Assertion on file descriptor write access

writeimg.c

@@ -74,6 +74,9 @@ int perform_write(write_job_t *job) {
     int block_fd = open(job->dev_name, O_RDWR);
     int file_fd = open(job->filename, O_RDONLY);
 
+    assert(block_fd >= 0);
+    assert(file_fd >= 0);
+
     /* TODO: Checks */
     assert(job->bufsize >= job->block_size);
 
@@ -202,10 +205,20 @@ int main(int argc, char *argv[]) {
         exit(EXIT_FAILURE);
     }
 
+    /* Seems to be the cleanest way to check for write perm on a blockdev */
+    int fd = open(wjob.dev_name, O_WRONLY);
+    if (fd < 0) {
+        printf("Cannot write to \"%s\", do you have write permissions?\n", wjob.dev_name);
+        exit(1);
+    }
+    close(fd);
+
     printf("Writing %s to %s\n", wjob.filename, wjob.dev_name);
 
     wjob.buffer = malloc(BLOCKSIZE);
     wjob.buffer2 = malloc(BLOCKSIZE);
+    assert(wjob.buffer);
+    assert(wjob.buffer2);
 
     wjob.bufsize = BLOCKSIZE;
     wjob.block_size = BLOCKSIZE;