fopen failing on variable filepath - c

Question

This function is passed the path of a text file(mapper_path) which contains paths to other text files on each line. I am supposed to open the mapper_path.txt file, then open and evaluate each of the paths within it (example in output).
fopen succeeds on the mapper_path file but fails on the paths which it contains.
In the failure condition, it prints the EXACT path I'm trying to open.
I'm working in C on windows and running commands on Ubuntu subsystem.
How can I properly read and store the sub-path into a variable to open it?
SOLVED with Rici's suggestion!
int processText(char * mapper_path, tuple * letters[])
{
char line[LINE_SIZE];
char txt_path[MAX_PATH];
FILE * mapper_fp = fopen(mapper_path, "r");
if(!mapper_fp)
{
printf("Failed to open mapper path: %s \n", mapper_path);
return -1;
}
//!!! PROBLEM IS HERE !!!
while(fgets(txt_path, MAX_PATH, mapper_fp))
{
//remove newline character from end
txt_path[strlen(txt_path)-1] = 0;
//open each txt file path, return -1 if it fails
FILE* fp = fopen(txt_path, "r");
if(!fp)
{
printf("Failed to open file path:%s\n", txt_path);
return -1;
}
//...more unimportant code
prints:
Failed to open filepath:
/mnt/c/users/adam/documents/csci_4061/projects/blackbeards/testtext.txt
This is the exact path of the file i am trying to open.

Answer 1

I suspect that the problem is related to this:
I'm working in C on windows and running commands on Ubuntu subsystem.
Presumably, you created the mapper.txt file using Windows tools, so it has Windows line endings. However, I think the Ubuntu subsystem does not know about Windows line endings, and so even though you open the file in mode 'r', it does not translate CR-LF into a single \n. When you then remove the \n at the end of the input, you still leave the \r.
That \r won't be visible when you print out the line, since all it does is move the cursor to the beginning of the line and the next character output is a \n. It's usually a good idea to surround strings with other text when you print debugging messages, since that can give you a clue about this sort of problem. If you'd used:
printf("Failed to open file path: '%s'\n", txt_path);
you might have seen the error:
'ailed to open filepath: '/mnt/c/users/adam/documents/csci_4061/projects/blackbeards/testtext.txt
Here, the hint that there is a \r at the end of the string is the overwriting of the first character of the message with the trailing apostrophe.
It's not quite accurate to say that fgets "adds a \n character to the end [of the line read]." It's more accurate to say that it doesn't remove that character, if it is present. It is quite possible that there isn't a newline at the end of the line. The line may be the last line in a text file which doesn't end with a newline character, for example. Or the fgets might have been terminated by reaching the character limit you supplied, rather than by finding a newline character.
So you are certainly better off using the getline interface, which has two advantages: (a) it allocates storage for the line itself, so you don't need to guess a maximum length in advance, and (b) it tells you exactly how many characters it read, so you don't have to count them.
Using that information, you can then remove a \n which happens to be at the end of the line, if there is one, and then remove the preceding \r, if there is one:
char* line = NULL;
size_t n_line = 0;
for (;;) {
ssize_t n_read = getline(&line, &n_line, mapper_fp);
if (n_read < 0) break; /* EOF or some kind of read error */
if (n_read > 0 && line[n_read - 1] == '\n')
line[nread--] = 0;
if (n_read > 0 && line[n_read - 1] == '\r')
line[nread--] = 0;
if (nread == 0) continue; /* blank line */
/* Handle the line read */
}
if (ferr(mapper_fp))
perror("Error reading mapper file");
free(line);