In-depth analysis of common interview questions for C/C ++ programmers

InC/C ++In the interview process of programmers, many interview questions seem simple, but they need profound basic skills to give a perfect answer. Enterprises require the interviewer to write the simplest strcpy function to see how technically the interviewer has achieved. Can we really write a strcpy function? We all think we can, but the strcpy we write may only get 2 out of 10 points. In this article, you can see examples of strcpy functions from 2 to 10 to see what level they belong.

Locate the wrong question

Question 1:

 
 

  
  
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Void test1 ()
  
  
{
  
  
Char string [10];
  
  
Char * str1 = "0123456789 ";
  
  
Strcpy (string, str1 );
  
  
}
 
 

Question 2:

 
 

  
  
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Void test2 ()
  
  
{
  
  
Char string [10], str1 [10];
  
  
Int I;
  
  
For (I = 0; I <10; I ++)
  
  
{
  
  
Str1 = 'a ';
  
  
}
  
  
Strcpy (string, str1 );
  
  
}
 
 

Question 3:

 
 

  
  
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Void test3 (char * str1)
  
  
{
  
  
Char string [10];
  
  
If (strlen (str1) <= 10)
  
  
{
  
  
Strcpy (string, str1 );
  
  
}
  
  
}
 
 

Answer:

Question 1 string str1 requires 11 bytes to be stored (including '\ 0' at the end), while string only has 10 bytes of space, strcpy will cause the array to cross-border;

For question 2, if the subject points out that the character array str1 cannot end in the array, it can give 3 points; if the subject points out strcpy (string, str1) calling makes the number of bytes replicated from the str1 memory to the string memory uncertain. It can be given 7 points. Based on this, it is pointed out that the strcpy function is working for 10 points;

For question 3, if (strlen (str1) <= 10) should be changed to if (strlen (str1) <10 ), the result of strlen does not count the 1 byte occupied by '\ 0.

Analysis:

Measure the test taker's knowledge about basic skills:

(1) The string ends with '\ 0;

(2) sensitivity to array out-of-bounds control;

(3) how the database function strcpy works. If the total score of a standard strcpy function is 10, the following are several different answers:

2 points

 
 

  
  
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Void strcpy (char * strDest, char * strSrc)
  
  
{
  
  
While (* strDest ++ = * strSrc ++ )! = '\ 0 ');
  
  
}
 
 

4 points

 
 

  
  
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Void strcpy (char * strDest, const char * strSrc)
  
  
// Add the source string to const, indicating that it is an input parameter and adds 2 points
  
  
{
  
  
While (* strDest ++ = * strSrc ++ )! = '\ 0 ');
  
  
}
 
 

7 points

 
 

  
  
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Void strcpy (char * strDest, const char * strSrc)
  
  
{
  
  
// Add non-0 assertions to the source and target addresses, and add 3 points
  
  
Assert (strDest! = NULL) & (strSrc! = NULL ));
  
  
While (* strDest ++ = * strSrc ++ )! = '\ 0 ');
  
  
}
 
 

10 points

 
 

  
  
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// For chained operation, add 3 points to return the destination address!
  
  
Char * strcpy (char * strDest, const char * strSrc)
  
  
{
  
  
Assert (strDest! = NULL) & (strSrc! = NULL ));
  
  
Char * address = strDest;
  
  
While (* strDest ++ = * strSrc ++ )! = '\ 0 ');
  
  
Return address;
  
  
}
 
 

We can clearly see from two to ten answers that the little strcpy has hidden so many xuanjicang! What a solid basic skill is required to write a perfect strcpy!

(4) Master strlen, which does not include '\ 0' at the end of the string '.

After reading strcpy versions with different scores, you can write a strlen function with 10 points. The perfect version is int strlen (const char * str). // enter the const parameter.

Question 4:

 
 

  
  
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Void GetMemory (char * p)
  
  
{
  
  
P = (char *) malloc (100 );
  
  
}
  
  
Void Test (void)
  
  
{
  
  
Char * str = NULL;
  
  
GetMemory (str );
  
  
Strcpy (str, "hello world ");
  
  
Printf (str );
  
  
}
 
 

Question 5:

 
 

  
  
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Char * GetMemory (void)
  
  
{
  
  
Char p [] = "hello world ";
  
  
Return p;
  
  
}
  
  
Void Test (void)
  
  
{
  
  
Char * str = NULL;
  
  
Str = GetMemory ();
  
  
Printf (str );
  
  
}
 
 

Question 6:

 
 

  
  
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Void GetMemory (char ** p, int num)
  
  
{
  
  
* P = (char *) malloc (num );
  
  
}
  
  
Void Test (void)
  
  
{
  
  
Char * str = NULL;
  
  
GetMemory (& str, 100 );
  
  
Strcpy (str, "hello ");
  
  
Printf (str );
  
  
}
 
 

Question 7:

 
 

  
  
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Void Test (void)
  
  
{
  
  
Char * str = (char *) malloc (100 );
  
  
Strcpy (str, "hello ");
  
  
Free (str );
  
  
... // Other omitted statements
  
  
}
 
 

Answer:

In question 4, the form parameter of the GetMemory (char * p) function is a string pointer. Modifying the form parameter within the function does not actually change the value of the input form parameter.

Char * str = NULL;

GetMemory (str );

The subsequent str is still NULL;

Question 5

Char p [] = "hello world ";

Return p;

The p [] array is the partial automatic variable in the function. After the function returns, the memory has been released. This is a common mistake made by many programmers. Its root cause is that they do not understand the survival of variables.

GetMemory in question 6 avoids Question 4. The input parameter of GetMemory is a pointer to the string pointer, but the request memory and the value assignment statement tiffanybracelets is executed in GetMemory.

* P = (char *) malloc (num );

If the memory application is successful, add:

If (* p = NULL)

{

... // Handle the Memory Request failure

}

Question 7 has the same problem as question 6.

Char * str = (char *) malloc (100 );

In addition, after free (str), str is not set to null, which may become a "wild" pointer. The following should be added:

Str = NULL;

In the Test function of question 6, the memory of malloc is not released.

Analysis:

Question 4 ~ 7. Measure the test taker's understanding about memory operations. Generally, 50-50 of the respondents with solid basic skills can answer these questions correctly ~ 60 error. However, it is not easy to answer the correct questions completely.

The memory operations are mainly focused on:

(1) pointer understanding;

(2) The survival time and scope of the variable;

(3) good dynamic memory application and release habits.

Let's take a look at the following program errors:

 
 

  
  
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Swap (int * p1, int * p2)
  
  
{
  
  
Int * p;
  
  
* P = * p1;
  
  
* P1 = * p2;
  
  
* P2 = * p;
  
  
}
 
 

In the swap function, p is a "wild" pointer, which may point to the system zone, causing program running to crash. In VC ++, the "Access Violation" error is prompted during DEBUG ". The program should be changed:

 
 

  
  
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Swap (int * p1, int * p2)
  
  
{
  
  
Int p;
  
  
P = * p1;
  
  
* P1 = * p2;
  
  
* P2 = p;
  
  
}
 
 

In short, the interview questions of C/C ++ programmers not only examine the knowledge level of the subject, but also examine the agile Thinking Ability of the subject. As a R & D Engineer, through in-depth analysis of these interview questions, you can further enhance your internal strength.

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