#2556 compile warnings and missing build dependencies

[Ian! D. Allen]

I have an issue with gnuplot, so I want to report it.
I want to run the latest version.
(I'm on Ubuntu 20.04.5 LTS with gnuplot 5.2 patchlevel 8 2019-12-01)

I download and extract the source gnuplot-5.4.5.tar.gz
I run: apt-get build-dep gnuplot
I run ./configure and get an error:

checking for LIBCERF... configure: WARNING:
Package requirements (libcerf) were not met:
No package 'libcerf' found

I have to run: apt install libcerf-dev
Now, ./configure gives no warnings.
Perhaps build-dep is only building for gnuplot 5.2 (and I'm building 5.4),
or perhaps build-dep is missing a dependency. You decide.

I run "make" and get these compile warnings:

wxterminal/wxt_gui.cpp: In function ‘void wxt_atexit()’:                        wxterminal/wxt_gui.cpp:4276:10: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [
-Wunused-result]
 4276 |   freopen("/dev/null","w",stdout);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
wxterminal/wxt_gui.cpp:4277:10: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result]
 4277 |   freopen("/dev/null","w",stderr);
      |

gplt_x11.c: In function ‘main’:                                                 gplt_x11.c:764:2: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result]
  764 |  freopen("/dev/null", "w", stderr);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gplt_x11.c: In function ‘process_event’:
gplt_x11.c:4663:12: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Wunused-result]                                     4663 |   if (cmd) system(cmd);
      |

qtterminal/QtGnuplotApplication.cpp: In member function ‘void QtGnuplotApplication::enterPersistMode()’:                                                        qtterminal/QtGnuplotApplication.cpp:94:9: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unuse
d_result [-Wunused-result]
   94 |  freopen("/dev/null","w",stdout);
      |  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
qtterminal/QtGnuplotApplication.cpp:95:9: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result]
   95 |  freopen("/dev/null","w",stderr);
      |

I got a working gnuplot 5.4.5 binary, but it was a bit ugly on the way.
I'm on Ubuntu 20.04.5 LTS

[Tatsuro MATSUOKA]

For libcerf, it is not an issue of gnuplot team but is an issue for packaging of gnuplot on the Ubuntu. Please ask on the Ubuntu package team.

[Tatsuro MATSUOKA]

On Ubuntu 22.04, gnuplot version is 5.4.3 and it lacks libcerf. Ubuntu gnuplot team have prepared gnuplot package without libcerf library.

[Ethan Merritt]

The warnings you show are just that - warnings, not errors. Recent versions of the C compiler have gotten very noisy in warning about trivialities. I usually set CFLAGS to "-Wall -g -Og -Wdeclaration-after-statement -Wuninitialized -Wunused-result -Wreturn -type" to suppress these.

As to the package dependencies for Ubuntu, that is out of our hands. Gnuplot will run just fine without libcerf, by the way. It just won't have built-in support for the set of special functions in that library.