How can I check the file extensions in c? - c

Question

I'm struggling with this for several days now.
I want to create a functions that goes through a directory, pick all the files with an extension *.csv and read the data in them.
I created a function that is supposed to check each file name that is legal, by checking that the string ends with .csv. To do this, I want to go to the end of the char * array. I tried this:
char * point = file_name;
while (point != "\0"){
point += 1;
}
which goes through the char * array without finding and "\0".
If I write
*point != "\0"
The compiler warns me that I'm comparing and int to a char.
I should note that I get the filename using
dirent->d_name

Answer 1

point != "\0" compares a pointer to another pointer, which is not what you want.
You want to compare whatever point points to , to a char with the value 0. So use e.g.
*point != '\0';
Note, if you want to find the end of the string, you could also do
point = file_name + strlen(filename);
If you want to check whether a string ends in .csv, you could also do something like
if((point = strrchr(filename,'.')) != NULL ) {
if(strcmp(point,".csv") == 0) {
//ends with csv
}
}
EDIT : fixed formatting

Answer 2

You must dereference the pointer to look at the value it points at:
while (*point != '\0')
point++;
Note that the right hand side is a character literal (single quotes), not a string (double quotes). This fixes the warning you got when trying to use a string.
Also note that it's really unnecessary to find the end of the string to check this. A better approach would be:
int ends_with(const char* name, const char* extension, size_t length)
{
const char* ldot = strrchr(name, '.');
if (ldot != NULL)
{
if (length == 0)
length = strlen(extension);
return strncmp(ldot + 1, extension, length) == 0;
}
return 0;
}
Call the above with e.g. ends_with("test.foo", "foo", 3) and you will get 1 returned, if no match is found it returns 0.
This is not faster, but it's a lot clearer since it operates at a higher level, using only well-known standard string functions.

Answer 3

To get the extension from a file name you can use the strrchr function to get the position of the last occurrence of a character in a string:
char * end = strrchr(filename, '.');
if(strcmp(end, ".csv") == 0)
{
// Yuppee! A CSV file! Let's do something with it!
}
(And have a look at what others have said, as they address several problems with your code.)

Answer 4

unwind is right, you just interpreted his code wrong.
What many (especially newbies; don't want to call you such) don't notice (especially when coming from other languages like JavaScript or PHP) is, that "" and '' aren't the same in C++. (Neither are they in PHP but the difference is bigger here.)
"a" is a string - essentially a const char[2].
'a' is a single char - it's just a char.
What you want is comparing a single character to a single character, so you have to use '\0'.
If you use "\0" you essentially compare your character to the address of the essentially empty string (which is an integer).

Answer 5

Try single quotes not double quotes. This limits the constant to a character. Double quotes are for strings not characters. You need to dereference point using * to get to the character:
char * point = file_name;
while (*point != '\0')
++point;

Answer 6

Based on #unwind's answer, I propose this version to check if a name ends with a specific extension. Note that the inputs are declared as const pointers pointing to const char.
int stringEndsWith(
char const * const name,
char const * const extension)
{
size_t length = 0;
char* ldot = NULL;
if (name == NULL) return 0;
if (extension == NULL) return 0;
length = strlen(extension);
if (length == 0) return 0;
ldot = strrchr(name, extension[0]);
if (ldot != NULL)
{
return (strncmp(ldot, extension, length) == 0);
}
return 0;
}