Print statement changing char array output - c

Question

I'm trying to convert some text (character by character) to its binary representation. For some reason the print statement printf("Hold is %d or %c: ", hold, hold); is changing the output of my function and I have no idea how to explain it. Any help would be greatly appreciated. The test file is just a text file with Hello, World! inside of it.
With it:
Hold is 72 or H: 01001000
Hold is 101 or e: 01100101
Hold is 108 or l: 01101100
Hold is 108 or l: 01101100
Hold is 111 or o: 01101111
Hold is 44 or ,: 00101100
Hold is 32 or : 00100000
Hold is 87 or W: 01010111
Hold is 111 or o: 01101111
Hold is 114 or r: 01110010
Hold is 108 or l: 01101100
Hold is 100 or d: 01100100
Hold is 33 or !: 00100001
Without it:
1000 �
0101 �
1100 �
1100 �
1111 �
1100 �
0000 �
0111 �
1111 �
0010 �
1100 �
0100 �
0001 �
Code
#include <stdio.h>
#include <string.h>
void decimal_to_binary(unsigned long num, FILE *out) {
int i = 255, a = 0;
char binarr[255];
for (i = 0; i < 255; i++) { binarr[i] = '0'; }
if (num != 0) {
while (num != 0) {
if (num % 2 == 0) {
binarr[i] = '0';
i--;
} else {
binarr[i] = '1';
i--;
}
num /= 2;
}
} else {
fprintf(out, "00000000");
}
fprintf(out, "%s ", binarr + strlen(binarr) - 8);
printf("%s\n", binarr + strlen(binarr) - 8);
memset(binarr, 0, sizeof(binarr));
}
int main(int argc, char *argv[]) {
int hold;
FILE *in = fopen(argv[1], "r");
FILE *out = fopen(argv[2], "w+");
while (!feof(in)) {
hold = fgetc(in);
if (hold > 0 && hold != 10){
printf("Hold is %d or %c: ", hold, hold);
decimal_to_binary(hold, out);
}
}
fclose(in);
fclose(out);
return 0;
}

Answer 1

Your decimal_to_binary function is incorrect:
you index beyond the end of the binarr array.
you do not null terminate this array to pass it to printf.
Here is a simpler and corrected version:
void decimal_to_binary(unsigned long num, FILE *out) {
int i = 256, a = 0;
char binarr[257];
memset(binarr, '0', sizeof(binarr) - 1);
binarr[i] = '\0';
while (num != 0) {
--i;
if (num % 2) {
binarr[i] = '1';
}
num /= 2;
}
if (i > 256 - 8) // print at least 8 bits
i = 256 - 8;
fprintf(out, "%s ", binarr + i);
printf("%s\n", binarr + i);
}
Your function main has problems too:
you test for end of file with feof(in). This is incorrect, you should instead check if hold is EOF.
hard coding the value of '\n' as 10 is bad practice.
Here is a correct version:
int main(int argc, char *argv[]) {
int hold;
FILE *in = fopen(argv[1], "r");
FILE *out = fopen(argv[2], "w+");
while ((hold = fgetc(in)) != EOF) {
if (hold != '\n') {
printf("Hold is %d or %c: ", hold, hold);
decimal_to_binary(hold, out);
}
}
fclose(in);
fclose(out);
return 0;
}

Answer 2

I decreased the extremely large array, made sure to terminate the string with a null character, zeroed the array, then printed it using fprintf. This solved the issue.
void decimal_to_binary(unsigned long num, FILE *out){
int i = 7, a = 0;
char binarr[9];
binarr[8]='\0';
for (a=7; a>=0; a--){ binarr[a] = '0'; }
if (num != 0) {
while (num!=0){
if (num%2 == 0){
binarr[i] = '0';
i--;
}
else { binarr[i] = '1'; i--; }
num /= 2;
}
} else { fprintf(out, "00000000"); }
fprintf(out, "%s ", binarr);
memset(binarr, 0, sizeof(binarr));
}

Answer 3

Your program has undefined behavior for couple of reasons.
You don't have a null terminated string. Calling strlen on such a string is cause for undefined behavior.
You are modifying binarr using an out of bounds index. That is also cause for undefined behavior.
I have my annotations to your function decimal_to_binary that point out where those errors are.
void decimal_to_binary(unsigned long num, FILE *out){
int i = 255, a = 0;
char binarr[255];
for (i=0; i<255; i++){ binarr[i] = '0'; }
// All the elements of binarr are set to '0'.
// It's not a null terminated string.
if (num != 0) {
while (num!=0){
// The value of i is 255 when this loop is
// entered the first time.
// Setting the value of binarr[255] is cause for
// undefined behavior.
if (num%2 == 0){
binarr[i] = '0';
i--;
}
else { binarr[i] = '1'; i--; }
num /= 2;
}
} else { fprintf(out, "00000000"); }
fprintf(out, "%s ", binarr + strlen(binarr) - 8);
printf("%s\n", binarr + strlen(binarr) - 8);
memset(binarr, 0, sizeof(binarr));
}
The fixes are simple.
Terminate string with the null character.
for (i=0; i<255; i++){ binarr[i] = '0'; }
i--;
binarr[i] = '\0';
Use the right index when modifying binarr in the while loop.
while (num!=0){
// Decrement the index before you assign to the next element.
// When the loop is entered the first time, i = 254, which
// is used to null terminate binarray.
// The next '1' or '0' needs to be placed at i = 253.
i--;
if (num%2 == 0){
binarr[i] = '0';
}
else {
binarr[i] = '1';
}
num /= 2;
}