The phenomenon of
I recently wrote a small program under Ubuntu that always reports the following errors when running:

/usr/include/c++/7/cstdlib:75:15: fatal error: stdlib.h: No such file or directory #include_next <stdlib.h>

The reason:
I found out that:

This is because GCC7 has included libstdlib.h for better optimization, and the C Library header stdlib.h USES Include_next, which is sensitive to the GCC system header path.

Solutions:
Method 1: remove the following line from the.pro file in Qt:

INCLUDEPATH +=/usr/include

Method 2:
add the following line to the Qt. Pro file:

QMAKE_CFLAGS_ISYSTEM = -I

Reference: https://stackoverflow.com/questions/52532936/usr-include-c-7-cstdlib7515-fatal-error-stdlib-h-no-such-file-or-directo