运行以下代码时，出现EXC_BAD_ACCESS错误

Question

这是相关的代码。当我尝试dest，src，start和replace的一些值时，在这些行上出现错误：

strncat(&returnV[i], &dest[i], 1);
strncat(&returnV[i], &src[i - start], 1);
strncat(&returnV[i], &src[i - start], 1);
strncat(&returnV[i], &dest[i - strlen(src) + replace], 1);

下面是相关代码的其余部分。

char * insertString(char * dest, char * src, int start, int replace) {

    static char *returnV = "";

    for(unsigned int i = 0; i < start; i++) {
        strncat(&returnV[i], &dest[i], 1);
    }
    for(unsigned int i = start; i < start + replace; i++) {
        strncat(&returnV[i], &src[i - start], 1);
    }
    for(unsigned int i = start + replace; i < (start + strlen(src)); i++) {
        strncat(&returnV[i], &src[i - start], 1);
    }
    for(unsigned long i = start + strlen(src); i < (strlen(src) + strlen(dest)); i++) {
        strncat(&returnV[i], &dest[i - strlen(src) + replace], 1);
    }

    return returnV;

}

我认为这是发生错误的地方，我试图将其修复为无济于事。如果需要的话，这是其余的代码：

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

char * insertString(char * dest, char * src, int start, int replace);

int main(int argc, const char * argv[]) {

    char * result;
    char dest[50];
    scanf("%s", dest);
    char src[50];
    scanf("%s", src);
    int start;
    scanf("%d", &start);
    int replace;
    scanf("%d", &replace);

    result = insertString(dest, src, start, replace);
    printf("%s", result);
    return 0;
}

char * insertString(char * dest, char * src, int start, int replace) {

    static char *returnV = "";

    for(unsigned int i = 0; i < start; i++) {
        strncat(&returnV[i], &dest[i], 1);
    }
    for(unsigned int i = start; i < start + replace; i++) {
        strncat(&returnV[i], &src[i - start], 1);
    }
    for(unsigned int i = start + replace; i < (start + strlen(src)); i++) {
        strncat(&returnV[i], &src[i - start], 1);
    }
    for(unsigned long i = start + strlen(src); i < (strlen(src) + strlen(dest)); i++) {
        strncat(&returnV[i], &dest[i - strlen(src) + replace], 1);
    }

    return returnV;

}

非常感谢您的帮助！