how can I run the 'cat' command from my program?

HarryBoy · 10-21-2008, 09:36 AM

Hi,

I have a platform which has several framebuffers like so:
fb0 = graphics
fb1 = video
fb2 = transparency

I need to change the transparency every now and again and I do this manually by:
cat FileContainingAlphavalues > /dev/fb/2

My question is how can I do this in my program?

i.e. how can I run the command:
cat FileContainingAlphavalues > /dev/fb/2
from within my c program?

Thanks

CRC123 · 10-21-2008, 09:48 AM

Use the system(const char *command) function.

Use this command for more info:

Code:

man 3 system

Hko · 10-21-2008, 10:02 AM

I wouldn't run the 'cat' program from inside your own program since the only thing 'cat' does in you your example commmands is to copy bytes from one file to another.

just doe the same thing in C itself, using either the low-level open(2), read(2) and write(2) combination, or use the stdio.h file handling functions if you're more confromatable with them.

HarryBoy · 10-21-2008, 10:06 AM

OK thanks guys. I'll try the open(2), read(2) and write(2) combination, or use the stdio.h file handling functions and see how I get on.

Hko · 10-21-2008, 02:47 PM

Here's a bare-bones file copy program I wrote. It shows how you can use open(2), read(2) and write(2) to 'cat' a file from within a C program.

(it not as complicated as it may first look: most of the lines are just comments and error checking/reporting code)

Code:

#include <stdio.h>
#include <string.h>
#include <errno.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define BLOCKSIZE 4096


int main(int argc, char *argv[])
{
    int  fdsrc, fddst;
    int  nbytes;
    char blockbuffer[BLOCKSIZE];

    if (argc != 3) {
        fprintf(stderr, "Usage:\n\t%s <src> <dest>\n", argv[0]);
        return 1;
    }
    
    /* Open source file for reading */
    fdsrc = open(argv[1], O_RDONLY);
    if (fdsrc < 0) {
        fprintf(stderr, "Error opening %s for reading: %s\n", argv[1], strerror(errno));
        return 2;
    }
    
    /* Open destination file for writing 
     * O_EXCL makes sure the file doesn't exist yet, so we
     * won't be overwriting anything...
     * (mode_t)0600 means the newcreated file will have its
     * permissions set to 0600 (rw-------).
     */
    fddst = open(argv[2], O_WRONLY | O_CREAT | O_EXCL, (mode_t)0600);
    if (fddst < 0) {
        fprintf(stderr, "Error creating %s for writing: %s\n", argv[2], strerror(errno));
        return 2;
    }

    /* Copy the file block by block in a loop
     * Endless loop ( for(;;) ) used here, but we get
     * out with 'break' when done.
     */
    for (;;) {
        nbytes = read(fdsrc, blockbuffer, BLOCKSIZE);
        if (nbytes == 0) { /* EOF, we are done. */
            break;
        }
        if (nbytes < 0) {  /* Error occurred */
            fprintf(stderr, "Error reading file: %s\n", strerror(errno));
            return 2;
        }

        /* Once here we have read a block of bytes succesfully.
         * The block may not be complete if it is the last
         * block. If that is the case, we will get out of
         * the loop next round, since then read() will return 0.
         * Note we are using 'nbytes' to specify number of
         * bytes to write, so if the block is not completely
         * full, we will also be writing the correct number 
         * of bytes.
         */
        if (write(fddst, blockbuffer, nbytes) < 0) {
            fprintf(stderr, "Error writing file: %s\n", strerror(errno));
            return 2;
        }
    }            
    return 0;
}