Jim basically has it right. With gcc -ansi gcc is being asked to supply a compilation environment as specified by an ISO C Standard. You will have to look at the gcc documentation to determine which version of the ISO C Standard that compiler flag implements.

The C Standard does not contain any definition for the fdopen() function. So, if you ask the compiler to give you a C Standard conforming environment, a function prototype for fdopen() can't be included in stdio.h.

The POSIX Standards do include a function prototype for fdopen() in stdio.h. If you want to build an application that conforms to the 2008 through 2017 versions of the POSIX Standards, you need to define the _POSIX_C_SOURCE macro to have the value 200809L before any header defined by the POSIX Standard is included and use a compiler that provides a C compiler conforming to the 1999 version of the ISO C Standard. On a UNIX, Linux, or BSD based system, this would typically be done with:

Code:

c99 -D _POSIX_C_SOURCE=200809L -o f f.c

Note also that using -c and -o in the same invocation of c99 produces unspecified results. Using both (as shown in post #1 might silently ignore one of those options, might produce a diagnostic message and do nothing else, might do whatever it was that the person who used that combination wanted to happen, or might do something else entirely at the discretion of the person who wrote the code for the compiler you're using.

Note also that using cc -o f.o f.c is always wrong. By convention a file with the extension .o is an object file created from a single source file. For example, the command:

Code:

cc -c f.c

will create a file named f.o if f.c does not contain any errors and will not create a runnable file. Using the -o. If the -c option is not included on the command line, the source files named on the command line will be compiled and the object files created from those sources file and any named object files will be linked together with default and named libraries to create a runnable file. By default that runnable file will be named a.out. The name of that runnable file can be overridden using the -o option (as shown in the c99 command I suggested above).