how to get text using fgets, and save std::string

Question

I read txt file with fgets, and save string

and i printf_s this string, but text is broken how can I fix this problem?

int main() {    
    char name[256] = "";

    int a = 0, b = 0, c = 0, d = 0;
    FILE *fp = 0;
    fopen_s(&fp, "a.txt", "r");
    if (fp == 0)
    {
        printf_s("can't open this file\n");
        return 0;
    }

    std::string st;
    fgets(name, 256, fp);
    st += name;
    printf_s("%s\n", name);

    fgets(name, 256, fp);
    st += name;
    printf_s("%s\n", name);

    fgets(name, 256, fp);
    st += name; 
    printf_s("%s\n", st, sizeof(st));

    fclose(fp);

    return 0;
}

"a.txt"'s text

hello world line 1 text text 2 line 3 line text

and in this code, I fgets several text line in char and add this text in string named st. can I insert all text of using fgets in string directly?

Answer 1

I think what you are looking for is a bit of pointer manipulation, but first:

1. Do not mix C++ code into your C program - std::string does not belong here.
2. Do not use sizeof on a string, there will be unexpected consequences. The length of the string is strlen(string), and the size is strlen(string)+1.

To keep appending to name, you can use something like:

fgets(name, 256, fp);
fgets(name+strlen(name), 256-strlen(name), fp);
fgets(name+strlen(name), 256-strlen(name), fp); //Repeat (loop?)

This way you will aggregate the file contents to name. Each time you are passing a pointer that points to the previous '\0' character, and overwriting it while making sure you do not read in total more than 256 characters (255 and a '\0').

You better make sure that:

1. fgets succeeds and does not return NULL.
2. feof(fp) is false.
3. You did not fill up name completely.