指针和字符串的分段错误

Question

I am writing a pointer version of the C function strcat. It copies the string t to the end of s. This is my solution:

/* strcat: a pointer version of the strcat (copy string t to the end of s) */
void strcat (char *s, char *t)
{
    while (*s++ != '\0')    /* find the end of s */
        ;

    while (*s++ = *t++)
    ;
}

I ran it and it crashed - the Code Blocks debugger called it a segmentation fault, and that this part of the function was causing the crash:

while (*s++ = *t++)

What have I done wrong?

Answer 1

Here is the fixed version and a test program:

#include <stdio.h>

void strcat (char *s, char *t)
{
    while (*s++) 
        ;

    s--;

    while (*s++ = *t++)
        ;
}

int
main(void)
{
    char str1[100] = "abc";
    char *str2 = "def";
    strcat(str1, str2);
    printf("%s\n", str1);
    return 0;
}

---

If you call that strcat() in the following way,

char *str1 = "abc";
char *str2 = "def";
strcat(str1, str2);

then your program may crash, because compilers usually put string literals in read-only memory region, try to write to these places will cause segment fault.