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I want to activate c99 mode in gcc compiler to i read in other post in this forum that -std should be equal to -std=c99 but i don't know how to set it to this value using command line so please help.
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C99 is substantially completely supported as of GCC 4.5 (with -std=c99 -pedantic-errors used; -fextended-identifiers also needed to enable extended identifiers before GCC 5), modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G).
The correct option is -std=c11 . However, it is not available in gcc 4.6 . You need at least gcc 4.7 to have this option supported. In some older versions like gcc 4.6 , the option -std=c1x was available with experimental (i.e., very limited) support of C11.
Instead of calling /usr/bin/gcc , use /usr/bin/c99 . This is the Single-Unix-approved way of invoking a C99 compiler. On an Ubuntu system, this points to a script which invokes gcc after having added the -std=c99 flag, which is precisely what you want.
Compile using:
gcc -std=c99 -o outputfile sourcefile.c
gcc --help lists some options, for a full list of options refer to the manual. The different options for C dialect can be found here.
As you are using make you can set the command line options for gcc using CFLAGS:
# sample makefile
CC = gcc
CFLAGS = -Wall -std=c99
OUTFILE = outputfile
OBJS = source.o
SRCS = source.c
$(OUTFILE): $(OBJS)
$(CC) $(CFLAGS) -o $(OUTFILE) $(OBJS)
$(OBJS): $(SRCS)
$(CC) $(CFLAGS) -c $(SRCS)
Addendum (added late 2016): C99 is getting kind of old by now, people looking at this answer might want to explore C11 instead.
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