7

Say there is a file called 12345.jpg. In C, how can I get the file extension so that I can compare with some file extension? If there are any inbuilt functions, kindly please let me know.

Improve this question
4
  • SO 290488 covers a lot of the same ground. Commented Jun 14, 2010 at 6:13
  • So is this - stackoverflow.com/questions/51949/… Commented Jun 14, 2010 at 6:46
  • Possible duplicate of Getting file extension in C Commented Mar 4, 2017 at 18:15
  • SO 51949, mentioned by Ofek Shilon, is a C++ question. The answers are not appropriate for C. SO 5309471, mentioned by nibot, was asked a year or so after this. There's a cross-reference back to this question, though. SO 290488, mentioned by YT, is tagged with both C++ and C, though the code it shows is pure C (that would also compile in C++, I think, based on eyeballing, not on testing it). Commented Nov 28, 2022 at 22:50

4 Answers 4

Reset to default
10

A function to do that, along with a test harness:

#include <stdio.h> #include <string.h> const char *getExt (const char *fspec) { char *e = strrchr (fspec, '.'); if (e == NULL) e = ""; // fast method, could also use &(fspec[strlen(fspec)]). return e; } int main (int argc, char *argv[]) { int i; for (i = 1; i < argc; i++) { printf ("[%s] - > [%s]\n", argv[i], getExt (argv[i])); } return 0; }

Running this with:

./program abc abc. abc.1 .xyz abc.def abc.def.ghi

gives you:

[abc] - > [] [abc.] - > [.] [abc.1] - > [.1] [.xyz] - > [.xyz] [abc.def] - > [.def] [abc.def.ghi] - > [.ghi]
Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

+1 for strrchr. Best method for finding the last instance of a character in a string.
Just in case anyone misses it. getExt() assumes that fspec is a basename, not full path. Consider for e.g., my.dir/file. Use basename() to get the basename from a full path.
6

Probably:

#include <string.h> char *extn = strrchr(filename, '.');

That will give you a pointer to the period of the extension, or a null pointer if there is no extension. You might need to do some more due diligence to ensure that there isn't a slash after the dot, amongst other things.

Improve this answer

4 Comments

strrchr(filename+1,'.'); would handle names like .svn that start with a dot but have no extension.
@drawnonward - indubitably, if you consider that to be a file name with no extension, rather than as an extension with no name. That is another of the 'due diligence' cases to be considered. However, if the OP is looking to compare the found extension with a fairly short list of known extensions, then the fact that .svn is not in the list means it probably won't matter anyway.
@drawnonward: I misread what you were saying - you make a valid point. However, that wouldn't help with absolute pathnames which start with a '/', or relative names containing a slash (subdir/.svn). There's a case for saying you need to find the start of the basename of the file (after the last slash, but you have to worry about trailing slashes and root) and then look for a dot not in the first position. I guess that means the parent directory '..' has the name 'dot' followed by an extension starting with a dot and nothing more...There's usually another curve ball to deal with.
@drawnonward Might cause segmentation fault if filename is empty string.
2

There's a portable CRT solution: _splitpath.

In windows there's also an undocumented shell32 API called PathGetExtension, but that's evil in so many ways that I probably shouldn't have noted that.

Improve this answer

5 Comments

This is obviously some new definition of the word "portable" of which I was previously unaware :-) With apologies to Douglas Adams of HHGTTG fame.
Indeed, never seen the word "portable" and a link to msdn in the same sentence:-)
My bad, my bad! Won't edit the answer, for others to enjoy :)
Well, yeah, the answer is still useful so I wouldn't edit it. If you're on an MS platform, it's probably preferable to use _splitpath than write your own (such as in my answer).
Of course - I considered removing 'portable' (where was my head, really?) but dropped it.
-1

Use strchr First array member will give you filename Second array member will give you extension

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.