How to write a file, then make it executable in C? - c

Question

I am trying to write a program that writes a file with some text in it, then makes that file executable. This is what I have:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main() {
char name[] = "foo";
FILE * fp;
fp = fopen(name, "w");
fprintf(fp, "This file should be executable.\n");
execl("/usr/bin/chmod", "/usr/bin/chmod", "+x", name, NULL);
return 0;
}
The problem I am having that running execl seems to remove the contents of the file I wrote. If I remove the execl, it works as expected, and writes a file with the desired text. But when I leave in the execl, it writes a file, makes it executable, but the file is blank. How do I make it so the file still has the text in it, and is executable?

Answer 1

The problem is that output to the file is buffered. When you call execl(), you replace the process with chmod, but never write the stdio buffer to the file.
You need to call fclose(fp) before execl() to force everything to be written. You could also use fflush(fp), but fclose() more complete.

Answer 2

Creating the file with the wrong permissions and then changing them is not what you want to do. Just create the file with the desired permissions in the first place. If you want the file to be executable you can (with one caveat mentioned below) run:
/* CAUTION: all error checking omitted for clarity */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
int
main(void)
{
char name[] = "foo";
FILE * fp;
int fd = open(name, O_RDWR | O_CREAT, 0777);
fp = fdopen(fd, "w");
fprintf(fp, "This file should be executable.\n");
return 0;
}
This will create the file with mode 0777 (modified by the umask) when executed. (eg, if umask is 111, the executable bits will not be set)
But, if you do want to create the file with the wrong permissions and then change them, don't exec out to /usr/bin/chmod. Just use chmod(2). eg chmod(name, 0777);
+x is convenient if you are just adding a permission, but since you are creating the file you are in complete control and you know what the permissions are.