C错答案

Question

I can't figure out what's wrong with my code

char* readString() {

    char* result;
    char line[SIZE];

    if(! fgets(line, sizeof(line), stdin))
        fprintf(stderr, "error\n");

    //printf 1
    printf("line length = %lu\n", strlen(line));

    result = line;

    return result;
}

int main() {

    char* myWord = readString();
    unsigned long len = (unsigned)strlen(myWord);

    //printf 2
    printf("myWord length = %lu\n", len);

    return 0;
} 

If printf1 and printf2 are both uncommented then I get true length. For example for word 'hello' I have output

line length = 6
myWord length = 6

but If I only comment printf1, for the same word 'hello' I have

myWord length = 16

Answer 1

Your code exhibits undefined behavior . You are returning a pointer to a variable declared on the stack. In this case, line or result .

Two possible fixes include:

1. Pass the char line[SIZE]; variable to the readString() function.
2. Allocate the memory for line on the heap using malloc() .

Answer 2

You return a dynamic value (stack allocated) which is deleted by return . You should either

• use a static string, but it won't support multithreading nor recursively

• let the caller define the string as

   char * readstring(char * line, size_t size) { // Your stuff char * result = fgets(line, size); // ... return result; }