#include <sys/file.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main( int argc, char * argv[] )
{
    int fd;
    const size_t buf_size = 40;
    char buf[ buf_size ];
    unsigned int n;
    ssize_t bytes;
    int rc;
    int exit_status = 0;
    off_t offset;
    size_t out_bytes;

    if ( argc != 2 )
    {
        fprintf( stderr, "usage: %s COUNT_FILE\n", argv[0] );
        exit_status = 1;
        goto done;
    }

    fd = open( argv[1], O_RDWR | O_CREAT, 0666 );
    if ( fd < 0 )
    {
        fprintf( stderr, "%s: open: %s\n", argv[1], strerror( errno ) );
        exit_status = 2;
        goto done;
    }

    rc = flock( fd, LOCK_EX );
    if ( rc != 0 )
    {
        fprintf( stderr, "%s: locking: %s\n", argv[1], strerror( errno ) );
        exit_status = 3;
        goto close_fd;
    }

    bytes = read( fd, buf, buf_size );
    if ( bytes < 0 )
    {
        fprintf( stderr, "%s: read: %s\n", argv[1], strerror( errno ) );
        exit_status = 4;
        goto unlock_fd;
    }
    else if ( bytes == 0 )
    {
        /* new file */
        n = 0;
    }
    else
    {
        if ( bytes >= buf_size )
            bytes = buf_size - 1;
        buf[bytes] = '\0';

        rc = sscanf( buf, "%u", &n );
        if ( rc != 1 )
        {
            fprintf( stderr, "%s: scanning '%s' for unsigned int\n", argv[1], buf );
            exit_status = 5;
            goto unlock_fd;
        }

        offset = lseek( fd, 0, SEEK_SET );
        if ( offset != 0 )
        {
            fprintf( stderr, "%s: lseek: %s\n", argv[1], strerror( errno ) );
            exit_status = 6;
            goto unlock_fd;
        }
    }

    rc = snprintf( buf, buf_size, "%u\n", ++n );
    if ( rc < 1 )
    {
        fprintf( stderr, "%s: snprintf: rc=%d\n", argv[1], rc );
        exit_status = 7;
        goto unlock_fd;
    }
    else if ( rc > buf_size )
    {
        fprintf( stderr, "%s: number too large for output buffer (%zu chars max)\n",
                 argv[1], buf_size );
        exit_status = 8;
        goto unlock_fd;
    }

    out_bytes = rc;

    rc = ftruncate( fd, 0 );
    if ( rc != 0 )
    {
        fprintf( stderr, "%s: truncate: %s\n", argv[1], strerror( errno ) );
        exit_status = 9;
        goto unlock_fd;
    }

    bytes = write( fd, buf, out_bytes );
    if ( bytes < 0 )
    {
        fprintf( stderr, "%s: write: %s\n", argv[1], strerror( errno ) );
        exit_status = 10;
        goto unlock_fd;
    }
    else if ( bytes != out_bytes )
    {
        fprintf( stderr, "%s: write: tried to write %zu bytes, wrote %d\n",
                 argv[1], out_bytes, (int)bytes );
        exit_status = 11;
        goto unlock_fd;
    }

    printf( "%u\n", n );

unlock_fd:
    flock( fd, LOCK_UN );

close_fd:
    close( fd );

done:
    return exit_status;
}