void* to char*

[James Purser]

I have a piece of c code I am trying to include in a c++/wxWidgets
project.

len = strlen(sha1_dir);
path = malloc(len + 40);
memcpy(path, sha1_dir, len);

When I try to compile the above using g++ I get "error: invalid conversion
from void* to char*"

I have tried casting the malloc as a char * but that results in a seg
fault at the memcpy line.

Any ideas would be much appreciated.

James Purser

 

[Axter]

I have a piece of c code I am trying to include in a c++/wxWidgets
project.

len = strlen(sha1_dir);
path = malloc(len + 40);
memcpy(path, sha1_dir, len);

When I try to compile the above using g++ I get "error: invalid conversion
from void* to char*"

I have tried casting the malloc as a char * but that results in a seg
fault at the memcpy line.

Any ideas would be much appreciated.

James Purser

Please post declaration for path, and code you use with the cast.

FYI: This code doesn't need cast in C, but would need it in C++

If you're putting this in a C++ file, I recommend you use new instead
of malloc.

 

[Rolf Magnus]

I have a piece of c code I am trying to include in a c++/wxWidgets
project.

len = strlen(sha1_dir);
path = malloc(len + 40);
memcpy(path, sha1_dir, len);

When I try to compile the above using g++ I get "error: invalid conversion
from void* to char*"

In C++, it's better to use new/delete instead of malloc/free.

I have tried casting the malloc as a char * but that results in a seg
fault at the memcpy line.

I can't see anything wrong with it. memcpy shouldn't crash AFAICS - unless
malloc failed to allocate the memory or sha1_dir doesn't point to the start
of a valid C style string.
You didn't copy the terminating '\0' character, so if you don't add anything
else and don't correctly terminate the string, the next function that
expects a C style string will probably crash.

 

[James Purser]

Please post declaration for path, and code you use with the cast.

FYI: This code doesn't need cast in C, but would need it in C++

If you're putting this in a C++ file, I recommend you use new instead
of malloc.

Full function declaration:

int init_db(char * dpath)
{
safe_create_dir(dpath);
char *sha1_dir, *path;
int len, i, err;
sha1_dir = getenv(DB_ENVIRONMENT);
if (!sha1_dir) {
sha1_dir = DEFAULT_DB_ENVIRONMENT;
err = 1;
}
len = strlen(sha1_dir);
path = malloc(len + 40);
memcpy(path, sha1_dir, len);

safe_create_dir(sha1_dir);
for (i = 0; i < 256; i++) {
sprintf(path+len, "/%02x", i);
safe_create_dir(path);
}
return err;
}

 

[James Purser]

Thanks for the help, changing it to new fixed the issue.

 

[Rolf Magnus]

Full function declaration:

int init_db(char * dpath)

Why not const char*?

{
safe_create_dir(dpath);
char *sha1_dir, *path;
int len, i, err;
sha1_dir = getenv(DB_ENVIRONMENT);
if (!sha1_dir) {
sha1_dir = DEFAULT_DB_ENVIRONMENT;
err = 1;

This is the only place where you write something to err. So if the getenv()
call succeeds, your function returns the value of an uninitialized
variable.

}
len = strlen(sha1_dir);
path = malloc(len + 40);

That would be ok in C, but in C++, you need to cast the value returned by
malloc, like:

path = static_cast<char*>(malloc(len + 40));

Or using new:

path = new char*[len + 40];

memcpy(path, sha1_dir, len);

safe_create_dir(sha1_dir);
for (i = 0; i < 256; i++) {
sprintf(path+len, "/%02x", i);
safe_create_dir(path);
}
return err;

You're having a memory leak here. The memory that 'path' points to is lost,
since the only pointer to it goes out of scope here, and you didn't free
the memory.

}

This all looks pretty much like C. Why are you running this through a C++
compiler?

 

[Owen Jacobson]

int init_db(char * dpath)
{
safe_create_dir(dpath);
char *sha1_dir, *path;

....

That would be ok in C, but in C++, you need to cast the value returned by
malloc, like:

path = static_cast<char*>(malloc(len + 40));

Or using new:

path = new char*[len + 40];

path = new char[len + 40]; // long day, rolf?

 

[Rolf Magnus]

Or using new:

path = new char*[len + 40];

path = new char[len + 40]; // long day, rolf?

Actually not so much. Just a stupid error, nothing more ;-)
Thx for the correction.