Using function to find max value from a file - c

Question

I am fairly new to programming so bear with me.
I'm trying to create some code that would read a text file that contains 3 numbers. I want to use a created function to find the max number. I get no errors when compiling but when I run the code the program crashes (no message recieved or anything, just simply file.exe has stopped working).
I would greatly appreciate help in tackling this problem.
Also I would like to avoid using arrays.
#include <stdio.h>
#include <stdlib.h>
int max(int a,int b,int c);
int main()
{
FILE *fpointer;
int a, b, c;
int maxNumber = max(a,b,c);
fpointer = fopen("marks.txt","r");
while(fscanf(fpointer,"%d %d %d",a,b,c)!=EOF) {
printf("%d",max(a,b,c));
}
fclose(fpointer);
return 0;
}
int max(int a,int b,int c){
if((a>b)&&(a>c))
return a;
if((b>a)&&(b>c))
return b;
if((c>a)&&(c>b))
return c;
}

Answer 1

I am fairly new to programming so bear with me.
OK, and we will, but no matter how hard we try, we cannot fix the Undefined Behavior you invoke with:
int maxNumber = max(a,b,c);
The values of a, b & c have not been initialized at the time you call max. This invokes Undefined Behavior. (attempting to access the value of an uninitialized object).
Second, also easily leading to Undefined Behavior is the failure to validate that fopen succeeds, and failing to validate that fscanf succeeds. Testing that fscanf (...) != EOF does not tell you anything about whether valid conversions actually took place. The return for fscanf is the successful number of conversions that took place -- based upon the number of conversion specifiers present in the format string (e.g. "%d %d %d" contains 3 conversion specifiers). So to validate that a, b & c all contain values, you must compare fscanf (...) == 3.
Putting those pieces together, you could do something similar to the following:
#include <stdio.h>
int max (int a, int b, int c);
int main (int argc, char **argv) {
int a, b, c, n = 0;
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
while (fscanf (fp, "%d %d %d", &a, &b, &c) == 3)
printf ("line[%2d] : %d\n", n++, max (a, b, c));
if (fp != stdin) fclose (fp); /* close file if not stdin */
return 0;
}
int max (int a, int b, int c)
{
int x = a > b ? a : b,
y = a > c ? a : c;
return x > y ? x : y;
}
Example Input
$ cat int3x20.txt
21 61 78
94 7 87
74 1 86
79 80 50
35 8 96
17 82 42
83 40 61
78 71 88
62 20 51
58 2 11
32 23 73
42 18 80
61 92 14
79 3 26
30 70 67
26 88 49
1 3 89
62 81 93
50 75 13
33 33 47
Example Use/Output
$ ./bin/maxof3 <dat/int3x20.txt
line[ 0] : 78
line[ 1] : 94
line[ 2] : 86
line[ 3] : 80
line[ 4] : 96
line[ 5] : 82
line[ 6] : 83
line[ 7] : 88
line[ 8] : 62
line[ 9] : 58
line[10] : 73
line[11] : 80
line[12] : 92
line[13] : 79
line[14] : 70
line[15] : 88
line[16] : 89
line[17] : 93
line[18] : 75
line[19] : 47
Look things over and let me know if you have further questions.

Answer 2

fscanf uses pointer arguments and you are passing the variable values. When the function tries to access the address a (which is in fact an uninitialized variable), it causes a segmentation fault (you are trying to access an invalid memory address) and your program crashes and exits.
You should instead pass the variable addresses to the pointer argument (e.g. &a - the address of variable a) , thus it will access a valid memory address.
while(fscanf(fpointer,"%d %d %d",&a,&b,&c)!=EOF) {
Other undefined behaviors might be avoided like initializing the variables and checking the return values correctly as the answer of #DavidC.Rankin describes in details.

Answer 3

All need to make it work is adding an & at this line...
while(fscanf(fpointer,"%d %d %d",&a,&b,&c)!=EOF) { ... }