C Unions with Files and buffer overflow? - c

Question

My program loads/saves double value into a text file using union.
I think I am having a buffer overflow, when I use fflush(stdin) it works, but I can't. The problem maybe somewhere else though. The code jumps without letting me enter a file to load from ( 2.000000 is autoinserted ).
Ps. Yes I know that tab[i] would be better than *(tab+i) but spare me, I just have to do it like this.
output :
Enter double value:8794.061758
Enter path to file you wish to save:valleykingstopwantreachspellshipcontinue
File saved
Enter a path to file you wish to load: 2.000000 ----- This value is inserted automatically ( I didnt write it)
Process finished with exit code 0
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
union double_to_char_t
{
double d;
char bytes[8];
};
int save_double(const union double_to_char_t *dtc, const char *filename)
{
if(dtc == NULL || filename == NULL) return 1;
FILE * f=fopen(filename,"w");
if(f == NULL) return 2;
int result = fwrite(dtc, sizeof(dtc), 1, f);
if( result != 1)
{
fclose(f);
return 3;
}
fclose(f);
return 0;
}
int load_double(union double_to_char_t *dtc, const char *filename)
{
if( dtc == NULL || filename == NULL) return 1;
FILE * f = fopen(filename,"r");
if( f == NULL ) return 2;
int result = fread(dtc, sizeof(dtc), 1, f);
if( result != 1)
{
fclose(f);
return 3;
}
fclose(f);
return 0;
}
int main()
{
union double_to_char_t unia;
char temp1[40] = {0};
char temp2[40] = {0};
char *input1 = temp1;
char *input2 = temp2;
printf("Enter double value: ");
if(scanf(" %lf",&(unia.d))!=1)
{
printf("Incorrect input");
exit(1);
}
printf("Enter path to file you wish to save:");
fgets(input1, 40, stdin);
int wynik = save_double(&unia,input1);
if(wynik > 0)
{
printf("Couldn't create file\n");
exit(5);
}
else
{
printf("File saved\n");
}
printf("Enter a path to file you wish to load: ");
fgets(input2, 40, stdin);
// scanf(" %39[^\n]", input2);
int score = load_double(&unia,input2);
if(score > 0 && score < 3)
{
printf("Couldn't open file\n");
exit(4);
}
if(score == 3)
{
printf("File corrupted\n");
exit(6);
}
printf("%f",unia.d);
return 0;
}

Answer 1

You used %39[^\n] to read the filename, then typed a 40 character filename. The first 39 characters were put into input1, and the last character was left in the input stream.
When you then tried to get the filename for input2, it read that last character. That's why it didn't wait for you to type anything -- there was still input available after the first filename was read.
So you saved to valleykingstopwantreachspellshipcontinu and loaded from e.
You need to ensure that your input buffer is longer than any potential input. If you use fgets() instead of scanf(), you can test whether the input ends with a newline. If not, you can report that the input was too long, read characters until the newline, then ask for input again. (The non-standard fflush(stdin) discards everything up to the next newline, which is why adding that fixed the problem.)
if (!fgets(input1, 40, stdin)) {
printf("input error\n");
exit(1);
}
if (input1[strlen(input1)-1] != '\n') { // check for trailing newline
printf("filename too long\n);
exit(3);
}
input1[strlen(input1)-1] = '\0'; // remove it before using as filename
You could also use the getline() function. This isn't standard C, but it's POSIX and widely implemented. It allocates the input buffer dynamically so any length can be accomodated.