How to safely escape a string from C++ - c++

Question

I'm writing a simple program to browse the local network and pass on filenames to mplayer using "system". However, sometimes filenames contain spaces or quotes.
Obviously I could write my own function to escape those, but I'm not sure exactly what characters do or do not need escaping.
Is there a function available in the CRT or somewhere in the linux headers to safely escape a string to pass to the command line ?

Answer 1

Other answers include this fork and exec solution, but I claim that this is the only right way to do it.
Escaping shell arguments is prone to bugs and a waste of time, just as trying to escape SQL parameters is a silly idea when safer and more efficient parameter binding APIs exist.
Here is a sample function:
void play(const char *path)
{
/* Fork, then exec */
pid = fork();
if( pid < 0 ) {
/* This is an error! */
return;
}
if( pid == 0 ) {
/* This is the child */
freopen( "/dev/null", "r", stdin );
freopen( "/dev/null", "w", stdout );
freopen( "/dev/null", "w", stderr );
execlp( "mplayer", "mplayer", path, (char *)0 );
/* This is also an error! */
return;
}
}

Answer 2

There isn't a single solution that works everywhere because different shells have different ideas of what special characters are and how they are interpreted. For bash, you could probably get away with surrounding the entire filename in single quotes after replacing every single quote in the file name with '"'"' (the first single quote stops the sequence, the "'" appends the literal single quote to the string, the final single quote starts the quoted sequence again). A better solution would be to find a way to call the program without using system, such as by using fork with one of the exec functions so there is no shell interpolation.

Answer 3

While i don't know a function that does this, you can surround each of your arguments with '...', and replace any ' in the original argument by '"'"' . like
system("mplayer 'foo'\"'\"' bar'"); will give a single argument to mplayer which is foo'bar and which is allowed to contain strange things like " or \n . Note the escaping before " above (\") is only to make it valid C++.
You should consider using a function that accepts the arguments each separate, thus avoiding such issues. Wikipedia has a good article on this about the famous fork-and-exec pattern. http://en.wikipedia.org/wiki/Fork-exec

Answer 4

And now here is a complete solution to the shell escape problem. Although this
doesn't answer the exact question of escaping a string for shell. It solves the problem of passing arguments to program. This solution is a POSIX portable way to execute commands with arguments properly passed into the command without worrying about needing to escape them.
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <sstream>
#include <string>
#include <sys/stat.h>
#include <vector>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
std::vector<std::string> split(std::string delimiter, std::string str){
std::size_t nextPos = 0;
std::size_t delimiterSize = delimiter.size();
std::vector<std::string> list;
while(true){
std::size_t pos = str.find(delimiter, nextPos);
std::string subStr;
if(pos == std::string::npos){
list.push_back(str.substr(nextPos));
break;
}
subStr = str.substr(nextPos, pos - nextPos);
list.push_back(subStr);
nextPos = pos + delimiterSize;
}
return list;
}
bool isFileExecutable(const std::string &file)
{
struct stat st;
if (stat(file.c_str(), &st) < 0)
return false;
if ((st.st_mode & S_IEXEC) != 0)
return true;
return false;
}
std::string ensureEndsWithSlash(std::string path){
if(path[path.length()-1] != '/'){
path += "/";
}
return path;
}
std::string findProgram(std::string name){
// check if it's relative
if(name.size() > 2){
if(name[0] == '.' && name[1] == '/'){
if(isFileExecutable(name)){
return name;
}
return std::string();
}
}
std::vector<std::string> pathEnv = split(":", getenv("PATH"));
for(std::string path : pathEnv){
path = ensureEndsWithSlash(path);
path += name;
if(isFileExecutable(path)){
return path;
}
}
return std::string();
}
// terminal condition
void toVector(std::vector<std::string> &vector, const std::string &str){
vector.push_back(str);
}
template<typename ...Args>
void toVector(std::vector<std::string> &vector, const std::string &str, Args ...args){
vector.push_back(str);
toVector(vector, args...);
}
int waitForProcess(pid_t processId){
if(processId == 0){
return 0;
}
int status = 0;
int exitCode = -1;
while(waitpid(processId, &status, 0) != processId){
// wait for it
}
if (WIFEXITED(status)) {
exitCode = WEXITSTATUS(status);
}
return exitCode;
}
/**
Runs the process and returns the exit code.
You should change it so you can detect process failure
vs this function actually failing as a process can return -1 too
#return -1 on failure, or exit code of process.
*/
template<typename ...Args>
int mySystem(Args ...args){
std::vector<std::string> command;
toVector(command, args...);
command[0] = findProgram(command[0]);
if(command[0].empty()){
// handle this case by returning error or something
// maybe std::abort() with error message
return -1;
}
pid_t pid = fork();
if(pid) {
// parent wait for child
return waitForProcess(pid);
}
// we are child make a C friendly array
// this process will be replaced so we don't care about memory
// leaks at this point.
std::vector<char*> c_command;
for(int i = 0; i < command.size(); ++i){
c_command.push_back(strdup(command[i].c_str()));
}
// null terminate the sequence
c_command.push_back(nullptr);
execvp(c_command[0], &c_command[0]);
// just incase
std::abort();
return 0;
}
int main(int argc, char**argv){
// example usage
mySystem("echo", "hello", "world");
}