/* is_writeable.c - demonstrate use of fcntl to query FILE * writeability
 * Andrew Ho (andrew@zeuscat.com)
 *
 * In a C API that takes a FILE *, you sometimes need to know whether
 * that FILE * is opened for reading, writing or both. The F_GETFL
 * argument to fcntl() gives you this. Unclear documentation is here:
 *
 * http://www.gnu.org/software/libc/manual/html_node/Getting-File-Status-Flags.html
 * http://www.gnu.org/software/libc/manual/html_node/Access-Modes.html
 *
 * Build: gcc -Wall -o is_writeable is_writeable.c
 * Usage: is_writeable
 */

#include <stdio.h>
#include <fcntl.h>

/* returns true if fh was opened as O_WRONLY or O_RDWR, see fcntl(2) */
int fh_is_writeable(FILE *fh) {
    int accmode = O_ACCMODE & fcntl(fileno(fh), F_GETFL);
    return accmode == O_WRONLY || accmode == O_RDWR ? 1 : 0;
}

int main(int argc, char **argv) {
    char  wronly_fn[L_tmpnam], rdwr_fn[L_tmpnam];
    FILE *rdonly, *wronly, *rdwr;

    /* open tempfiles in each of O_RDONLY, O_WRONLY, O_RDWR */
    if(!(rdonly = fopen("/etc/hosts", "r"))) {
        perror("could not open /etc/hosts for reading");
        return 1;
    }
    if(!tmpnam(wronly_fn)) {
        fprintf(stderr, "could not create temporary filename\n");
        return 1;
    }
    if(!(wronly = fopen(wronly_fn, "w"))) {
        fprintf(stderr, "could not open %s for writing: ", wronly_fn);
        perror(NULL);
        return 1;
    }
    if(!tmpnam(rdwr_fn)) {
        fprintf(stderr, "could not create temporary filename\n");
        return 1;
    }
    if(!(rdwr = fopen(rdwr_fn, "w+"))) {
        fprintf(stderr, "could not open %s for update: ", rdwr_fn);
        perror(NULL);
        return 1;
    }

    /* for debugging, confirm constant values from fcntl.h */
    fprintf( stderr, "O_RDONLY = 0x%04x, O_WRONLY = 0x%04x, O_RDWR = 0x%04x\n",
        O_RDONLY, O_WRONLY, O_RDWR );

    /* display fh_is_writeable() status for each of stdio filehandles */
    fprintf( stderr, "stdin (%d) = %d, stdout (%d) = %d, stderr (%d) = %d\n",
        fileno(stdin),  fh_is_writeable(stdin),
        fileno(stdout), fh_is_writeable(stdout),
        fileno(stderr), fh_is_writeable(stderr) );

    /* display fh_is_writeable() status for each filehandle we opened */
    fprintf( stderr, "rdonly (%d) = %d, wronly (%d) = %d, rdwr (%d) = %d\n",
        fileno(rdonly), fh_is_writeable(rdonly),
        fileno(wronly), fh_is_writeable(wronly),
        fileno(rdwr),   fh_is_writeable(rdwr) );

    /* clean up and exit */
    fclose(rdonly);
    fclose(wronly);
    fclose(rdwr);
    return 0;
}