appending in C failing, simply overwriting

Question

Instead of appending "abc" (and thus eventually getting a file full of abcabcabc....), no matter how many times I run this, myfile only containst "abc".... how can I append?

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

int main(){
    char strng[10];
    strcpy(strng,"abc");
    FILE *my_file;
    my_file = fopen("myfile","a+");
    if (my_file == NULL){ printf("problem\n");}
    fwrite(strng, sizeof(strng), 1, my_file);
    printf("appending %s\n",strng);
    fclose(my_file);
}

Answer 1

Apart from the fact that:

fwrite(strng, sizeof(strng), 1, my_file);

should be:

fwrite(strng, strlen(strng), 1, my_file);

it works for me.

Answer 2

Don't use sizeof, use strlen. sizeof is 10, so you're writing "abc\0\0\0\0\0\0\0".

Answer 3

That program works and does what you said to do: append 10 characters (sizeof(strng)) to file, including \0 and rest of the array. Change sizeof to strlen.

Besides that you should not only print "problem" when you can't open file, but also not write to it.

And last problem in your code - you declared main as returning int, but end program without setting return code. If you end program correctly, always return EXIT_SUCCESS.

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

#define BUF_SIZE 10
char buf[BUF_SIZE];

int main(int ac, char **av) {
    FILE *my_file;

    strncpy(buf, "abc", BUF_SIZE-1);
    buf[BUF_SIZE-1] = '\0';
    my_file = fopen("myfile", "a+");
    if (my_file == NULL) {
        fprintf(stderr, "problem opening file\n");
        return EXIT_FAILURE;
    } else {
        fwrite(buf, strlen(buf), 1, my_file);
        printf("appending %s\n", buf);
        fclose(my_file);
        return EXIT_SUCCESS;
    }
}

Answer 4

When you fopen the file, the stream position is at the beginning of the file. Surely you have to seek to the end before writing in order to get the string appended?

Answer 5

I got it to work using strlen instead of sizeof. Thanks for the tips everyone!