Tag: attribute constructor

In this video tutorial lets look at using #pragma preprocessor directive.

#pragma Directive

#pragma is a special purpose directive which can be used to turn on and off certain features.

Note: #pragma commands or keywords vary from one compiler to another. So make sure to check the keywords and/or syntax for the compiler you are using.

Video Tutorial: #pragma Preprocessor Directive: C Program

Source Code: #pragma Directive: C Program

#include<stdio.h>

void init();
void end();

#pragma startup init
#pragma exit end

int main()
{
    printf("We're inside main method\n");

    return 0;
}

void init()
{
    printf("We're inside init method\n");
}

void end()
{
    printf("We're inside end method\n");
}

Output:
We’re inside init method
We’re inside main method
We’re inside end method

#pragma startup and #pragma exit are keywords.

#pragma startup followed by a function name: this makes the function name specified to be executed before main method.

#pragma exit followed by a function name: this makes the function name specified to be executed just before program termination.

Note:
#pragma startup and #pragma exit doesn’t work in GCC compiler. So the equivalent behavior can be obtained by using below syntax:

Source Code: For GCC Compiler: C Program

#include<stdio.h>

void __attribute__((constructor)) init();
void __attribute__((destructor)) end();

int main()
{
    printf("We're inside main method\n");

    return 0;
}

void init()
{
    printf("We're inside init method\n");
}

void end()
{
    printf("We're inside end method\n");
}

Output:
We’re inside init method
We’re inside main method
We’re inside end method

__attribute__((constructor)) acts like #pragma startup and __attribute__((destructor)) acts like #pragma exit.

#include<stdio.h>

void init();
void end();

int main()
{
    printf("We're inside main method\n");

    return 0;
}

void __attribute__((constructor)) init()
{
    printf("We're inside init method\n");
}

void __attribute__((destructor)) end()
{
    printf("We're inside end method\n");
}

Output:
We’re inside init method
We’re inside main method
We’re inside end method

We could even use __attribute__((constructor)) and __attribute__((destructor)) in function definition, as shown in above source code.

Source Code: Suppressing warnings using #pragma Directive: C Program

#include<stdio.h>

#pragma warn -rvl /* No Return Value */#pragma warn -par /* Parameter Not Used */#pragma warn -rch /* Unreachable Code */
int main()
{
    int count = 1;

    printf("%d\n", count);

    return 0;

    count++;

}

int total()
{
    printf("Sum of a and b is c\n");
}

void end(int x)
{
    printf("We're inside end method\n");
}

Output:
1

Here function total has a return type of integer, but it doesn’t return any value from inside, so it must throw “no return value” warning. But that warning is suppressed by #pragma warn -rvl .

Function end takes a integer argument, but the value is nowhere used inside the function definition. Compiler should throw “Parameter Not Used” warning, but it’s suppressed by #pragma warn -par.

Inside main method, there is count++ after return. This code is never executed. So the compiler should throw “Unreachable Code” warning, which is suppressed by #pragma warn -rch.

Note: -(minus or subtraction) symbol before rvl, par and rch means compiler removes the warning. + sign indicated the warning will be shown in the editor after compilation of the program.

Also note that,

#pragma warn -rvl /* No Return Value */#pragma warn -par /* Parameter Not Used */#pragma warn -rch /* Unreachable Code */

doesn’t work on GCC compiler.

Important Note: The functions init() and end() should not receive any arguments and should not return any value, if we want to handle their invocation via #pragma.

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