带字符串的 Printf 不起作用（2 种方式：函数返回值和一个值）

Question

So, my problem is whenever I want to print %s (char *), the program shows an empty string. Here is my code and I numbered all the questions inside the comments in the program

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *decToBinary(char *num, int size);

int main(){
    char *n;
    n = (char *)malloc(5);  
    //memset(n, 0, 5);
    printf("Enter n: ");

    // getting the number as a string
    scanf("%s", n);
    int size = 33;
    char *binNum;
    binNum = (char *)malloc(size);  
    memset(binNum, 0, size);

    //copying the (char *) which is returned by function to char binNum[33] 
    strcpy(binNum, decToBinary(n, size));

    //sprintf(binNum, "%s", decToBinary(n, size));

    // Printing the FUNCTION variable as a character in a loop. WORKS(WHY?)(3)
    for(int i = 0; i < size; i++){
        printf("With function: %c", decToBinary(n, size)[i]);
    }
    //Printing the variable binNum as a string <--------- DOES NOT WORK. JUST EMPTY MESSAGE(WHY?)(4)
    printf("In main: %s\n", binNum);

    // Printing the variable binNum as a character in a loop <------------ DOES NOT WORK(5)
    for(int i = 0; i < size; i++){
        printf("with variable: %c", binNum[i]);
    }
    printf("\n");
}

char *decToBinary(char *num, int size){
    // convert decimal to binary
    int i = size - 2;
    int remainder;
    int decimal;
    char *binary;
    int n;
    n = atoi(num); 
    binary = (char *)malloc(size);
    binary[size - 1] = '\0';
    memset(binary, 0, size);
    do{
        remainder = n % 2;
        n /= 2;
        binary[i] = remainder + '0';
        i--;
    }while(n);
    printf("decToBinary = %s\n", binary); // does not work as string (1)
    for(int j = 0; j < size; j++){
        printf("%c", binary[j]); // but this works(2)
    }
    printf("\n");
    return binary;   
}

• (1) The string is not displayed in the function. Even if I flush the buffer by using "\\n"
• (2) But when I use "for" loop and display each element of the array using %c - it works. But this is not a good way to make it work
• (3) In main() function printf of every element of return value from the function works fine even though I do not get the idea how it can work, because as I understood,the function is invoked 33 times and the value is returned 33 times and here I just read one element by one. That's bad practice I think.
• (4) So I copied the returned value from function to the variable binNum and tried to display it using %s, but again empty string is displayed.
• (5) Same thing with variable but using "for" loop and %c. Also did not work

So, my question is how to manage these problems?

Answer 1

If you change your code a bit

    for(int j = 0; j < size; j++){
        printf("%c 0x%02x\n", binary[j], binary[j]); // but this works(2)
    }

you will see the leading '\\0' in your result:

With function: 1decToBinary =
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
 0x00
1 0x31
0 0x30
0 0x30
0 0x30
0 0x30
0 0x30
0 0x30
0 0x30
0 0x30
1 0x31
1 0x31
 0x00

So there is something wrong in your function decToBinary .

You count with i from the end of the array, so depending on the input you may not reach the beginning of the array.

You can try return binary+i+1; at the end of decToBinary . This will return a pointer to the character you have written last. Note that you should not iterate over size characters of the result but only up to a '\\0 .

This is not the final solution because you use malloc to allocate the character array in decToBinary . This means you should return the pointer you got from malloc and call free in the calling function.

To allow this you can use an additional loop that copies all characters from index i+1 up to a '\\0' to the start of the array.

    for(i++, j=0; binary[i]; i++, j++) {
        binary[j] = binary[i];
    }
    binary[j] = '\0';

    /* ... */

    return binary;

Additional note:

Code like this creates a memory leak because you don't free the results of repeated calls to decToBinary :

    for(int i = 0; i < size; i++){
        printf("With function: %c", decToBinary(n, size)[i]);
    }