C - 在离开while循环后覆盖数组中的值

Question

I have some strings I add into an array in a while loop. Everything is good and seems added inside the while loop, but when I leave the while loop and loop through the array to inspect the values, the last value added appears in all the indexes.

Here is the code:

    char buffer[50];
    char *keys[1000] = {""}; 
    int count = 0;

    FILE *file = fopen(argv[1], "r");
    if (file == NULL) {
            return 1;
    } 

    while (fgets(buffer, 100, file) != NULL) {
            buffer[strlen(buffer) - 1] = '\0';           
            printf("Buffer being read: %s\n", buffer);
            keys[count] = buffer;
            printf("Buffer after storing: %s, index: %d\n", keys[count], count);
            count++;
            if (count == 3) {
                    break;
            }
    }

    printf("Reading from stored array\n");
    printf("%s, index: %d\n", keys[0], 0);
    printf("%s, index: %d\n", keys[1], 1);
    printf("%s, index: %d\n", keys[2], 2);

And here is the output:

Buffer being read: 0DJz9J/xOBK/chDRPFvOwg==
Buffer after storing: 0DJz9J/xOBK/chDRPFvOwg==, index: 0
Buffer being read: IHKA7WHFdhQIRpqFmqT1ew==
Buffer after storing: IHKA7WHFdhQIRpqFmqT1ew==, index: 1
Buffer being read: TtD7k1Z+4PFLfl46xWOZgQ==
Buffer after storing: TtD7k1Z+4PFLfl46xWOZgQ==, index: 2
Reading from stored array
TtD7k1Z+4PFLfl46xWOZgQ==, index: 0
TtD7k1Z+4PFLfl46xWOZgQ==, index: 1
TtD7k1Z+4PFLfl46xWOZgQ==, index: 2

As you can see, after I leave the while loop, somehow the values are magically over ridden.

Any ideas what could be causing this?

Thanks

Answer 1

You have multiple problems in your code:

buffer is defined with a size of 50 and you pass a maximum size of 100 to fgets :

fgets(buffer, 100, file);

You should instead increase the buffer size and use sizeof to keep the use and definition in sync:

fgets(buffer, sizeof buffer, file);

Furthermore, you should not assume that the buffer contains a final '\\n' after a successful fgets . If the file does not end with a linefeed, the final read will not store one into the buffer. A simple and reliable way to remove the final '\\n' if present is this:

buffer[strcspn(buffer, "\n")] = '\0';

You store the address of buffer in all your entries in the keys array:

keys[count] = buffer;

Be aware that this does not duplicate the contents of the array, but just stores its address. So all entries in keys point to the same buffer, that has the contents of the last line read (or even undefined contents if end of file was reached). You can fix this by allocating a copy of the string:

keys[count] = strdup(buffer);

But remember to free all these strings before returning for this function.