C - MemncpyStrncpy(也试过strncat)复制的字符比应该的少了1个，希望得到帮助。

Question

就像我说的，memncpy()(在main中间)复制的字符比应该少了1个，不知道为什么。我加了注释和一张图片，让它更容易理解。

    #define BIT_AMOUNT 4

char * randomBinaryGenerator(char * random){

    int randomNum, i;
    char temp;
    char * tempPtr = (char *)malloc(1 * sizeof(char));

    for(i = 0; i <= BIT_AMOUNT - 1; i++){

        randomNum = rand() % 2;
        temp = randomNum + '0';

        tempPtr = NULL;
        tempPtr = &temp;
        strcat(random, tempPtr);
    }

    return random;
}

int main(){

    srand(time(0));
    char str_bin_bitKey[BIT_AMOUNT] = "";

    char * random = (char *)malloc(BIT_AMOUNT * sizeof(char));
    char * bin_bitKey = (char *)malloc(BIT_AMOUNT * sizeof(char));

    printf("\nSize of str_bin_bitKey: %ld, Size of bin_bitKey: %ld\n", sizeof(str_bin_bitKey), sizeof(bin_bitKey));

    bin_bitKey = randomBinaryGenerator(random);    //generates 4 bit long binary number
    memcpy(str_bin_bitKey, bin_bitKey, BIT_AMOUNT);//copies 1 character less

    printf("\nbin_bitKey:     %s\n", bin_bitKey);    //4 bits
    printf("\nstr_bin_bitKey: %s\n", str_bin_bitKey);//3 bits???

    long long dec_bitKey = 0;//unimportant for now .... convertBinaryToDecimal(bin_bitKey); 

    printf("\ndec_bitKey: %lld\n\n", dec_bitKey);

    free(random);
    return 0;
}

这是输出的结果，你可以看到str_bin_bitKey是3个字符，而不是4个。

感谢大家的帮助

Answer 1

一些问题...

在 main数组指针的大小需要考虑到nul终结符，所以它们需要是 BIT_AMOUNT + 1

在 main，你的 memcpy 是否 不 复制nul终止符。使用 strcpy 来代替。

添加起始nul最容易的方法是(例如)。

*random = 0;

不要投 malloc: 我是否要投掷malloc的结果？

sizeof(char) 是 始终 1 (根据定义)，不管实际的，依赖于架构的大小(例如 char 实际上是16位）。) 所以，不要使用 sizeof(char)

在 randomBinaryGenerator, tempPtr 泄漏内存。不需要 malloc [甚至 tempPtr 根本]。使用 char temp[2]; 而不是。

sizeof(bit_bitKey)总是恒定的，因为它的大小是由 指针 和 不 它所指向的事物（即它是 不 BIT_AMOUNT).

randomBinaryGenerator 需要几乎完全重做。

---

这是你的代码的注释和修正版本。我添加了:

#if 0
// old/original code
#else
// new/fixed code
#endif

来帮助显示这些变化

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#define BIT_AMOUNT 4

char *
randomBinaryGenerator(char *random)
{

    int randomNum, i;
#if 0
    char temp;
    char *tempPtr = malloc(1);
#else
    char temp[2];
#endif

#if 1
    // add nul terminator
    *random = 0;
    temp[1] = 0;
#endif

#if 0
    for (i = 0; i <= BIT_AMOUNT - 1; i++) {
#else
    for (i = 0; i < BIT_AMOUNT; i++) {
#endif
        randomNum = rand() % 2;
#if 0
        temp = randomNum + '0';
        tempPtr = NULL;
        tempPtr = &temp;
        strcat(random, tempPtr);
#else
        temp[0] = randomNum + '0';
        strcat(random, temp);
#endif
    }

    return random;
}

int
main(void)
{

    srand(time(0));

// NOTE/BUG: need space for EOS terminator
#if 0
    char str_bin_bitKey[BIT_AMOUNT] = "";
    char *random = malloc(BIT_AMOUNT);
    char *bin_bitKey = malloc(BIT_AMOUNT);
#else
    char str_bin_bitKey[BIT_AMOUNT + 1] = "";
    char *random = malloc(BIT_AMOUNT + 1);
    char *bin_bitKey = malloc(BIT_AMOUNT + 1);
#endif

// NOTE/BUG: sizeof(bit_bitKey) is the size of the _pointer_ and _not_ what
// it points to (i.e. it is _not_ BIT_AMOUNT)
#if 0
    printf("\nSize of str_bin_bitKey: %ld, Size of bin_bitKey: %ld\n",
        sizeof(str_bin_bitKey), sizeof(bin_bitKey));
#endif

    // generates 4 bit long binary number
    bin_bitKey = randomBinaryGenerator(random);

    // copies 1 character less
#if 0
    memcpy(str_bin_bitKey, bin_bitKey, BIT_AMOUNT);
#else
    strcpy(str_bin_bitKey, bin_bitKey);
#endif

    // 4 bits
    printf("\nbin_bitKey:     %s\n", bin_bitKey);
    // 3 bits???
    printf("\nstr_bin_bitKey: %s\n", str_bin_bitKey);

    // unimportant for now .... convertBinaryToDecimal(bin_bitKey);
    long long dec_bitKey = 0;

    printf("\ndec_bitKey: %lld\n\n", dec_bitKey);

    free(random);

    return 0;
}

---

这是一个经过清理和改进的版本。请注意 randomBinaryGenerator 更快更好 橆 使用 strcat 完全没有。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#define BIT_AMOUNT 4

char *
randomBinaryGenerator(char *random)
{

    int randomNum, i;

    for (i = 0; i < BIT_AMOUNT; i++) {
        randomNum = rand() % 2;
        random[i] = randomNum + '0';
    }

    // add nul terminator
    random[i] = 0;

    return random;
}

int
main(void)
{

    srand(time(0));

    char str_bin_bitKey[BIT_AMOUNT + 1];
    char *random = malloc(BIT_AMOUNT + 1);
    char *bin_bitKey = malloc(BIT_AMOUNT + 1);

    // generates 4 bit long binary number
    bin_bitKey = randomBinaryGenerator(random);

    strcpy(str_bin_bitKey, bin_bitKey);

    // 4 bits
    printf("\nbin_bitKey:     '%s'\n", bin_bitKey);

    // 3 bits???
    printf("\nstr_bin_bitKey: '%s'\n", str_bin_bitKey);

    // unimportant for now .... convertBinaryToDecimal(bin_bitKey);
    long long dec_bitKey = 0;

    printf("\ndec_bitKey: %lld\n\n", dec_bitKey);

    free(random);

    return 0;
}