Get current execution path from within C library - c

Question

I'm writing a logger library in C and am currently trying to get better backtrace output by using addr2line. In order to do so, I need to be able to get the path of the current executable. At the moment I am just concerned about linux, but will be shooting for Mac OS support as well.
For linux support I'm trying to use readlink() and /proc/self/exe to resolve the current executable's path:
static char** getPrettyBacktrace( void* addresses[], int array_size ) {
// Used to return the strings generated from the addresses
char** backtrace_strings = (char**)malloc( sizeof( char ) * array_size );
for( int i = 0; i < array_size; i ++ ) {
backtrace_strings[i] = (char*)malloc( sizeof( char ) * 255 );
}
// Will hold the command to be used
char* command_string = (char*)malloc( 255 );
char* exe_path = (char*)malloc( 255 );
// Used to check if an error occured while setting up command
bool error = false;
// Check if we are running on Mac OS or not, and select appropriate command
char* command;
#ifdef __APPLE__
// Check if 'gaddr2line' function is available, if not exit
if( !system( "which gaddr2line > /dev/null 2>&1" ) ) {
command = "gaddr2line -Cfspe";
// TODO: get path for mac with 'proc_pidpath'
} else {
writeLog( SIMPLOG_LOGGER, "Function 'gaddr2line' unavailable. Defaulting to standard backtrace. Please install package 'binutils' for better stacktrace output." );
error = true;
}
#else
// Check if 'addr2line' function is available, if not exit
if( !system( "which addr2line > /dev/null 2>&1" ) ) {
command = "addr2line -Cfspe";
if( readlink( "/proc/self/exe", exe_path, sizeof( exe_path ) ) < 0 ) {
writeLog( SIMPLOG_LOGGER, "Unable to get execution path. Defaulting to standard backtrace." );
error = true;
}
} else {
writeLog( SIMPLOG_LOGGER, "Function 'addr2line' unavailable. Defaulting to standard backtrace. Please install package 'binutils' for better stacktrace output." );
error = true;
}
#endif
// If an error occured, exit now
if( error ) {
free( backtrace_strings );
free( command_string );
free( exe_path );
return NULL;
}
for( int i = 0; i < array_size; i++ ) {
// Compose the complete command to execute
sprintf( command_string, "%s %s %X", command, exe_path, addresses[i] );
// Execute the command
FILE* line = popen( command_string, "r" );
// Get the size of the command output
int line_size = fseek( line, 0, SEEK_END );
// Read the output into the return string
fgets( backtrace_strings[i] , line_size, line );
// Close the command pipe
pclose( line );
}
return backtrace_strings;
}
The path being returned by readlink() is: /home/nax��?. the first part is correct: /home/na, but everything after that is pure gibberish.
Why am I unable to get the current execution path in this way?

Answer 1

char* exe_path = (char*)malloc( 255 );
// ...
readlink( "/proc/self/exe", exe_path, sizeof( exe_path ) )
exe_path is a pointer, so it's size will be equal to sizeof(char*) (4 or 8), not 255.
change exe_path to char[255] or change the call to sizeof
btw, readlink does not append the NULL byte, so you should do something like this:
len = readlink( "/proc/self/exe", exe_path, sizeof( exe_path ) )
exe_path[len] = 0;