为字符串分配内存

Question

I try to allocate memory , store a string then print it but I think it didn't work.

#include<stdio.h>
#include<windows.h>
int main()
{
    char* allochere;
    allochere = malloc(sizeof(char));
    *allochere = "Hello";
    printf("%s",allochere);


    return 0;
}

Answer 1

The assignment to *allochere only assigns a value to the first character of the uninitialized string. The value assigned is nonsensical to boot, since the right-hand side of the assignment is a pointer. (If you turn on compilation warnings, the compiler will warn you of problems with such an assignment.)

To correctly initialize the string, you need to call strcpy to copy the string:

strcpy(allochere, "Hello");

Also, you need to allocate allochere with the correct size to hold the string you intend to put there:

allochere = malloc(strlen("Hello") + 1);

Answer 2

There are several mistakes here.

First of all, you should know that C-strings end with a zero byte . Thanks to this, you don't have to store string's length, you're just saying "here's the end of my string" using the zero byte.

Secondly, you call malloc() in a wrong way. If you want to allocate memory for an N-letter string, you should write malloc(N + 1); (+1 because the zero byte has to have its cell, too).

Lastly, you should use a function like strcpy to copy the string to its destination. In this case, you should call strcpy(allochere, "Hello") .

Your code should look like this:

char* allochere;
allochere = malloc(6*sizeof(char));
strcpy(allochere, "Hello");
printf("%s", allochere);

...

// don't forget to deallocate the memory
free(allochere);

Answer 3

That code doesn't make any sense.

char *allochere;

'allochere' is a pointer to a char, in other words, a string.

allochere = malloc(sizeof(char));

Now you allocate a single character, and point 'allochere' to that. BTW, sizeof(char) is always 1.

*allochere = "Hello";

Now you set the first char of 'allochere' to a 'const char *', a compiler would barf here. Now allochere points to random data, not a string.

Let's see some alternatives that work.

char *allochere;
allochere = "Hello";
printf("%s", allochere);

char *allochere;
allochere = strdup("Hello");
printf("%s", allochere);

char *allochere;
allochere = malloc(strlen("Hello") + 1);
strcpy(allochere, "Hello");
printf("%s", allochere);

Answer 4

You are not allocating enough memory to hold the word "Hello" - you need to allocate space for 5 chars plus the terminating byte, so you should replace the malloc call with:

allochere = malloc(6);

You can't assign a string literal to allochere because you will miss the pointer to the allocated memory. Instead, after allocating enough space, use strcpy:

strcpy(allochere, "Hello");

Answer 5

You need something like this.

#include<stdio.h>
#include<windows.h>

int main()
{
    char* allochere;

    allochere = malloc(strlen("Hello") + 1);

    strcpy(allochere, "Hello");
    printf("%s\n", allochere);

    return 0;
}

Answer 6

In this case, you need to actually copy the data directly from the string constant "Hello" into your allocated space. However, before we even get there, there are a couple of issues.

1. Make sure you're allocating enough space.

   You are allocating the sizeof one character, not enough to hold the entire string (5 characters in "Hello" plus one more for the null terminator). You can do this with

    allochere = malloc(6); 

   or use the common convention of

    allochere = malloc(strlen("Hello") + 1); 

   To be more clear about the size and purpose you're allocating the memory for.

2. You need to copy the data because string assignment doesn't understand exactly what you're trying to do. By doing

    *allochere = "Hello"; 

   You're actually assigning the value at allochere to be the address of "Hello" . If instead you had done this:

    allochere = "Hello"; 

   That might actually work (on some systems -- I'm not sure and haven't tested it), but that's because you are assigning and address to an address.

   However, you are malloc ing data which means you want to copy the data. Thus, you will want to use something like strcpy to copy from the location into your allocated string. Hence:

    strcpy(allochere, "Hello"); 

3. The last problem is going to be actually seeing the output. Because you're using printf , you will need to flush the data to the screen. printf will automatically flush the data if you have a newline ( '\\n' ) character, but you could also use fflush() .