Root21 - 4 months ago 28

C Question

I'm writing a program to convert base-10 numbers into their equivalents in other bases, but I keep getting a segmentation fault when I try to run it.

This is the code so far:

`#include <stdio.h>`

int base;

int num;

char* num2[50];

int x =0;

int main(){

printf("Input a number in base 10 and a base (2-16) that you would like to convert to in this format: 100 2\n");

fflush(stdout);

scanf("%d%d",&num, &base);

printf("%d%d", num, base); fflush(stdout);

while ((num % base) > 1){

printf("%d", x); fflush(stdout);

if(num % base == 10)

num2[x] = "A";

else if (num % base == 11)

num2[x] = "B";

else if (num % base == 12)

num2[x] = "C";

else if (num % base == 13)

num2[x] = "D";

else if (num % base == 14)

num2[x] = "E";

else if (num % base == 15)

num2[x] = "F";

else

*num2[x] = num % base;

num = num % base;

x++;

printf("%d, %s", num, num2[x]); fflush(stdout);

}

while (x >= 0){

printf("%s", num2[1]);

fflush(stdout);

x--;

}

printf("\n");

fflush(stdout);

return 0;

}

Through checking the errors with gdb and print statements, I've come to the conclusion that the error stems from something in line 14:

`while ((num % base) > 1) {`

I'm relatively new to C, most of what I've done has been in Java.

Answer

Both of the earlier answers are right (i.e. @Sourav and @Tony). The exact reason among these 2 depends on the input provided during an execution instance.

Say, your input is num=3, base=16.

Then, your program will crash at the line -

`*num2[x] = num % base;`

num2[x] points to NULL since 'num2' is a global array.

Say, your input is num=13, base=16.

Then, your program will crash at the line -

`num2[x] = "D";`

This is because 'num' remains a constant at 13 (

`num = num % base`

) and as a result 'x' would eventually be a value way beyond 49 and`num2[x]`

will be accessing an illegal location.

Also, your program is logically flawed; you will not get the output that you desire with your current approach. Your program needs to be tweaked slightly -

```
char num2[50]; /* Array of chars, instead of array of pointers to char. */
```

. . .

```
/* Add check to verify that base is a value between 2 and 16. */
while ((num % base) > 0){ /* Greater than 0, not 1. */
if(num % base == 10)
num2[x] = 'A'; /* Use a character instead of a string. */
else if (num % base == 11)
num2[x] = 'B';
else if (num % base == 12)
num2[x] = 'C';
else if (num % base == 13)
num2[x] = 'D';
else if (num % base == 14)
num2[x] = 'E';
else if (num % base == 15)
num2[x] = 'F';
else
num2[x] = (num % base) + '0'; /* Convert the number to its character form. */
num /= base; /* Duh! */
printf("%d, %c", num, num2[x]); fflush(stdout); /* %c instead of %s*/
x++;
}
while (x >= 0){
printf("%c", num2[x]); fflush(stdout); /* %c instead of %s*/
x--;
}
```