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c - 无法打印 typedef 结构的 char* 类型字段

问题描述

以下代码将十进制转换为二进制并将其二进制表示存储到结构中。get_binary_representation()可以正常工作，您可以通过取消注释打印val. 但是，当我尝试打印存储在里面的二进制表示时，binary什么都没有被打印出来。

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <inttypes.h>
#include <stdbool.h>


typedef struct {
    char* val;
    int   length;
} binary;


char* get_binary_representation(uint16_t val, int length) {
    
    char* result; 

    if (length < 1)
        return NULL;

    if (val == 0) {
        result = (char*) malloc(sizeof(char) * 2);
        result[0] = '0';
        result[1] = '\0';
        return result;
    }

    result = (char*) malloc(sizeof(char) * length);
    result[length - 1] = '\0';

    for (int i = length; i > 0; i--) {
        result[i] = ((val & 1) == 1) ? '1' : '0';
        // printf("Bit %d = %c\n", i, result[i]);
        val >>= 1;
    }

    return result;
}


binary i16_to_binary(uint16_t* val) {
    binary result; // tried replacing it with pointer and using malloc, didn't help
    result.val = get_binary_representation(*val, 16);    
    /* There is a function that finds length of number in binary 
       but I excluded it from MRE for simplicity sake. 
       16 for length because uint16_t is 16 bit integer*/
    return result;
}


int main(int argc, char** argv) {
    uint16_t a = 16; uint16_t b = 254; uint16_t c = 37;
    
    binary a_bin = i16_to_binary(&a);
    binary b_bin = i16_to_binary(&b);
    binary c_bin = i16_to_binary(&c);
    
    /* should print 0000 0000 0001 0000, 0000 0000 0111 1110, 0000 0000 0010 0101
       without spaces and commas */
    printf("%s %s %s\n", a_bin.val, b_bin.val, c_bin.val);

    return 0;
}

标签： cgccstructchar

解决方案

当心：

  • 不要导致缓冲区溢出。只有索引0（length-1包括两者）可用于带有length元素的数组。result[i]时超出范围i = length。
  • 初始化所有要读取的元素。reuslt[0]未初始化。
  • malloc()家庭铸造结果被认为是一种不好的做法。

那个部分：

    if (val == 0) {
        result = (char*) malloc(sizeof(char) * 2);
        result[0] = '0';
        result[1] = '\0';
        return result;
    }

    result = (char*) malloc(sizeof(char) * length);
    result[length - 1] = '\0';

    for (int i = length; i > 0; i--) {
        result[i] = ((val & 1) == 1) ? '1' : '0';
        // printf("Bit %d = %c\n", i, result[i]);
        val >>= 1;
    }

应该：

    if (val == 0) {
        result = malloc(sizeof(char) * 2); /* no casting the result */
        result[0] = '0';
        result[1] = '\0';
        return result;
    }

    result = malloc(sizeof(char) * (length + 1)); /* no casting the result and allocate enough elements */
    result[length] = '\0'; /* put NUL to proper place */

    for (int i = length - 1; i >= 0; i--) { /* adjust the indice */
        result[i] = ((val & 1) == 1) ? '1' : '0';
        // printf("Bit %d = %c\n", i, result[i]);
        val >>= 1;
    }

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