Input C++ Vector into C function

Question

I have a std::vector<float> containing sound data. Without copying its data, I'd like to use this vector as input to the sonicChangeFloatSpeed function of the Sonic library. This method expects a float* as first argument and mutates the input array. After completion, the pointer in first argument would point to the result data.

data gives me access to the internal array of a C++ vector and with assign, I can replace the vector's contents. Hence, I tried the following:

float* ptr = vec.data();
int num_samples = sonicChangeFloatSpeed(ptr, vec.size(), 1., 1.5, 1., 1., 0, 41000, 1);
vec.assign(ptr, ptr + num_samples);

But when I run this program, I get the error double free or corruption (!prev) with a SIGABRT at this location. What is the problem of this approach and how would this question be solved more appropriately?