I need to get the first item from an string arrays in C, this is my code:

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


int main(){

char* codes[16]={"VVVV","VVVB","VVBV","VVBB","VBVV","VBVB","VBBV","VBBB","BVVV","BVVB","BVBV","BVBB","BBVV","BBVB","BBBV","BBBB"};
char code[4];
int ret=0;
int j = 0;


printf("Enter code to search: ");
scanf("%s",code);

for( j=0; j<16; j  )
{
        printf("\ncode to search is:  %s, j value: %d, codes[j]: %s", code,j, codes[j]);    
ret = strcmp(code,codes[j]);
        if (ret == 0)
    {
        printf("\nValue of index: %d\n",j);
    }
}

}

When I enter the first code to search (VVVV) I get a void value in position 0, and It doesn't return the index 0. Another values works but the first doesn't.

This is the output:

Enter code to search: VVVV

code to search is:  VVVV, j value: 0, codes[j]: 
code to search is:  VVVV, j value: 1, codes[j]: VVVB
code to search is:  VVVV, j value: 2, codes[j]: VVBV
code to search is:  VVVV, j value: 3, codes[j]: VVBB
code to search is:  VVVV, j value: 4, codes[j]: VBVV
code to search is:  VVVV, j value: 5, codes[j]: VBVB
code to search is:  VVVV, j value: 6, codes[j]: VBBV
code to search is:  VVVV, j value: 7, codes[j]: VBBB
code to search is:  VVVV, j value: 8, codes[j]: BVVV
code to search is:  VVVV, j value: 9, codes[j]: BVVB
code to search is:  VVVV, j value: 10, codes[j]: BVBV
code to search is:  VVVV, j value: 11, codes[j]: BVBB
code to search is:  VVVV, j value: 12, codes[j]: BBVV
code to search is:  VVVV, j value: 13, codes[j]: BBVB
code to search is:  VVVV, j value: 14, codes[j]: BBBV
code to search is:  VVVV, j value: 15, codes[j]: BBBB

What could be wrong?

CodePudding user response：

Dude I have tried your code and it works completely fine , no it does not return void value at index zero.

Cause at iteration j=0, for "VVVV" ret = strcmp(code,codes[j]); ,it will return (ret=0), So 'if' statement would work just fine and index zero will be printed as a result.You need to be careful and to post the exact code that produces the exact result you show.

CodePudding user response：

Your code has a buffer overflow, which is a very dangerous kind of undefined behaviour.

The following line

char code[4];

declares an array of char that can hold up to 3 characters plus a null-terminator character, which add up to 4 characters total. This is because C strings are null-terminated.

When you attempt to scan a 4 character string (don't forget the 5th \0 that's added automatically) into a 3 character buffer (plus the 4th \0), your code overflows. Therefore, you need to change the line above to

char code[5];

That said, your code is far from being trouble-safe. And yes, I mean that scanf() call. I suggest you read this post as it explains the pitfalls of that function, and provides alternative solutions.

Here is a better version of your code:

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

int main(void)
{
    char *codes[] = { // Let your array be flexible in case you want to add to it later.
        "VVVV", "VVVB", "VVBV", "VVBB",
        "VBVV", "VBVB", "VBBV", "VBBB",
        "BVVV", "BVVB", "BVBV", "BVBB",
        "BBVV", "BBVB", "BBBV", "BBBB"
    };
    
    // Computes the size (number of items) of a STATIC array.
    // Also, use size_t as it's the appropriate type for sizes.
    size_t ncodes = sizeof(codes) / sizeof(codes[0]);
    
    char code[5]; // <-- 4 characters   \0
    printf("Enter code to search: ");
    scanf("%4[^\n]", code);
    
    bool found = false;
    
    for(size_t j = 0; j < ncodes;   j) {
        if (strcmp(code, codes[j]) == 0) {
            printf("Code found at index: %ld\n", j);
            found = true;
            break; // Stop looping when you find the first occurence
        }
    }
    
    if (!found) {
        printf("Code %s not found.\n", code);
    }
}