There are some Common Bugs in C Programming.

Most of the contents are directly from or modified from Prof. . Most credits should go to him.

For all the following problems, answer the output message of the code, unless they are specified differently. If there are any exception(s) (or segmentation faults), indicate where it is (or they are). Most importantly, you need to explain the reason.

I don't want to write down the complete answer.

If you have some questions, please leave a message below. Thank you!

Problem 1:

//case 1#include 
    
     int main(void){  char *start = "this is a string";  char *start2 = strtok(start, " ");  return 0;}
    

//case 2#include 
    
     int main(void){  char start[] = "this is a string";  char *start2 = strtok(start, " ");  start2 = "this is a string";  start2[4] = '\0';  return 0;}
    

What happens and why?

Hint 1:

Maybe you can get an answer from ， and .

"this is a string" in char *start = "this is a string" is stored in the code segment。It can not be modified, so the program will cause a runtime error.  For case 1, modified ``char *start``to ``char start[]``

Problem 2:

#include 
    
     int division(int *a, int *b){  return *a/*b /* a simple division */;}int main(){  int a = 6;  int b = 2;  int *aptr = &a;  int *bptr = &b;  printf("%d\n", division(aptr, bptr));}
    

What's wrong? How to fix it?

Problem 3:

#include 
    
     #include 
     
      int main(void){  char string[] = "this is a string";  char *start;  start = string;  start = strtok(start, " ");  while (start != NULL) {    printf("%s\n", start);    start = strtok(NULL, " ");  }  start = string;  start = strtok(start, " ");  while (start != NULL) {    printf("%s\n", start);    start = strtok(NULL, " ");  }  return 0;}
     
    

Show output and explain the difference of the two loops.

Hint 3:

See the definition of function

Problem 4:

#include 
    
     int main(void){  FILE *fp;  char c;  int count;  int i;  fp = fopen("file", "wb");  for (i = 0; i < 256; i++)    fputc(i, fp);  fclose(fp);  fp = fopen("file", "rb");  count = 0;  while ((c = fgetc(fp)) != EOF)    count++;  printf("count = %d\n", count);  return 0;}
    

Explain why one is missing.

Hint 4:

Check the ASCII number of the EOF

Problem 5:

#include 
    
     int main(){  float a = 1.134;  float b = 3.402;  if (a * 3 == b)    printf("yes");  else    printf("no");}
    

What happens and why?

Problem 6:

#include 
    
     int main(){  long int lab_tel = 035731603;  printf("my lab’s telephone number is %ld\n", lab_tel);  return 0;}
    

Why is our lab number incorrect?

Hint 6:

Octal & Decimal

Problem 7:

#include 
    
     int main(){  int a[10];  if (a == &a)    printf("yes\n");  else    printf("no\n");  if (a + 1 == &a + 1)    printf("yes\n");  else    printf("no\n");}
    

What happens and why?

Hint 7:

You may find the answer in or .

Problem 8:

#include 
    
     int main(void){  FILE *fp;  fp = fopen("file", "w");  fputs("hello\n", fp);  fputs("hello", fp);  fputs("hello\n", fp);  fclose(fp);  return 0;}
    

Answer the lengths of “file” in Unix and Windows, and explain.

Hint 8:

Windows: \CR\LF

Unix: \LF

Problem 9:

#include 
    
     int main(void){  FILE *fp;  char c;  fp = fopen("file", "wb");  for (c = 0; c < 256; c++) {    fputc(c, fp);  }  fclose(fp);  return 0;}
    

NEVER NEVER run this. Otherwise, your hard disk will crash.

Just tell what is wrong with this program.

Problem 10:

#include 
    
     #define inc(x) ((x)++)#define square(x) (x * x)int main(){  int i = 3;  int j = 4;  printf("%d\n", square(i + j));  printf("%d %d\n", square(inc(i)), i);}
    

Explain the result, and how to fix it.

Problem 11:

struct csie {  char c;  short s;  int i;  double e;};  struct ceis {  char c;  double e;  int i;  short s;};  int main(void){  printf("csie = %d\n", sizeof(struct csie));  printf("ceis = %d\n", sizeof(struct ceis));  return 0;}

Explain the result.

Hint: this is very very important to the program development in our lab.

Problem 12:

#include 
    
     #include 
     
      int main(void){  char source[] = "This is a string.";  char destination[4];  int i = 5;  strcpy(destination, source);  printf("i is %d\n", i);  printf("source is [%s]\n", source);  printf("destination is [%s]\n", destination);  return 0;}
     
    

What happens and why? How to fix it?

Problem 13: (The examples are given by Ting-Fu Liao.)

/// header.h#include 
    
     static int val = 0;void set(int x) ;
    

/// impl.c#include "header.h"void set(int x){   val = x ;}

/// main.c#include "header.h"int main() {  set(100);  if ( val == 100 )     printf("val == 100\n");  else    printf("val != 100\n");  return 0;}

Separate them into three files. What happens and how to fix it?

Problem 14:

#include 
    
     int main(){  char filename[80];  FILE *fp;  printf("input file name: ");  fgets(filename, 79, stdin);  fp = fopen(filename, "r");  // try assert(fp != NULL);  fclose(fp);}
    

Why can’t you open the file? How to fix it.

Hint 14:

See the definition of

Problem 15:

int main(){  int i = 2147483647;  unsigned int ui = 2147483647;  if (i + 1 < 0)         printf("i + 1 < 0\n");     if (ui + 1 > 0)    printf("ui + 1 > 0\n");  if (ui + 1 > i + 1)    printf("ui + 1 > i + 1\n");  return 0;}

why?

Problem 16:

int main(){  unsigned int ui = 2147483647;  if (ui + 1 > 0)    printf("ui + 1 > 0\n");   if (ui + 1 < -1)    printf("ui + 1 < -1\n");  return 0;}

why?

Problem 17:

#include 
    
     int main(){  int i = -13;  if ((i / 2) == (i >> 1))    printf("yes\n");  else    printf("no\n");  return 0;}
    

why?

Problem 18:

// for qsort, read http://www.cplusplus.com/reference/clibrary/cstdlib/qsort//* qsort example */#include 
    
     #include 
     
      int values[] = {-2147483640, 50, 100};int compare (const void * a, const void * b){  return ( *(int*)a - *(int*)b );}int main (){  int n;  qsort (values, 3, sizeof(int), compare);  for (n = 0; n < 3 ; n++)     printf ("%d ",values[n]);  return 0;}
     
    

What happens and why? How to fix it?

Hint 18:

Integer overflow.

Problem 19:

#include 
    
     #include 
     
      int main(void){  FILE *fp;  int c;  fp = fopen(__FILE__, "r");  assert(fp != NULL);  while ((c = fgetc(fp)) != EOF)    putchar(c);  fclose(fp);  return 0;}
     
    

What is the output?

use "gcc –E test.c" to see what happens.

Problem 20:

#include 
    
     #define SWAP(x, y) x ^= y ^= x ^= ymain(){  int i = 3;  int j = 5;  printf("%d\n", i);  printf("%d\n", j);  SWAP(i, j);  printf("%d\n", i);  printf("%d\n", j);  SWAP(i, i);  printf("%d\n", i);}
    

What happens and why? Is strange skills good?

Hint 20:

Don't zuosi !

Problem 21:

#include 
    
     main(){  int i = 3;  i = i++ + ++i;  printf("%d\n", i);}
    

Try this in both Visual C++ 6.0 and gcc.

Problem 22:

#include 
    
     int main(){  int type = 10;  int i =10;  switch (type) {  case 1:    i = 0;    printf("i = %d\n", i);    break;  case 2:    i = 4;    printf("i = %d\n", i);    break;  defualt:    i = 5;    printf("i = %d\n", i);    break;  }  return 0;}
    

What happens ? why??? amazing?? easy.

Problem 23:

#include 
    
     int *bar(int t){  int i = t;  int *temp = &i;  printf("temp is %d, (*temp) is %d\n", temp, *temp);  return temp;}void foo(int a, int b){  int i;  int *temp = &i;  *temp = a+b; }int main(){  int *a;  a = bar(10);  printf("a is %d, (*a) is %d \n", a, *a);  foo(10, 20);  printf("a is %d, (*a) is %d \n", a, *a);}
    

What happens and how to fix it?

Problem 24:

#include 
    
     int main(){  char i = 1;  char j;  scanf("%d", &j);  if (i & j)    printf("yes.\n");  else    printf("no.\n");  return 0;}
    

Input

3

What happens and how to fix it?

Problem 25:

The sub1 below may result in a run-time error. Why?

int& sub1 ( int& a , int& b ){    int c = a − b ;    return c ;}

The sub2 below does not result in a run-time error, but there may be some other

problem. What is the problem?

int& sub2 ( int& a , int& b ){    int ∗ pc = new int ;    ∗pc = a − b ;    return (∗ pc ) ;}

Hint 25:

stack & heap

Problem 26:

#include 
    
     #include 
     
      #include 
      
     
    

Show the output of the translated program, and run it. Also importantly, you need to give a scenario when/where you would use it in this way. Another small case is as below. (The examples are given by Ting-Fu Liao.)

#define test(x) x _x=a; pr##x##f(#x" %d",_x);#include 
    
     int main(a){  test(int);  return 0;}