fopen with path stored in variable

Question

I have saved paths in a text file and would like to read them out. With these paths I want to read new text files, but if I call fopen () with a variable, the program only crashes. The paths I save are absolute. I have already tried to read the files without variables, which also worked.

FILE *fp;
FILE *variable;
char file[256];

fp = fopen("C:\\Example\\Example.txt","r");

if(fp != NULL)
{
    while(fgets(file, 256, fp) != NULL)
    {
         variable = fopen(("%s", file), "r");
         // another while loop which reads out the content of the variablefile 
         fclose(variable);
    }
    fclose(fp);
}

I found that somewhere on the internet while I was searching how I can pass a variable to fopen(). — Raphael Wirnsberger, Jul 24 '19 at 09:08
You will probably also want to remove the newline character left in by the [`fgets`](https://en.cppreference.com/w/c/io/fgets) call. — Sander De Dycker, Jul 24 '19 at 09:22
I tried with variable = fopen(file, "r") and it worked on my pc. Probably as @SanderDeDycker pointed out you need to remove the newline character. — Diego Chinellato, Jul 24 '19 at 09:27
After `variable = fopen(file, "r");` add something like `if(variable == NULL) { perror("fopen"); fprintf(stderr, "file = \"%s\"\n", file); }` to see what is wrong. (probably trailing newline) — Bodo, Jul 24 '19 at 09:29
I tested it and if I have only one line in my paths file, it works. But how can I do it with more than one? Separate them by ";" would be an option but.. — Raphael Wirnsberger, Jul 24 '19 at 09:34

score 1 · Accepted Answer · answered Jul 24 '19 at 09:42

1

There are two problems in your code. The first is this line

variable = fopen(("%s", file), "r");

I don't know where you found this notation, but fopen requires two parameters. From man: FILE *fopen(const char *pathname, const char *mode). Basically two string, one for the path and the other for the opening mode So the correct call to fopen would be:

variable = fopen(file, "r");

Also, fgets stores any newline read into the buffer. Simply remove this newline before opening the file:

char *newline = strchr(Name, '\n');
if (newline)
    /*if a newline is found, we remove it*/
    *pos = '\0';
else
    /*error: input too long for buffer */

answered Jul 24 '19 at 09:42

Diego Chinellato

165
1
9

thanks for your reply. I solved it already. I used a for loop and checked the char[] if there is anywhere a "\n" and replaced it with "\0". – Raphael Wirnsberger Jul 24 '19 at 09:47
1

That's roughly the same solution. Consider marking this as the accepted answer if it solves your problem. – Diego Chinellato Jul 24 '19 at 09:53

fopen with path stored in variable

1 Answers1