Reentrant Code and Compiler Flags

Reentrant Code and Compiler Flags

Depending upon the platform and compiler, it may be necessary to compile with special flags to indicate that the application is multithreaded. On some platforms, gcc has the flag -pthread, which links in the POSIX threading library and defines _REENTRANT. The Solaris Studio compiler has the flag -mt, which performs the same task; only the flag defined is __REENTRANT.

Defining the preprocessor flag includes a set of “reentrant” functions to go with the usual set. The reentrant versions are denoted by the _r postfix. The reason these func-tions exist is that many of the C library functions are stateful, meaning that a call to the function sets up some state that is then used in later function calls. This statefulness means that multiple threads cannot call the function at the same time without the state becoming corrupted. The reentrant versions of the functions typically use a structure that the calling thread provides to hold the state for that calling thread. The code in Listing 5.75 uses the readdir() call to print out the contents of the current directory.

Listing 5.75   Multithread Unsafe Code for Listing a Directory

#include <dirent.h> #include <stdio.h>

int main()

{

DIR * directory = opendir( "." );

while ( 1 )

{

struct dirent * entry = readdir( directory );

if (entry!=0)

{

printf( "%s\n", entry->d_name );

}

else

{

closedir( directory );

break;

}

}

}

The code works when compiled as a single-threaded application but could, on some operating systems, fail if multiple threads were to use the readdir() function simultane-ously. The code can easily be modified to use the reentrant version of readdir(), readdir_r(). Listing 5.76 shows the modified code.

Listing 5.76   Using Multithread-Safe Code to Print Out a Directory Listing

#include <dirent.h> #include <stdio.h> #include <stdlib.h>

int main()

{

DIR * directory = opendir( "." );

struct dirent * entry; struct dirent * result;

entry = (struct dirent*)malloc( sizeof(struct dirent) + FILENAME_MAX + 1 );

while ( ( readdir_r( directory, entry, &result ) ==0 ) && result )

{

printf( "%s\n", entry->d_name );

}

closedir( directory );

free( entry );

}

The reentrant version of readdir() places the results into a dirent structure that is passed into the call. However, not all operating systems declare the dirent structure with storage for the filename by default. The code shown will compile on Solaris with _POSIX_C_SOURCE defined to be greater than or equal to 199506L.