Why does C print an extra line when reading from a file? - c

Question

I'm brand new to C and trying to learn how to read a file. My file is a simple file (just for testing) which contains the following:
this file
has been
successfully read
by C!
So I read the file using the following C code:
#include <stdio.h>
int main() {
char str[100];
FILE *file = fopen("/myFile/path/test.txt", "r");
if(file == NULL) {
puts("This file does not exist!");
return -1;
}
while(fgets(str, 100, file) != '\0') {
puts(str);
}
fclose(file);
return 0;
}
This prints my text like this:
this file
has been
successfully read
by C!
When I compile and run it, I pipe its output to hexdump -C and can see an extra 0a at the end of each line.
Finally, why do I need to declare an array of chars to read from a file? What if I don't know how much data is on each line?

Answer 1

fgets() reads up to the newline and keeps the newline in the string and puts() always adds a newline to the string it is given to print. Hence you get double-spaced output when used as in your code.
Use fputs(str, stdout) instead of puts(); it does not add a newline.
The obsolete function gets() — removed from the 2011 version of the C standard — read up to the newline but removed it. The gets() and puts() pair worked well together, as do fgets() and fputs(). However, you should certainly NOT use gets(); it is a catastrophe waiting to happen. (The first internet worm in 1988 used gets() to migrate — Google search for 'morris internet worm').
In comments, inquisitor asked:
Why does the line need to be read into a char array of a specific size?
Because you need to make sure you don't overrun the space that is available. C does not do automatic allocation of space for strings. That is one of its weaknesses from some viewpoints; it is also a strength, but it routinely confuses newcomers to the language. If you want the input code to allocate enough space for a line, use the POSIX function getline().
So is it better to just read and output until I hit a '\0' since I won't always know the amount of chars on a given line?
No. In general, you won't hit '\0'; most text files do not contain any of those. If you don't want to allocate enough space for a line, then use:
int c;
while ((c = getchar()) != EOF)
putchar(c);
which reads one character at a time in the user code, but the underlying standard I/O packages buffer the input up so it isn't too costly — it is perfectly feasible to implement a program that way. If you need to work on lines, either allocate enough space for lines (I use char buffer[4096]; routinely) or use getline().
And Charlie Burns asked in a comment:
Why don't we see getline() suggested more often?
I think it is not mentioned all that often because getline() is relatively new, and not necessarily available everywhere yet. It was added to POSIX 2008; it is available on Linux and BSD. I'm not sure about the other mainline Unix variants (AIX, HP-UX, Solaris). It isn't hard to write for yourself (I've done it), but it is a nuisance if you need to write portable code (especially if 'portable' includes 'Microsoft'). One of its merits is that it tells you how long the line it read actually was.
Example using getline()
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
char *line = 0;
size_t length = 0;
char const name[] = "/myFile/path/test.txt";
FILE *file = fopen(name, "r");
if (file == NULL)
{
fprintf(stderr, "%s: failed to open file %s\n", argv[0], name);
return -1;
}
while (getline(&line, &length, file) > 0)
fputs(str, stdout);
free(line);
fclose(file);
return 0;
}

Answer 2

fgets saves the newline character at the end of the line when reading line by line. This allows you to determine wether actually a line was read or just your buffer was too small.
puts always adds a newline when printing.
Either trim off the newline from fgets or use printf
printf("%s", str);