CODEWITHSUNDEEP
cfilefile-ioiotruncate
user6005857
user6005857

Reputation: 631

Command line arguments and ftruncate in C

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);
}

Upvotes: 1

Views: 1598

Answers (2)

David Ranieri
David Ranieri

Reputation: 41046

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.

Upvotes: 2

blatinox
blatinox

Reputation: 843

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.

Upvotes: 1

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