Why does opendir() work for one string but not another?

Question

I would like to open a directory using opendir but am seeing something unexpected. opendir works for the string returned from getcwd but not the string from my helper function read_cwd, even though the strings appear to be equal.

If I print the strings, both print /Users/gwg/x, which is the current working directory.

Here is my code:

char real_cwd[255];
getcwd(real_cwd, sizeof(real_cwd));

/* This reads a virtual working directory from a file */
char virt_cwd[255];
read_cwd(virt_cwd);

/* This prints "1" */
printf("%d\n", strcmp(real_cwd, virt_cwd) != 0);

/* This works for real_cwd but not virt_cwd */
DIR *d = opendir(/* real_cwd | virt_cwd */);

Here is the code for read_cwd:

char *read_cwd(char *cwd_buff)
{
    FILE *f = fopen(X_PATH_FILE, "r");
    fgets(cwd_buff, 80, f);
    printf("Read cwd %s\n", cwd_buff);
    fclose(f);
    return cwd_buff;
}

If `strcmp(real_cwd, virt_cwd) != 0` then the strings are *not* equal — Tavian Barnes, Jan 05 '15 at 00:25
Ah. It returns `0` if they *are* equal? Why do they look the same? — jds, Jan 05 '15 at 00:27
Is there a newline at the end of the string? fgets will preserve it, so you have to manually remove it. — Tony, Jan 05 '15 at 00:27
@gwg Yes, 0 means the strings *are* equal. -1 means the first string is ordered first, and 1 means the second string is ordered first. — Tavian Barnes, Jan 05 '15 at 17:51

score 3 · Accepted Answer · answered Jan 05 '15 at 00:34

3

The function fgets includes the final newline in the buffer — so the second string is actually "/Users/gwg/x\n".

The simplest (but not necessarily the cleanest) way to solve this issue is to overwrite the newline with a '\0': add the following at the end of the function read_cwd:

n = strlen(cwd_buff);
if(n > 0 && cwd_buff[n - 1] == '\n')
    cwd_buff[n - 1] = '\0';

answered Jan 05 '15 at 00:34

jch

5,382
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[`strlen()`](http://stackoverflow.com/a/27729970/2410359) (like this answer) and [`strchr()`](http://stackoverflow.com/a/2693826/2410359), IMO, are the 2 best clean ways to remove `'\n'` after `fgets()`. – chux - Reinstate Monica Jan 05 '15 at 00:39

score 1 · Answer 2 · answered Jan 05 '15 at 00:31

fgets() includes the newline.

Parsing stops if end-of-file occurs or a newline character is found, in which case str will contain that newline character. — http://en.cppreference.com/w/c/io/fgets

You should trim the white space on both ends of the string when reading input like this.

score 1 · Answer 3 · answered Jan 05 '15 at 00:32

From the fgets man page:

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte (aq\0aq) is stored after the last character in the buffer.

You need to remove the newline character from the string you are reading in.

Why does opendir() work for one string but not another?

3 Answers3