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Asked: 2026-05-13T15:20:04+00:00

When I compile a program using just gcc code.c There are no messages, and

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When I compile a program using just

gcc code.c

There are no messages, and an output file is generated successfully. The outputted file works. However, when I try to the same cygwin installation’s gcc compiler in an IDE (I’ve tried Netbeans and Dev-C++), I get the following errors

main.cpp:27: error: `exit' undeclared (first use this function)
main.cpp:27: error: (Each undeclared identifier is reported only once for each function it appears in.)
main.cpp:77: error: `write' undeclared (first use this function)
main.cpp:78: error: `close' undeclared (first use this function)

I don’t see what’s different. Why does it not compile?

OK, the issue was that in the IDE, the file had a .cpp extension, whereas when I was compiling from a terminal, it had a .c extension. So, my new question is why does it not compile when it’s treated as a c++ file. Isn’t C a subset of C++?

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1 Answer

  1. Editorial Team

    C++ is stricter then C. Where C allows you to call a function without a prototype, C++ does not allow this.

    To solve the problem, you want to add:

    #include <stdlib.h>

    Also, when compiling at the command line. Make sure to use the -Wall flag so you’ll get important warnings:

    gcc -Wall code.c
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