fgets causing an infinite for loop in C

Question

It seems my implementation of fgets() is incorrect here, would very much appreciate some extra eyes to look over what I've done!

Here's the code

int main(int argc, const char* argv[]){ int numIntegers; char buffer[20]; int intArray[10]; //if no argument is passed in, terminate if (argc == 1){ printf("no argument given, terminating..\n"); return EXIT_FAILURE; } else{ numIntegers = atoi(argv[1]); //we only want numbers greater than 0 if (numIntegers <= 0){ printf("# must be greater than 0\n"); return EXIT_FAILURE; } else{ printf("Enter %d integer values to place in array: \n", numIntegers); for (int i = 0; i < numIntegers; i++){ fgets(buffer, numIntegers, stdin); intArray[i] = atoi(buffer); printf("Index is = %d \n", i); } } } //for (int i =0; i < numIntegers; i++){ // printf("Index[%d] = %d \n", i, intArray[i]); //} }

Here's the output, the line with no other text besides an integer is user input. Notice how the value of i resets. The issue only occurs when I give an initial argument of anything more than 10. It turns the for loop into an endless loop, for whatever reason.

$ ./a.out 11 Enter 11 integer values to place in array: 5 Index is = 0 2 Index is = 1 1 Index is = 2 2 Index is = 3 3 Index is = 4 4 Index is = 5 123 Index is = 6 123 Index is = 7 123 Index is = 8 1 Index is = 9 2 Index is = 2 2 Index is = 3 3 Index is = 4 5 Index is = 5 1 Index is = 6 12 Index is = 7

This is a fairly localized question, but the explanation of what is going on (beyond the standard "check your bounds" advice) is kind of cool. Make sure to read all the way to the end of my answer. — Floris
– Floris, Commented Oct 8, 2013 at 22:11

Floris · Accepted Answer · 2013-10-08 22:10:21Z

You are using

fgets(buffer, numIntegers, stdin);

The second parameter should be the size of the buffer - in your case, 20. That is at least one obvious problem...

The next problem: you are allowing numIntegers to be greater than 10 - so you will be writing values beyond the end of your intArray. Need to fix that too...

if(numIntegers > 10) { printf("cannot have number greater than 10!\n"); // abort, retry, ignore... }

In fact - here is your code, with the bugs ironed out: note the use of defined sizes for BUFSIZE and MAXNUM just so you don't have to change it in multiple places if you change your mind...

#include <stdio.h> #define BUFSIZE 20 #define MAXNUM 10 #define EXIT_FAILURE 0 int main(int argc, const char* argv[]){ int i, numIntegers; char buffer[BUFSIZE]; int intArray[MAXNUM]; //if no argument is passed in, terminate if (argc == 1){ printf("no argument given, terminating..\n"); return EXIT_FAILURE; } else{ numIntegers = atoi(argv[1]); //we only want numbers greater than 0 if (numIntegers <= 0 || numIntegers > MAXNUM){ printf("# must be greater than 0 and less than %d!\n", MAXNUM); return EXIT_FAILURE; } else{ printf("Enter %d integer values to place in array: \n", numIntegers); for (i = 0; i < numIntegers; i++){ fgets(buffer, BUFSIZE, stdin); intArray[i] = atoi(buffer); printf("Index is = %d \n", i); } } } }

Finally - you may wonder why your integer counter seems to "reset"? Well - your intArray is a block of 10 integers on the stack; and when you declare loop variable i, it occupies the next place in memory (as int intArray[10]; was the last time a variable was declared before you got to the for loop) - which you happen to get to when you "index" to intArray[10] (a memory location you are not allowed to access, but you did anyway). You happened to enter the value 2 - and thus, i was reset to 2...

If you had declared i at the start of the program (as I did, since my compiler doesn't "do" C99 by default - I'm that old!), the problem would have shown up differently - or not at all.

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