Copying valid strings to 2d array in C

Question

I am checking if a function returns true, it prints out valid strings according some other function I got. At the moment, it's printing it out correctly but it is also printing empty lines which seem to correspond to the invalid strings.

How can I make these empty lines go away?

Here is my code:

int main()
{
    int i, count = 0;
    char input[10];
    char validStr[10][60] = {""};
    for (i = 0; i < 60; ++i){
        if(fgets(input,10, stdin) == NULL){
            break;
        }
        input[strcspn(input,"\n")] = '\0';
        if(checkIfValid(input)){
            memcpy(validStr[i],input,sizeof(input));
            count++;
        }
    }
    printf("%d\n",count);
    for (int j = 0 ; j < count; ++j){
        printf("%s\n",validStr[j]); 
    }
}

The count indicates it is printing only the valid strings but as you can tell by the pic it prints white lines.

Note: For various reasons the program needs to follow the current order so the output is printed after the first for loop.

Thanks in advance!

10 chars is too short by 1 for a 10-character string (no room for '\0'). Don't skimp on input buffer size. char input[1024]; is fine (reduce accordingly on embedded devices) Note how you allows 60 chars for each string in char validStr[10][60] = {""}; Instead of memcpy(validStr[i],input,sizeof(input)); use strcpy (validStr[i],input); — David C. Rankin
– David C. Rankin, Commented Nov 18, 2021 at 7:12
I am allowing 10 characters and 60 strings, not the other way around. — RandomDeveloper
– RandomDeveloper, Commented Nov 18, 2021 at 7:17
Oops sorry, you had [10][60] to begin with -- just swap the bounds around :) — David C. Rankin
– David C. Rankin, Commented Nov 18, 2021 at 7:25

selbie · Accepted Answer · 2021-11-18 07:21:24Z

1

Instead of this:

    if(checkIfValid(input)){
        memcpy(validStr[i],input,sizeof(input));
        count++;
    }

This:

    if(checkIfValid(input)){
        memcpy(validStr[count],input,sizeof(input));
        count++;
    }

As others have pointed out in the comments, you want to safely secure that string copy. May I suggest:

    if(checkIfValid(input)){
        char* dst = validStr[count];
        size_t MAXLEN = 10;
        strncpy(dst, input, MAXLEN);
        dest[MAXLEN-1] = '\0';
        count++;
    }

edited Nov 18, 2021 at 7:21

answered Nov 18, 2021 at 7:19

selbie

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1 Comment

RandomDeveloper Over a year ago

and voila! thanks!

David C. Rankin · Accepted Answer · 2021-11-18 07:29:52Z

Continuing from the comment, if you want to store the entire string, you need to provide adequate space for the nul-terminating character.

AAAAAAAAAA
QELETIURTE
...

contain strings that are 10 characters long and will not fit in input as declared char[10].

Instead of looping with a for, allow the return from fgets() control your read-loop and keep count as a condition controlling the loop to ensure you protect your array bounds, e.g.

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

#define MAXC 128    /* if you need a constant, #define one (or more) */
#define NSTR  10

int checkIfValid (const char *s) { return 1; (void)s; }

int main(void)
{
    size_t count = 0;
    char input[MAXC];
    char validStr[NSTR][MAXC] = {""};
    
    while (count < NSTR && fgets (input, sizeof input, stdin)) {
        input[strcspn(input,"\n")] = '\0';
        if(checkIfValid(input)){
            strcpy (validStr[count], input);
            count++;
        }
    }
    
    printf ("%zu\n",count);
    for (size_t j = 0 ; j < count; ++j) {
        printf("%s\n",validStr[j]); 
    }
}

(adjust your array declaration for 60 strings of 10 characters each)

If you want to cut off at 9 characters and ensure the stings are nul-terminated, @selbie has that covered.

Example Use/Output

With your data (as good as I could read it) in dat/validstr.txt you could do:

$ ./bin/validstring <dat/validstr.txt
6
AAAAAAAAAA
QELETIURTE
321qweve
sdsdsdfFF
GRSGGFDDSS
toLotssAAA

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