使用递归反转字符串 - 获取“线程 1：EXC_BAD_ACCESS（代码 = 1，地址 = 0x0）”错误

Question

I'm trying to reverse a string with the following code but am getting an error Thread 1: EXC_BAD_ACCESS (code=1, address=0x0) .

I understand that it's telling me that I'm trying to access a NULL pointer but I fail to see where:

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

int empty(char s[]){
    return strcmp(s, "") == 0 ? 1 : 0;
}

char getHead(char s[]){
    char *dup = (char *)malloc(sizeof(char));
    strcpy(dup, (s + 0));
    return *dup;
}

char * getTail(char s[]){
    char *dup = malloc(sizeof(char) * strlen(s) - 1);
    for(int i = 0; i < strlen(s) - 1; i++){
        dup[i] = s[i+1];
    }
    return dup;
} 

char * ReverseStringHeadTail(char s[]){
    if(empty(s))
    {
        return NULL;
    }
    else
    {
        char head = getHead(s);
        char *tail = getTail(s);
        return strcat(ReverseStringHeadTail(tail), head); // Error: Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
    }
}

int main(int argc, const char * argv[]) {
    char string[] = "string";
    printf("%s\n", ReverseStringHeadTail(string));

    return 0;
}

Thanks!

Answer 1

The function empty can be written simpler.

int empty( const char *s )
{
    return *s == '\0';
}

The function getHead is invalid and does not make sense. Its allocates memory for one character but is trying to copy a whole string. And its call results in a memory leak.

You can just write

char getHead( const char *s )
{
    return *s;
}

The function getTail is also invalid. For example if the string is for example "A" then the function allocates a memory of 0 bytes. In any case the allocated memory will not contain a string.

The function strcat may not be used with an object of the type char or with a null-pointer.

It seems what you mean is the following.

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

char * ReverseStringHeadTail( const  char *s )
{
    if ( *s == '\0' ) return calloc( 1, 1 );

    char tmp[2] = { *s, '\0' };

    char *p = ReverseStringHeadTail( s + 1 );
    p = realloc( p, strlen( p ) + sizeof( tmp ) );

    return strcat( p, tmp );
}

int main(void) 
{
    const char *s = "12345";

    puts( s );

    char *p = ReverseStringHeadTail( s );

    puts( p );

    free( p );

    return 0;
}

The program output is the following

12345
54321

Pay attention to that the function does not check whether the memory was allocated successfully.

Answer 2

char *revm(char *buff, const char *str, const size_t len)
{
    if(!len) len = strlen(str);
    if(!buff)
    {
        buff = calloc(len + 1, 1);
    }
    if(buff && len)
    {
        *buff = *(str + len - 1);
        *(buff + len - 1) = *str;
        if(len > 2) rev(buff + 1, str + 1, len - 2);
    }
    return buff;
}

int main(void)
{

    char *str = "1234567890";

    char *str1 = rev(NULL, str, 0);
    printf("%s\n", str1);
    return EXIT_SUCCESS;
}

or if you do not want to reverse the string given (it has to be writable)

char *rev(char *str, size_t len)
{
    if(!len) len = strlen(str);

    if(len)
    {
        int tmp = *str;
        *str = *(str + len - 1);
        *(str + len - 1) = tmp;
        if(len > 2) rev(str + 1, len - 2);
    }
    return str;
}


int main(void)
{

    char str[] = "123456789";

    char *str1 = rev(str, 0);
    printf("%s\n", str1);
    return EXIT_SUCCESS;
}