Segmentation fault occurring in for loop (no body statements executed) C

Question

I am trying to write a program to convert a hexadecimal number to a decimal, and then print the decimal value. The problem I am facing is that my for loop causes a segmentation fault.

  printf("no segmentation fault so far..."); /*this printed*/
  for (i=0; (c=getchar()) != '\n'; i++) {
    printf("no segmentation fault after entering for loop"); /*this didn't print*/

I know this because as you can see from my code above all the code before the for loop were ran but the body wasn't executed. There could be a possibility that the for loop test case was not met and therefore it was skipped. To test for this, I put printf statements in other parts of my program as well. However, only the one above the for loop was executed. Here's my full code if it helps:

#include <stdio.h>
#include <ctype.h>
#include <math.h>
#include <stdlib.h>
#define MAXLEN 100

int htoi(char *s, int lim);
void copy(char *to, char *from);

int main(void) {
  char *hexArray = (char *)malloc(MAXLEN);
  htoi(hexArray, MAXLEN);
  return 0;
}

int htoi(char s[], int lim) {
  double decOutput;
  int i = 0;
  int c;
  int size = MAXLEN;
  printf("no segmentation fault so far...");
  for (i=0; (c=getchar()) != '\n'; i++) {
    printf("no segmentation fault after entering for loop");
    if (isdigit(c) || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F')) {
      if(i >= size - 1) {
        size = size + MAXLEN;
        char *tmp = (char *)malloc(size);
        copy(tmp, s);
        free(s);
        s = tmp;
        printf("Buffer size increased to %d\n", size);
      }
      s[i] = c;
    }
    else {
      printf("INVALID INPUT!!! Decimal output may be inaccurate.");
    }
  }

  printf("exited loop");

  for(i = 0; s[i] != '\0'; i++) {
    printf("entered second for loop");
    if (s[i] != '0' || (s[i+1] != 'x' && s[i+1] != 'X')) {
      double y = pow(16, (double)i);
      double x = s[i];
      decOutput = decOutput + pow(x, y);
    }
  }
  printf("%s", decOutput);

  return decOutput;
}

void copy(char *to, char *from) {
  int i = 0;
  while((to[i] = from[i]) != '\0') {
    i++;
  }
}

Answer 1

You can simply catch errors or useful warning from gcc compiler with this command.

gcc t.c -o t -l m  -Wall -Wextra 

I compiled your code and gcc produce this output.

t.c: In function ‘htoi’:
t.c:54:10: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘double’ [-Wformat=]
   printf("%s", decOutput);
          ^
t.c:19:24: warning: unused parameter ‘lim’ [-Wunused-parameter]
 int htoi(char s[], int lim) {

The correct way of print a double variable with printf is change printf("%s", decOutput); to printf("%f", decOutput); concentrate on double format specifier is %f in printf(). (read about that on true way of printing a double )

By add this change and attach simple message state that end of htoi function your code converts to:

//getchar() != EOF

#include <stdio.h>
#include <ctype.h>
#include <math.h>
#include <stdlib.h>

#define MAXLEN 100

int htoi(char *s, int lim);
void copy(char *to, char *from);

int main(void) {
  char *hexArray = (char *)malloc(MAXLEN);
  htoi(hexArray, MAXLEN);
  return 0;
}

int htoi(char s[], int lim) {
  double decOutput;
  int i = 0;
  int c;
  int size = MAXLEN;
  printf("no segmentation fault so far...");
  for (i=0; (c=getchar()) != '\n'; i++) {
    printf("no segmentation fault after entering for loop");

    if (isdigit(c) || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F')) {
      if(i >= size - 1) {
        size = size + MAXLEN;
        char *tmp = (char *)malloc(size);
        copy(tmp, s);
        free(s);
        s = tmp;
        printf("Buffer size increased to %d\n", size);
      }
      s[i] = c;
    }
    else {
      printf("INVALID INPUT!!! Decimal output may be inaccurate.");
    }
  }

  printf("exited loop");

  for(i = 0; s[i] != '\0'; i++) {
    printf("entered second for loop");
    if (s[i] != '0' || (s[i+1] != 'x' && s[i+1] != 'X')) {
      double y = pow(16, (double)i);
      double x = s[i];
      decOutput = decOutput + pow(x, y);
    }
  }

  printf("%f", decOutput);

  printf("\nend htoi\n");
  return decOutput;
}

void copy(char *to, char *from) {
  int i = 0;
  while((to[i] = from[i]) != '\0') {
    i++;
  }
}

work further on it.