結構成員char *在函數調用中初始化為參數

Question

這更多是出於好奇，而不是我想要或需要它的方式。 但是，當我做一些樣機並想測試某些東西時，我最終得到了這樣的東西……並且想知道為什么它不能按我預期的那樣工作。

typedef struct {
    char *a;
    char *b;
    char *c;
}mystruct;

void init_chars (char *arg)
{
    arg = malloc (sizeof (char)*10);
    arg = "0123456789";
    printf ("%s\n", arg);
}

int main ()
{
    mystruct *msp = malloc (sizeof (mystruct));
    init_chars (msp->a);
    init_chars (msp->b);
    init_chars (msp->c);
    printf ("%s, %s, %s\n", msp->a, msp->b, msp->c);
    return 0;
}

打印...

0123456789

0123456789

0123456789

(null),(null),(null)

Answer 1

將值傳遞給函數參數時，C中有兩件事

1. 傳遞價值
2. 通過參考

您正在按值傳遞，因此您要將一些未初始化的值傳遞給該函數，並在該函數外部看不見的函數中對其進行初始化，因此您試圖在main()使用未初始化的值，這將導致未定義的行為。

arg = "0123456789";

如果要將字符串復制到某個內存位置，則需要memcpy()或strcpy()

注意：使用未初始化的值會導致未定義的行為。

Answer 2

您的代碼有幾個問題。 這是固定代碼：

/* Don't forget to include <stdio.h>, <stdlib.h> and <string.h> */

typedef struct {
    char *a;
    char *b;
    char *c;
}mystruct;

void init_chars (char **arg) /* This should be char** as the address of a char* is passed */
{
    *arg = malloc ( /*sizeof(char)* */ 11); /* sizeof(char) is 1 always. You don't need it */
                                            /* Note the change of `arg` to `*arg` too */
                                            /* And that I've used 11 and not 10 because there needs to be space for the NUL-terminator */

    if(*arg == NULL) /* If the above malloc failed */
    {
        printf("Oops! malloc for *arg failed!");
        exit(-1);
    }

    //arg = "0123456789"; /* You just lost the allocated memory as you make the pointer point to a string literal */

    strcpy(*arg, "0123456789"); /* Use strcpy instead */
    printf ("%s\n", *arg);      /* *arg here */
}

int main ()
{
    mystruct *msp = malloc (sizeof (mystruct));

    if(msp == NULL) /* If the above malloc failed */
    {
        printf("Oops! malloc for msp failed!");
        return -1;
    }

    init_chars (&(msp->a));
    init_chars (&(msp->b)); 
    init_chars (&(msp->c));  /* Pass address of variables rather than their value */

    printf ("%s, %s, %s\n", msp->a, msp->b, msp->c);

    free(msp->a);
    free(msp->b);
    free(msp->c);

    free(msp);    /* Free everything after use */

    return 0;
}

Answer 3

typedef struct {
    char *a;
    char *b;
    char *c;
}mystruct;

void init_chars (char **arg)
{
    *arg = malloc (sizeof (char)*11);
    if(*arg == NULL)
    {
        printf("malloc failed!");
        exit(-1);
    }
    strcpy(*arg,"0123456789");
    printf ("%s\n", *arg);
}

int main ()
{
    mystruct *msp = malloc (sizeof (mystruct));
    if(msp == NULL)
    {
        printf("malloc failed");
        exit(-1);
    }
    init_chars (&msp->a);
    init_chars (&msp->b);
    init_chars (&msp->c);
    printf ("%s, %s, %s\n", msp->a, msp->b, msp->c);
    free(msp->a);
    free(msp->b);
    free(msp->c);
    free(msp)
    return 0;
}

輸出：

0123456789

0123456789

0123456789

0123456789、0123456789、0123456789

與您的代碼進行比較，您將理解為什么