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When using a library it is essential to include the appropriate header files, in order to declare the function arguments and return values with the correct types. Without declarations, the arguments of a function can be passed with the wrong type, causing corrupted results.
The following example shows another program which makes a function call
to the C math library. In this case, the function pow
is used to
compute the cube of two (2 raised to the power of 3):
#include <stdio.h> int main (void) { double x = pow (2.0, 3.0); printf ("Two cubed is %f\n", x); return 0; }
However, the program contains an error—the #include
statement
for math.h is missing, so the prototype double pow (double
x, double y)
given there will not be seen by the compiler.
Compiling the program without any warning options will produce an executable file which gives incorrect results:
$ gcc badpow.c -lm
$ ./a.out
Two cubed is 2.851120 (incorrect result, should be 8)
The results are corrupted because the arguments and return value of the
call to pow
are passed with incorrect types.6 This can be detected by turning on the warning option
-Wall:
$ gcc -Wall badpow.c -lm badpow.c: In function `main': badpow.c:6: warning: implicit declaration of function `pow'
This example shows again the importance of using the warning option -Wall to detect serious problems that could otherwise easily be overlooked.
The actual output shown above may differ, depending on the specific platform and environment.
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