sprintf into char* var[1] fails with Segmentation fault-CodePudding

consider code:

using std::cout; using std::cerr;
using std::endl; using std::string;
using std::vector;

// . . .

    char* envp[10];

    vector<string> lines;
    char* c_line = nullptr;
    size_t len = 0;

    while ((getline(&c_line, &len, input_file)) != -1) {
        string line;
        lines.push_back(line.assign(c_line));
    }

    fclose(input_file);
    free(c_line);

    sprintf(envp[0], "SERVER=%s", lines[0].data());

    // printing envp[0] here OK

    sprintf(envp[1], "DOMAIN=%s", lines[1].data());

    // printing envp[1] never happened - Segmentation fault prints in output instead

I am C# dev I havent used C for couple decades. Something obvious is missing. Mem allocation?

P.S. I am mixing "old" char * for strings with std strings as the app uses 3rd party dll with chars*

EDIT declaring char envp[10][512]; fails down the line when I try to assing to the 3rd party property somedllObj.envps = envp; with cannot convert char[10][512] to char**

CodePudding user response：

I do not recommend mixing std::string and old C-strings in such a wild manner. Instead I'd rely on C classes as long as possible:

std::ifstream inputFile("path to file");
if(!inputFile)
{
    // error handling
}

std::vector<std::string> lines;
std::string tmp;
while(std::getline(inputFile, tmp))
{
    lines.emplace_back(std::move(tmp)); // since C  11, not copying the strings...
}

if(!inputFile.eof())
{
    // not the entire file read
    // -> error handling!
}

// TODO: size check; what, if not sufficient lines in file?

lines[0] = std::string("SERVER=%s")   lines[0];
lines[1] = std::string("DOMAIN=%s")   lines[1];

std::vector<char*> envp;
envp.reserve(lines.size());
for(auto& l : lines) // C  11: range based for loop...
{
    // note that this ABSOLUTELY requires the library not
    // modifying the strings, otherwise undefined behaviour!
    envp.emplace_back(const_cast<char*>(l.c_str()));
}

// so far, pure C  ...
// now we just get the pointers out of the vector:

libraryFunction(envp.size(), envp.data());

Bonus: No manual memory management at all...

(Note: Assuming the strings are not modified in the library!)

CodePudding user response：

Assuming servers and domains are in the input file, they do not have whitespaces, thus getline is not necessary.

vector<string> lines;
std::ifstream input_file("path to file");
std::copy(std::istream_iterator<std::string>(input_file),
          std::istream_iterator<std::string>(),
          std::back_inserter(lines));
lines[0] = "SERVER="s   lines[0];
lines[1] = "DOMAIN="s   lines[1];

std::vector<char*> envp;
for(auto& line : lines)
    envp.emplace_back(line.data());
envp.emplace_back(nullptr);  // Usually env arrays end with nullptr, thus  1 to array size
somedllObj.envps = envp.data();