Command line arguments and ftruncate in C

Question

I am trying to make a C program that changes the size of input file to desired size by truncating or extending the input file using ftruncate. It must also use command line arguments.

For example, the following is valid input:

./changefilesize data.txt 100

If the size of data.txt = 100, then return save file, if more than 100 then cut off the end, if less than 100, then extend the file size to 100.

I am having trouble dealing with input arguments and using ftruncate. The only information I found about ftruncate is basically the man info which says:

#include <unistd.h>
int ftruncate(int fildes, off_t length);
int truncate(const char *path, off_t length);

Here is what I have so far:

#include <stdio.h>
#include <unistd.h>
int main (int argc, char *argv[]) {
    if ( argc != 2 ) {
        printf("2 inputs expected\n");
    }
    else {
        int length;
        length = atoi (argv[1]);
        FILE *file = fopen(argv[0], "r");
        int ftruncate(int file, off_t length);
        fclose(file);
    }
}

If I enter ./changefilesize data.txt 100, I get 2 inputs expected and I don't understand why.

Edit: updated code based on answers:

#include <stdio.h>
#include <unistd.h>
int main (int argc, char *argv[]) {
    long length;
    length = atoi (argv[2]);
    FILE *file = fopen(argv[1], "w");
    ftruncate (fileno(file), length);
    fclose(file);
}

That is impossible to get if you print `argc` correctly. You should get a number. — 2501, Jun 10 '16 at 08:38
Oh, I see. The output is 1. Why is there already one input argument? — user6005857, Jun 10 '16 at 08:44

David Ranieri · Accepted Answer · 2016-06-10T09:14:10.143

2

The string pointed to by argv[0] represents the program name, so you are receiving 3 arguments: changefilesize, data.txt and 100.

and here

    FILE *file = fopen(argv[0], "r");
    int ftruncate(int file, off_t length);
    fclose(file);

the second line is a prototype (not a call to ftruncate) change to

    FILE *file = fopen(argv[1], "w"); /* 1 instead of 0 and "w" instead of "r" */
    ftruncate(fileno(file), 100);
    fclose(file);

Note that with ftruncate the file must be writable.

edited Jun 10 '16 at 09:14

answered Jun 10 '16 at 08:53

David Ranieri

39,972
7
52
94

I updated my code and added it to the question. When I enter `./changefilezise data.txt 100` data.txt becomes a 100 byte file but its contents are replaced by invisible characters, i.e. the file is empty but has size 100 bytes. data.txt has size 201 and consists of 200 '1' characters and a new line character. How can I preserve the contents? – user6005857 Jun 10 '16 at 09:05
If the file previously was larger than length, the extra data is discarded. If it was previously shorter than length, it is unspecified whether the file is changed or its size increased. If the file is extended, the extended area appears as if it were zero-filled. – David Ranieri Jun 10 '16 at 09:07

score 1 · Answer 2 · edited May 23 '17 at 10:28

1

The first argument of your program is its name (in argv[0]). So if you give 2 arguments, argc is 3 (path + 2 arguments).

See this question for more details.

edited May 23 '17 at 10:28

Community

1
1

answered Jun 10 '16 at 08:42

blatinox

833
6
18

So I fixed that problem but now when I run the program, the output file is not changed to have size 50. – user6005857 Jun 10 '16 at 08:46
@user6005857 That is a different question. Please ask a new question here: https://stackoverflow.com/questions/ask – 2501 Jun 10 '16 at 08:48

Command line arguments and ftruncate in C

2 Answers2