sprintf doesn't add strings with escape character

Question

I have this function which gets the given directory and filename and concat it by checking if the end of the directory contains / character and concatenating the string's for condition. My problem is that when i give the function a directory name and filename if the filname has \ it does not concat the string correctly. but when if the file does not contain any \ the output will be given.How can i fix the issue?.

Here is my C Code.

char * find_endwith_slash(char *dir_path, char *file_name)
{
    char full_path[1024];
    if (dir_path[strlen(dir_path) - 1] != '/')
    {
        sprintf(full_path, "%s/%s", dir_path, file_name);
    }
    else
    {
        sprintf(full_path, "%s%s", dir_path, file_name);
    }

    char * full_name = malloc(strlen(full_path) * sizeof(char));
    full_name = strdup(full_path);

    return full_name;



}

Note: You're allocating memory for `full_name`, then losing track of that by using `strdup()` which does its own allocation. — tadman, Dec 28 '22 at 16:40
This could be as easy as a couple of `strcat()` calls. You're repeating yourself in the `sprintf` variants. — tadman, Dec 28 '22 at 16:41
How are you calling `find_endwith_slash()` in your `main()` function? — Gaurav Pathak, Dec 28 '22 at 16:42
`dir_path[strlen(dir_path) - 1]` is UB when `dir_path[0] == 0`. — chux - Reinstate Monica, Dec 28 '22 at 17:10
`sprintf(full_path, "%s/%s", dir_path, file_name);` risks buffer overflow. — chux - Reinstate Monica, Dec 28 '22 at 17:11
@tadman *...consider using `MAX_PATH`* [`MAX_PATH` isn't really safe](https://stackoverflow.com/a/837855/4756299). And [POSIX `PATH_MAX` doesn't even have to exist](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html): "A definition of one of the symbolic constants in the following list **shall be omitted** from the header on specific implementations where the corresponding value is equal to or greater than the stated minimum, .... The actual value supported for a specific pathname shall be provided by the `pathconf()` function." — Andrew Henle, Dec 28 '22 at 21:00

chux - Reinstate Monica · Answer 1 · 2022-12-28T22:15:31.087

1

A more proper approach to conditionally concatenating path and file name:

char * find_endwith_slash_alt(const char *dir_path, const char *file_name) {
  size_t d_length = strlen(dir_path);
  size_t f_length = strlen(file_name);
  if (d_length > 0 && dir_path[d_length-1] == `/`) {
    d_length--;
  } 
  char *both = malloc(d_length + 1 + f_length + 1);
  if (both) {
    memcpy(both, dir_path, d_length);
    both[d_length] = '/';
    memcpy(both + d_length + 1, file_name, f_length + 1);
  }
  return both;
}

edited Dec 28 '22 at 22:15

answered Dec 28 '22 at 17:21

chux - Reinstate Monica

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What if `const char *file_name = "Fil\\eName"` such as `find_endwith_slash("Directory", "Fil\\eName");`? – Gaurav Pathak Dec 28 '22 at 17:30
@GauravPathak The result is `"Directory/Fil\\eName"`. If the path or file name are ill formed, the result is ill formed. Alternatively code could validate the path and file names, yet validation is really outside the goal (and OS specific) of this function and better handled in a separate validation function and called as needed. – chux - Reinstate Monica Dec 28 '22 at 17:46

sprintf doesn't add strings with escape character

1 Answers1