Pseudoportable C script pattern

Question

(See the next iteration.)

From time to time, while working with a command line in *nix family of operating systems, we have to write those scripts doing a rather specific task. Usually we use bash + utility programs to do the job. However, I had to ask myself: how to implement such a script in C? After an invocation of such a script, it should perform the following:

1. Create a temporary source file \$S\$,
2. Dump the C code to \$S\$,
3. Compile \$S\$ to the program \$P\$,
4. Run \$P\$ passing the arguments to it and caching its exit status,
5. Remove \$S\$ and \$P\$,
6. Return the cached exit status of \$P\$.

Code

#! /bin/bash

# Below, XXXX asks for random characters in order to make a unique file name. 
# Mac OSX seems to ignore XXXX, yet Ubuntu requires them.

# Create a temporary file name for the source code:
TMP_SOURCE_FILE="$(mktemp -t sourceXXXX).c"
# Create a temporary file name for the executable file:
TMP_PROGRAM_FILE="$(mktemp -t programXXXX)"

# Write the source code into the temporary source file:
cat > $TMP_SOURCE_FILE <<- END_OF_SOURCE
#include <stdio.h>

int main(int argc, char* argv[]) 
{
    int i;

    puts("Hello, world! I am a pseudoportable C program.");
    for (i = 1; i < argc; ++i) 
    {
        printf("Argument %d: %s\n", i, argv[i]);
    }

    return argc - 1;
}
END_OF_SOURCE

# Compile and run:
gcc $TMP_SOURCE_FILE -o $TMP_PROGRAM_FILE
$TMP_PROGRAM_FILE $@
EXIT_STATUS=$?

# Clean the source and the binary:
rm $TMP_SOURCE_FILE
rm $TMP_PROGRAM_FILE

# Delegate the exit status of the C program to calling bash:
exit $EXIT_STATUS

So, what do you think? How can I improve anything? Also, can you come with an example of the situation where this "C script" pattern is more preferable than bash + command line utilities, Python, etc.?

Answer 1

It's not guaranteed that $TMP_PROGRAM_FILE is on the $PATH, so you need to either set the $PATH or use an absolute path for $TMP_PROGRAM_FILE.

You have problems with mktemp.

• -t option: It means different things on GNU/Linux mktemp(1) and Mac OS X mktemp(1).

  On GNU/Linux:

  -t    interpret TEMPLATE as a single file name component,
        relative to a directory: $TMPDIR, if set; else the
        directory specified via -p; else /tmp [deprecated]
  

  On OS X:

  mktemp [-t prefix] template …
  mktemp -t prefix
  
      -t prefix
              Generate a template (using the supplied prefix and TMPDIR if set) to create a filename tem-plate. template.
              plate.
  

  Perhaps the confusion is the reason why the GNU/Linux man page indicates that it is deprecated. I think you just want to run mktemp TEMPLATE without the -t option.

• Cleanup: Use trap "rm $TMP_SOURCE_FILE $TMP_PROGRAM_FILE" EXIT as a more robust way to delete the temporary files, in case your script aborts before reaching the rm commands.

• .c suffix: By writing TMP_SOURCE_FILE="$(mktemp -t sourceXXXX).c", you end up getting mktemp to create the file sourceXXXX. That file does not get cleaned up. Also, when you do cat > $TMP_SOURCE_FILE, you may be creating sourceXXXX.c, or worse, truncating an existing file.

  On GNU/Linux, a remedy would be to use the --suffix .c option. Unfortunately, the BSD/OS X version doesn't support it. One workaround is to rename the file after creation using mv -i — which might fail, but at least it's secure.

  A better workaround would be to use gcc -x c to tell GCC that it is C source code, without considering the filename extension.

  An even better solution would be…

• Avoid $TMP_SOURCE_FILE altogether: gcc can take its source code from its standard input! Use - as the filename.

  gcc -o "$TMP_PROGRAM_FILE" -x c - <<- END_OF_SOURCE
  …
  END_OF_SOURCE