strncpy functions produces wrong file names

Question

I am new in C and writing a code to help my data analysis. Part of it opens predetermined files.

This piece of code is giving me problems and I cannot understand why.

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


#define MAXLOGGERS 26

// Declare the input files
char inputfile[];
char inputfile_hum[MAXLOGGERS][8];

// Declare the output files
char newfile[];
char newfile_hum[MAXLOGGERS][8];

int main()
{
    int n = 2;
    while (n > MAXLOGGERS)
    {
        printf("n error, n must be < %d: ", MAXLOGGERS);
        scanf("%d", &n);
    }

    // Initialize the input and output file names
    strncpy(inputfile_hum[1], "Ahum.csv", 8);
    strncpy(inputfile_hum[2], "Bhum.csv", 8);
    strncpy(newfile_hum[1], "Ahum.txt", 8);
    strncpy(newfile_hum[2], "Bhum.txt", 8);


    for (int i = 1; i < n + 1; i++)
    {

        strncpy(inputfile, inputfile_hum[i], 8);

        FILE* file1 = fopen(inputfile, "r");
        // Safety check
        while (file1 == NULL)
        {
            printf("\nError: %s == NULL\n", inputfile);
            printf("\nPress enter to exit:");
            getchar();
            return 0;
        }

        strncpy(newfile, newfile_hum[i], 8);

        FILE* file2 = fopen(newfile, "w");
        // Safety check
        if (file2 == NULL)
        {
            printf("Error: file2 == NULL\n");
            getchar();
            return 0;
        }

        for (int c = fgetc(file1); c != EOF; c = fgetc(file1))
        {
            fprintf(file2, "%c", c);
        }

        fclose(file1);
        fclose(file2);
    }
//  system("Ahum.txt");
//  system("Bhum.txt");
}

This code produces two files but instead of the names:

Ahum.txt
Bhum.txt

the files are named:

Ahum.txtv
Bhum.txtv

The reason I am using strncpy in the for loop is because n will actually be inputted by the user later.

Answer 1

I see at least three problems here.

The first problem is that your character array is too small for your strings. "ahum.txt", etc. will need to take nine characters. Eight for the actual text plus one more for the null terminating character.

The second problem is that you have declared the character arrays "newfile" and "inputfile" as empty arrays. These also need to be a number able to contain the strings (at least 9). You're lucky to have not had a crash from overwriting memory out the program space.

The third and final problem is your use of strcpy(). strncpy(dest, src, n) will copy n characters from src to dest, but it won't copy final null terminator character if n is equal or less than size of the src string.

From strncpy() manpage: https://linux.die.net/man/3/strncpy

The strncpy() function ... at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

Normally what you would want to do is have "n" be the size of the destination buffer minus 1 to allow for the null character.

For example: strncpy(dest, src, sizeof(dest) - 1); // assuming dest is char array