Command line arguments, reading a file

Question

If i entered into the command line C: myprogram myfile.txt

How can I use myfile in my program. Do I have to scanf it in or is there an arbitrary way of accessing it.

My question is how can I use the myfile.txt in my program.

int
main(){
    /* So in this area how do I access the myfile.txt 
    to then be able to read from it./*

Is your question about how to read files, or about how to get the filename from the argument list? — Barmar, Jun 01 '13 at 05:39
Note that if you're on a unix-like system you could run your program as `myprogram < myfile` and the contents of the file will be fed into stdin. — John Carter, Jun 01 '13 at 05:47

score 24 · Accepted Answer · edited May 23 '17 at 12:10

24

You can use int main(int argc, char **argv) as your main function.

argc - will be the count of input arguments to your program.
argv - will be a pointer to all the input arguments.

So, if you entered C:\myprogram myfile.txt to run your program:

argc will be 2
argv[0] will be myprogram.
argv[1] will be myfile.txt.

More details can be found here

To read the file:
FILE *f = fopen(argv[1], "r"); // "r" for read

For opening the file in other modes, read this.

edited May 23 '17 at 12:10

Community

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answered Jun 01 '13 at 05:39

srikanta

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How to pass the `txt` file name to another function, not to the `main`? – Sigur Mar 23 '17 at 22:20

Nobilis · Answer 2 · 2013-06-01T05:53:44.183

Declare your main like this

int main(int argc, char* argv [])
- argc specified the number of arguments (if no arguments are passed it's equal to 1 for the name of the program)
- argv is a pointer to an array of strings (containing at least one member - the name of the program)
- you would read the file from the command line like so: C:\my_program input_file.txt
Set up a file handle:

File* file_handle;
Open the file_handle for reading:

file_handle = fopen(argv[1], "r");
- fopen returns a pointer to a file or NULL if the file doesn't exist. argv1, contains the file you want to read as an argument
- "r" means that you open the file for reading (more on the other modes here)
Read the contents using for example fgets:

fgets (buffer_to_store_data_in , 50 , file_handle);
- you need a char * buffer to store the data in (such as an array of chars), the second argument specifies how much to read and the third is a pointer to a file
Finally close the handle

fclose(file_handle);

All done :)

score 3 · Answer 3 · answered Jun 01 '13 at 05:56

This is the Programming 101 way. It takes a lot for granted, and it doesn't do any error-checking at all! But it will get you started.

/* this has declarations for fopen(), printf(), etc. */
#include <stdio.h>

/* Arbitrary, just to set the size of the buffer (see below).
   Can be bigger or smaller */
#define BUFSIZE 1000

int main(int argc, char *argv[])
{
    /* the first command-line parameter is in argv[1] 
       (arg[0] is the name of the program) */
    FILE *fp = fopen(argv[1], "r"); /* "r" = open for reading */

    char buff[BUFSIZE]; /* a buffer to hold what you read in */

    /* read in one line, up to BUFSIZE-1 in length */
    while(fgets(buff, BUFSIZE - 1, fp) != NULL) 
    {
        /* buff has one line of the file, do with it what you will... */

        printf ("%s\n", buff); /* ...such as show it on the screen */
    }
    fclose(fp);  /* close the file */ 
}

score 0 · Answer 4 · edited May 23 '17 at 11:33

0

Command line arguments are just plain C-strings. You can do whatever you want with them. In your case you might want to open a file, read something from it and close it.

You might find this question (and answer) useful.

edited May 23 '17 at 11:33

Community

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answered Jun 01 '13 at 05:39

akalenuk

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score 0 · Answer 5 · answered Jun 01 '13 at 05:55

All the suggestion you received about using the command line are correct,but It sounds to me you can also consider to use a typical pattern that is read the stdin instead of a file, then drive your app by piping, for example cat myfile > yourpgm. You then can use scanf to read from the stdin. In an analogous way you can use stdout/stderr to produce the output.

Command line arguments, reading a file

5 Answers5