C99 code vs. gcc defaults?

[joe-lawrence]

Internal CI testing on RHEL-7.9 popped up this build error for #1415:

gcc -MMD -MP -I../kmod/patch -Iinsn -Wall -Wsign-compare -Wconversion -Wno-sign-conversion -g -Werror   -c -o create-diff-object.o create-diff-object.c
create-diff-object.c: In function ‘kpatch_create_pfe_sections’:
create-diff-object.c:3831:4: error: ‘for’ loop initial declarations are only allowed in C99 mode
    for (int i=0; i<16; i++) {
    ^
create-diff-object.c:3831:4: note: use option -std=c99 or -std=gnu99 to compile your code

Up to now, we haven't merged any C99-isms like this into the code base, but it seems that default gcc options in distros like Fedora and the Ubuntu? github Travis CI must be assuming C99, so I never saw this error until the internal CI ran on RHEL-7.

A few things we could do:

• Add -std=gnu99 to kpatch-build Makefile
• Add -std=gnu89 to kpatch-build/Makefile
• Add -std=gnu89 test to .travis.yml to catch them earlier in the PR

[joe-lawrence]

@jpoimboe : any preferences given the maturity of the code base?

[jpoimboe]

I think we can add -std=gnu11 to kpatch-build/Makefile as that's what the kernel uses and we tend to follow kernel style.