Within C fork, printf() is not executing after while() loop - c

Question

I am working on writing a custom shell for a school project, and I need to be able to run external commands via the "execv" function. I need my command to either run successfully with the appropriate output, or state that the command was not found. Here is my code (with some printf() output for debugging) at this point:
/* Create a child process */
pid_t pid = fork();
/* Check if the fork failed */
if (pid >= 0)
{
if (pid == 0)
{
/* This is the child process - see if we need to search for the PATH */
if( strchr( command.args[0], '/' ) == NULL )
{
/* Search the PATH for the program to run */
char fullpath[ sizeof( getenv("PATH") ) ];
strcpy( fullpath, getenv("PATH") );
/* Iterate through all the paths to find the appropriate program */
char* path;
path = strtok( fullpath, colon );
while(path != NULL)
{
char progpath[COMMAND_SIZE];
/* Try the next path */
path = strtok( NULL, colon );
strcpy(progpath, path);
strcat(progpath, "/");
strcat(progpath, command.args[0]);
/* Determine if the command exists */
struct stat st;
if(stat(progpath, &st) == 0)
{
/* File exists. Set the flag and break. */
execv( progpath, command.args );
exit(0);
}
else
{
printf("Not found!\n");
}
}
printf("%s: Command not found!\n", command.args[0]);
}
else
{
...
}
/* Exit the process */
exit(EXIT_FAILURE);
}
else
{
/* This is the parent process - wait for the child command to exit */
waitpid( pid, NULL, 0 );
printf("Done with fork!\n");
}
}
else
{
/* Could not fork! */
printf("%s: %s > Failed to fork command!\n", command.args[0], strerror(errno) );
}
And here is the output:
john#myshell:/home/john/project>dir
/usr/local/sbin/dir: Not found!
/usr/local/bin/dir: Not found!
/usr/sbin/dir: Not found!
/usr/bin/dir: Not found!
/sbin/dir: Not found!
/bin/dir: Found!
makefile makefile~ myshell.c myshell.c~ myshell.x
Done with fork!
john#myshell:/home/john/project>foo
/usr/local/sbin/foo: Not found!
/usr/local/bin/foo: Not found!
/usr/sbin/foo: Not found!
/usr/bin/foo: Not found!
/sbin/foo: Not found!
/bin/foo: Not found!
/usr/games/foo: Not found!
Done with fork!
john#myshell:/home/john/project>
The known command "dir" is being found and executed properly. The output is great. However, when I use the fake "foo" command, I expected it to not find the command (which is clearly doesn't), complete the "while" loop, and execute the following "printf" command. This being said, I expected to see the following near the end of the output:
foo: Command not found!
I have tried using a boolean and integer value as a "flag" to determine if the command was found. However, no code seems to run outside the while loop at all. If I remove the "exit(0)", the "printf" command still doesn't run. I am stuck and baffled as to why the code outside the while loop doesn't seem to run at all. I also don't know if this is a problem with the way I am forking or if this has to do with the output buffer.
Am I doing this the wrong way, or how can I ensure that the "Command not found" message always runs exactly one time if the command was not found?

Answer 1

There's an error in your code -- you are using strcpy() and causing a buffer overrun:
// Note the declaration of getenv():
char *getenv(const char *name);
Therefore sizeof(getenv("PATH")) == sizeof(char*), which is probably 4 or 8.
/* Search the PATH for the program to run */
char fullpath[ sizeof( getenv("PATH") ) ]; // allocate fullpath[4] or [8]
strcpy(fullpath, getenv("PATH")); // overrun... copy to 4-8 char stack buffer
// UNDEFINED behavior after this - Bad Things ahead.
You could use malloc() instead to allocate fullpath on the heap dynamically:
char* fullpath = malloc(strlen(getenv("PATH")) + 1); // +1 for terminating NUL
strcpy(fullpath, getenv("PATH")); // OK, buffer is allocated large enough
// ... use fullpath ...
// Then when you are done, free the allocated memory.
free(fullpath);
// And as a general habit you want to clear the pointer after freeing
// the memory to prevent hard-to-debug use-after-free bugs.
fullpath = 0;