Function "strcmp()" working without adding "string. h"

Question

I wrote an algorithm in C that uses the function strcmp(). Even forgetting to add the string.h the algorithm worked.

I would like to understand how it worked since I only found this function in the library string.h.

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


typedef struct Produto{
    char nome[30];
    float preco;
}Produto;

int compara_nome(const void * A, const void* B){
Produto * pointerA=(Produto *) A;
Produto * pointerB=(Produto *) B;

return strcmp (pointerA->nome,pointerB->nome);/* como isto está funcionando sem o string.h ?*/


}
int compara_preco(const void * A, const void * B){
    Produto * pointerA=(Produto *) A;
    Produto * pointerB=(Produto *) B;
return (pointerA->preco-pointerB->preco);

}


int main(){
 static Produto Estoque[10]={{"Leite",5.50},{"Donuts",23.6},{"Detergente",4.15},{"Acucar",7.84},
 {"Brigadeiro",12.30},{"Limão",3.48},{"Morango",6.21},{"Tomate",4.12},{"Feijao",3.10},{"Skol",7.89}};

 qsort(Estoque,10,sizeof(Produto),compara_nome);

 int i;
 for(i=0;i<10;i++){
    printf("Nome: %-10s\t  Preco:%5.2f \t\n",Estoque[i].nome,Estoque[i].preco);
 }
qsort(Estoque,10,sizeof(Produto),compara_preco);    
printf("-----------------------------------------------");
printf("\n");
 for(i=0;i<10;i++){
    printf("Nome: %-10s\t  Preco:%5.2f \t\n",Estoque[i].nome,Estoque[i].preco);
 }  

    return 0;
}

Answer 1

What’s in the header file is just the signing function, not its implementation. If the implementation is available, the linkediting will work. In this case the compiler recognizes that it is possible to call it without further checks and trusts that it will work. The ideal is not to do so because calling yourself wrong will give error at runtime, an error that could be detected in the compilation if had the signature evaluated.

C has a lot of things that work, but there’s no guarantee it’s right. Remember that C is a portable assembly and not a high-level language with all possible checks. That’s why I always say:

Especially in C people need to get used to the philosophy of understanding what is doing and happening and not rely on the outcome. In modern C this is considered a mistake. It is possible to connect an analysis in the compiler not to let happen, I advise to do it, otherwise it will deal with something like "works on my machine" which is always a programmer problem.

Signing is just a contract to be followed, it is not something that executes a code. Execution will be inserted normally and you hope it works.