fprintf在特定字符串上失败，而外壳函数使用它时没有崩溃

Question

The short version: I have a C++ code that uses a C call to fprintf(stdout, some_cpp_str.c_str()) and crashes during it. The first 4 calls are fine, only the 5th crashes, and I have no idea why _{(suspecting unreadable char inside the string)} . The 1st code I posted was mostly C, so I posted another one, with only C++ except for the fprintf (code added at the bottom of the question). The crashes occur (consistently) on an embedded device. On my own PC the code runs fine

---

The long version:

I have a code that reads lines from text, and pushes them into a string vector. TO check my progress, I also fprintf them to the screen after the vector is populated:

int main(){
    char    err_msg[256], * line = NULL, *in_file = "...", *keyword = "blah";
    size_t  len = 0;
        ssize_t num_bytes_read;
    int i = 1;
    std::vector<std::string> lines_vector;

    FILE * fp = fopen(in_file, "r");
    if (!fp) {
        fprintf(stdout,"can't open file %s for reading\n", in_file);
        goto EXIT;
    }

    while ((num_bytes_read = getline(&line, &len, fp)) != -1) {
        /* if found keyword inside line */
        if (strstr(line, keyword)) {
            /* add 3 lines (entry heading, entry body, newline)*/
            lines_vector.push_back(std::string(line));
            for(int lines_to_copy = 2; lines_to_copy > 0; lines_to_copy--) {
                if((num_bytes_read = getline(&line, &len, fp)) == -1) {
                    fprintf(stdout,"can't read line from %s\n", in_file);
                    goto EXIT;
                }
                lines_vector.push_back(std::string(line));
            }
        }
    }
    fprintf(stdout,"finished reading from file\n");

EXIT:
    fclose(fp);
    free(line);

    for (std::vector<std::string>::iterator it = lines_vector.begin() ; it != lines_vector.end(); ++it, ++i) {
        fprintf(stdout, "%d)", i);
        fprintf(stdout, "%s", (*it).c_str());
    }
    return 0;
}

This works fine on my VM, but I also run it on an embedded device, where it always crashes on a specific line. The line is:

certificates local generate name localcert common-name sf country(region) AB auto-regenerate-days 12 auto-regenerate-days-warning 11 e-mail X@Y.com locality(city) Z organization Q organization-unit T scep-password-string 57E6CA35452E72E4D1BC4518260ABFC7 scep-url http://0.0.0.0/X/Y/ state(province) s

I don't think there is a problem in the line itself (as it doesn't crash on my VM). When trying to print it to a file instead of to the screen, it doesn't crash:

for (std::vector<std::string>::iterator it = lines_vector.begin(); it != lines_vector.end(); ++it){
    sprintf(tmp, "echo \"%s\" >> /X/Y/Z.txt", (*it).c_str());
    OS_run(tmp);  // run this command on sh shell
}

Since it crashes only on my embedded and not my VM, I thought the file is somehow corrupted. Could it be that the string has an invalid char inside that crashes fprintf , but not echo ?

---

I tried translating this code into proper C++, but I still get a crash in the middle of the last string. I know mixing C/C++ is not good, but shouldn't c_str() be a proper interface between std::string and char * (which fprintf expects)?

If not this, then what could possibly crash during the fprintf ?

int main()
{
    std::vector<std::string> lines_vector;
    std::ifstream infile(in_file);
    std::string line;
    int counter = 1;

    while (std::getline(infile, line))  {
        if (line.find(keyword, 0) != std::string::npos) {
            lines_vector.push_back(line);
            for(int lines_to_copy = 2; lines_to_copy > 0; lines_to_copy--)  {
                std::getline(infile, line);
                lines_vector.push_back(line);
            }
        }
    }

    for (std::vector<std::string>::iterator it = lines_vector.begin(); it != lines_vector.end(); ++it){
        fprintf(stdout, "%d)%s", counter++, (*it).c_str());
    }
}

Answer 1

On an embedded device, you can expect that dynamic memory allocation fail. That means that you absolutely must control all possible allocations (you should anyway even on non embedded device, but the crash risk is much lower...). You really should have:

while ((num_bytes_read = getline(&line, &len, fp)) != -1) {
    ...
}
if (line == NULL) {
    perror("getline could not allocate buffer");
}

This will not fix anything, but at least you will know what happens.

---

I have respected your coding style here, making heavy use of the C library and also using goto . But I must advise you not to do that in C++ programs.

C library used to be included in C++ standard library because early C++ implementations were lacking too many functionalities. In modern C++ goto is to be banned, as are all raw C strings and C io functions (except in very special use cases). And C++ come with a version of getline (in header <string> ) that directly fills a std::string . You really should try to avoid C construct if learning C++.

---

Per Ben Voigt's comment, there are correct use case to use old style C library if you want to avoid dynamic allocation. But in that case, you should also avoid std::string and std::vector