Remove strcat_alloc()

[jngrad]

Utils::strcat_alloc() appends a C-string to a char array with manual memory management. This utility function is one of the last consumers of the ESPResSo manual memory management toolset (#2900), the others being EK and LB that are currently being rewritten in the waLBerla branch.

Here is a typical use of Utils::strcat_alloc():

double p3m_mc_time(char **log, const int mesh[3], int cao, ...) {
  char buffer[200];
  sprintf(buffer, "%-4d %-3d cao too large for this mesh\n", mesh[0], cao);
  *log = strcat_alloc(*log, buffer);
}

The same can be achieved with a std::string:

#include <string>
double p3m_mc_time(std::string &log, const int mesh[3], int cao, ...) {
  char buffer[200];
  sprintf(buffer, "%-4d %-3d cao too large for this mesh\n", mesh[0], cao);
  log += buffer;
}

Affected files: p3m.cpp, p3m-dipolar.cpp, mmm1d.cpp to remove calls to Utils::strcat_alloc(), and electrostatics.pxd, magnetostatics.pxd to replace char ** log by string &log from libcpp.string. After that, the utility function and its unit test can be removed.

The sprintf() statements do not need to be modified. They have two modern std::string equivalents, but they cannot be used out-of-the-box: std::format is a C++20 feature, boost::format uses a different syntax. Anyway, these sprintf() calls are found in functions that are probably going to be rewritten in Python in the future, so further beautification isn't worth it.

[fweik]

Shouldn't it first be discussed if a string log is really the best way to report back from the tuning?

[jngrad]

I would be fine with a logger or a printf. Also, there are 2 memory leaks with the tuning code, but that shouldn't surprise anyone...

diff --git a/src/core/electrostatics_magnetostatics/p3m.cpp b/src/core/electrostatics_magnetostatics/p3m.cpp
index 70918abbc..139cf58d5 100644
--- a/src/core/electrostatics_magnetostatics/p3m.cpp
+++ b/src/core/electrostatics_magnetostatics/p3m.cpp
@@ -1140,4 +1140,12 @@ int p3m_adaptive_tune(char **log) {
           time_best);
   *log = strcat_alloc(*log, b);
+  constexpr size_t N = 500*1024*1024;
+  char buffer[N];
+  for (size_t i = 0; i < N; ++i)
+    buffer[i] = '\n';
+  buffer[N-1] = '\0';
+  printf("(allocating %.0f MB in the core)\n", sizeof(buffer) / 1024. / 1024.);
+  *log = strcat_alloc(*log, buffer);
+  (*log)[0] = '\0';
   return ES_OK;
 }

import os, psutil, espressomd.electrostatics
process = psutil.Process(os.getpid())
ram = lambda: print(f'{process.memory_info().rss / 1024. / 1024.:.0f} MB')
system = espressomd.System(box_l=3*[1], time_step=0.01)
system.part.add(pos=[[0,0,0],[1,0,0]], q=[-1,1])
ram()  # should print 88 MB
p3m = espressomd.electrostatics.P3M(prefactor=1.0, accuracy=1e-2)
system.actors.add(p3m)  # start tuning
ram()  # should print 98 MB

Output:

87 MB
(allocating 500 MB in the core)
1097 MB