
    }
}

Answer 1

It is a mistake to write:

  char *fileName = (char*)malloc(100);
  ...
  if (sscanf(input, "Z %s", &fileName ))

With the above, &fileName is the address of the variable in which you have stored a memory location. If you attempt to write data from the input string to that location, you are overwriting the only copy you have of the address...but you are not writing to the allocated location. (In other words, if the input string is Z abc , then the scanf is essentially equivalent to fileName = 0x616263, and you have a memory leak.) If you want to write to the location that you have allocated, use:

sscanf(input, "Z %99s", fileName)

Answer 2

After seeing why it is easy to miss the fact that fileName is already a pointer and there is NO & before it in the sscanf() argument list, it appears you want to do something similar to:

#include <stdio.h>

#define MAXC 128

int main (void) {
    
    char input[MAXC], fileName[MAXC];
    
    for (;;) {
        fputs ("\nEnter Student's Grade(s) >>> ", stdout);
        if (!fgets (input, MAXC, stdin)) {
            puts ("(user canceled input)");
            break;
        }
        if (*input == '\n')
            break;
        
        if (sscanf (input, "Z %127[^\n]", fileName) == 1)
            printf ("filename: %s\n", fileName);
        else
            puts ("not filename input");
    }
}

(by using "%[^\n]" (with field-width) you can handle filenames with spaces included, but note while "Z%s" would work because "%s" discards leading whitespace, "%[..]" does not -- so the spaces before it is mandatory in this circumstance)

Note, there is no need to allocate for input or fileName (though I would suggest a minimum of 1024 for each, the 128 above covers you if you are on an embedded system)

Example Use/Output

To end input, simply press Enter alone on an empty-line:

$ ./bin/zfilename

Enter Student's Grade(s) >>> 10, 30, 90, 20
not filename input

Enter Student's Grade(s) >>> Z george.txt
filename: george.txt

Enter Student's Grade(s) >>> Z file with spaces.txt
filename: file with spaces.txt

Enter Student's Grade(s) >>>

Look things over and let me know if you have further questions.