Empty an array of struct containing char*

Question

I have a struct which contains a set of char* properties.

struct gcp_values {
char* srcX;
char* srcY;
char* dstX;
char* dstY;
};

well I fill it in a loop from a set of qt objects

std::vector<GeoRefLibrary::gcp_values> gcpvalues(vl.size());

    for(int i=0;i<vl.size();i++) {
        gcppoint_ *a = qobject_cast<gcppoint_ *>(vl.at(i).value<QObject *>());
        gcpvalues[i].srcX= strdup(QString::number(a->row()).toStdString().c_str());
        gcpvalues[i].srcY= strdup(QString::number(a->column()).toStdString().c_str());
        gcpvalues[i].dstX= strdup(QString::number(a->lon()).toStdString().c_str());
        gcpvalues[i].dstY= strdup(QString::number(a->lat()).toStdString().c_str());
    }

when I run valgrind test it results these outpt

12 bytes in 4 blocks are definitely lost in loss record 813 of 19,623
14 bytes in 4 blocks are definitely lost in loss record 889 of 19,623
16 bytes in 1 blocks are definitely lost in loss record 3,621 of 19,623
32 bytes in 4 blocks are definitely lost in loss record 8,851 of 19,623
36 bytes in 4 blocks are definitely lost in loss record 9,134 of 19,623

these errors are at those lines in loop. I tried to delete [] gcpvalues but it returned wrong delete/free error. I searched and found this post too

delete[] an array of objects

but cant find the solutions,can some one please help me with this problem?

I also tried

    struct gcp_values {
    QString srcX;
    QString  srcY;
   QString  dstX;
    QString  dstY;
};

and it returns the same errors

I changed the loop into this and it seems errors are gone

for(int i=0;i<vl.size();i++) {
    gcppoint_ *a = qobject_cast<gcppoint_ *>(vl.at(i).value<QObject *>());

      char* srcX = strdup(QString::number(a->row()).toStdString().c_str());
      char* srcY = strdup(QString::number(a->column()).toStdString().c_str());
      char* dstX = strdup(QString::number(a->lon()).toStdString().c_str());
      char* dstY = strdup(QString::number(a->lat()).toStdString().c_str());
      gcpvalues[i].srcX=srcX;
      gcpvalues[i].srcY= srcY;
      gcpvalues[i].dstX= dstX;
      gcpvalues[i].dstY= dstY;
    if(srcX) { free(srcX);}
    if(srcY) { free(srcY);}
    if(dstX) { free(dstX);}
    if(dstY) { free(dstY);}
}

The `strdup` documentation specifically says: *The returned pointer must be passed to free to avoid a memory leak.* — Nico Schertler, Nov 11 '17 at 09:57
This problem could be trivially avoided by just putting `QString`s in your `struct`. Manual memory management is difficult and unnecessary. Don't do it. — nwp, Nov 11 '17 at 10:41
@nwp then later how can we convert Qstring to char* as I need a char* in later functions. — Majid Hojati, Nov 11 '17 at 10:55
Write a destructor for your struct that frees the char arrays. `delete[]` should work as well. — Nico Schertler, Nov 11 '17 at 10:57
Same as you did now. `QString srcX; something_that_takes_char_pointer(srcX.toStdString().c_str());`. Just make sure `something_that_takes_char_pointer` doesn't do something stupid like calling `free` on that pointer. Although the better solution would be to change the function to take a string instead. — nwp, Nov 11 '17 at 10:58
@nwp as I tried befor .c_str() returns const char* and it later I have to allocate memory to cnovert const char to char — Majid Hojati, Nov 11 '17 at 11:01
@nwp as I tried also std::string srcX also returns the same error. does it make any changes if I use Qstring instead? — Majid Hojati, Nov 11 '17 at 11:03
_I need a char* in later function_ why? Why don’t they use std::string? — , Nov 11 '17 at 11:05
@manni66 in fact it is a library which I am using (gdal) and it accepts char * as input. — Majid Hojati, Nov 11 '17 at 11:08
With C++ 17 you can get a non const char* to the std::string data with data(). — , Nov 11 '17 at 11:08
The gdal C++ API uses const char*. Do you refer to an out parameter? — , Nov 11 '17 at 11:12
@manni66 but I need its opossite, as c_str() returns const char* and I need to have a char * — Majid Hojati, Nov 11 '17 at 11:12
@manni66 yes, in this function GDALGeneralCmdLineProcessor it gets a char** as input of commandline — Majid Hojati, Nov 11 '17 at 11:14

score 1 · Accepted Answer · answered Nov 11 '17 at 16:08

1

strdup is a POSIX function, and POSIX functions which allocate memory (and do not provide type-specific deallocation functions such as fclose and freeaddrinfo) expect the caller to eventually call free to deallocate the allocated memory.

POSIX does not use C++ memory allocation, so operator delete[] is not correct here.

answered Nov 11 '17 at 16:08

Florian Weimer

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Thanks it was the main problem, I changed the loop and it seems it works. but if I free values in loop does it work? – Majid Hojati Nov 11 '17 at 16:43
It will work as long as you free each pointer value exactly once. – Florian Weimer Nov 11 '17 at 16:43
Of course, this is not really about POSIX, but rather any C API, since C does not have automatic dellocation via RAII like C++ does. – underscore_d Nov 11 '17 at 16:56
@underscore_d, `strdup` is a POSIX function. For some reason, it is not part of any C standard. – Florian Weimer Nov 11 '17 at 17:00
Sure, but I mean that what you said applies to all(?) other C functions that return newly allocated things and thus need freed later, not just POSIX ones. – underscore_d Nov 11 '17 at 17:02

Empty an array of struct containing char*

1 Answers1