How to Use strdup Functions

Function Name: strdup

Function: copy the string to the new position.

Usage: char * strdup (char * Str );

 

This function is interpreted:

The strdup () function returns a pointer Toa new string which is

Duplicate of the string S. Memory for thenew string is obtained

Malloc (3), and can be freed with free (3 ).

The strndup () function is similar, but onlycopies at most N charac-

TERs. If S is longer than N, only ncharacters are copied, and a Termi-

Nating NUL is added.

 

Strdup function prototype:

Strdup () is mainly used to copy a copy of string s, which is returned by the function return value. This copy has its own memory space and is irrelevant to S. After the strdup function is used to copy a string, remember to delete the dynamically applied memory in the function. The number of records of the strdup function cannot be null. If it is null, a segment error is reported, because this function contains the strlen function, the number of records of this function cannot be null.

How strdup works:

Char * _ strdup (const char * s)

{

Size_t Len = strlen (s) + 1;

Void * New = malloc (LEN );

If (New = NULL)

Return NULL;

Return (char *) memcpy (new, S, Len );

}

 

Instance 1:

C/C ++ code

# Include <stdio. h>

# Include <string. h>

# Include <alloc. h>

Int main (void)

{

Char * dup_str, * string = "ABCDE ";

Dup_str = strdup (string );

Printf ("% s \ n", dup_str); free (dup_str); Return 0;

}

 

Instance 2:

# Include <stdio. h>

# Include <stdlib. h>

# Include <string. h>

Unsigned int test ()

{

Charbuf [] = "Hello, world! ";

Char * pb = strndup (BUF, strlen (BUF ));

Return (unsignedint) (PB );

}

 

Int main ()

{

Unsigned int PCH = test ();

Printf ("testing: % s \ n", (char *) PCH );

Free (void *) PCH );

Return 0;

}

Use strndup in the test function, and the test function can still operate the memory and release it.

The problem extended from this problem is how to make the memory data obtained by the function outgoing but still available.

Now I only think of two solutions: an external variable, such as passing a memory block pointer to the function. However, you have to pass enough memory, that is, you cannot know the buffer size required by this function in advance.

Another method is to apply for static variables in the function, which is also a global variable. However, the disadvantage of this method is that when the function is executed multiple times, the data in the static variable is overwritten. Another method of this type is to use global variables, but this is very similar to the use of static variables. The difference is that global variables can be operated and controlled, while static variables are assumed not to pass out the function, it cannot be controlled. Another method is described above, which is implemented using the memory in the heap, but there is a critical situation. Strdup allocates space from the heap! Strdup calls malloc, so it needs to be released! For stacks: stacks are managed by programmers. For example, new and malloc are allocated on stacks!

Stack is managed by the compiler.