Not including stdlib.h does not produce any compiler error!

Question

Hopefully this is a very simple question. Following is the C pgm (test.c) I have.

#include <stdio.h>
//#include <stdlib.h>

int main (int argc, char *argv[]) {
    int intValue = atoi("1");
    double doubleValue = atof("2");
    fprintf(stdout,"The intValue is %d and the doubleValue is %g\n", intValue, doubleValue);
    return 0;
}

Note that I am using atoi() and atof() from stdlib.h, but I do not include that header file. I compile the pgm (gcc test.c) and get no compiler error!

I run the pgm (./a.out) and here is the output, which is wrong.

The intValue is 1 and the doubleValue is 0

Now I include stdlib.h (by removing the comments before the #include) and recompile it and run it again. This time I get the right output:

The intValue is 1 and the doubleValue is 2

How come the compiler did not complain about not including the stdlib.h and still let me use the atoi(), atof() functions?

My gcc info:

$ gcc --version
gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-27)

Any thoughts appreciated!

Answer 1

For historical reasons -- specifically, compatibility with very old C programs (pre-C89) -- using a function without having declared it first only provokes a warning from GCC, not an error. But the return type of such a function is assumed to be int, not double, which is why the program executes incorrectly.

If you use -Wall on the command line, you get a diagnostic:

$ gcc -Wall test.c
test.c: In function ‘main’:
test.c:5: warning: implicit declaration of function ‘atoi’
test.c:6: warning: implicit declaration of function ‘atof’

You should use -Wall basically always. Other very useful warning options for new code are -Wextra, -Wstrict-prototypes, -Wmissing-prototypes, -pedantic, and -Wwrite-strings, but compared to -Wall they have much higher false positive rates.

Tangentially: never use atoi nor atof, they hide input errors. Use strtol and strtod instead.

Answer 2

If you don't specify otherwise, I believe a C compiler will just guess that undeclared functions take the form extern int foo(). Which is why atoi works and atof doesn't. Which compiler flags were you using? I suggest using -Wall to turn on a bunch of gcc warnings, which should include referencing undeclared functions.