c- segmentation fault error due to calling a function that has text that user put in the terminal-CodePudding

Summary

I am trying to pass the text that user has put in the terminal and pass it to the function named printString(). I don't fully understand C but I think I has to do with the pointer not being in the heap. Any help would be appreacitaed!

#include <stdio.h>
void printString();
int main (int argc, char* argv[]) {
        int commandAmount = 1;
        while (commandAmount < argc) {
        printString(commandAmount, &argv);
        }
        return 0;
}
void printString(int commandAmount, char* argv[]) {
        printf("the word is %s," , argv[commandAmount]);
}

./shortExample example

Segmentation fault (core dumped)

CodePudding user response：

The prototype void printString(); does not match the actual implementation. It should have been:

void printString(int commandAmount, char* argv[]);

You could also skip the prototype and just implement the function before main. Your loop while (commandAmount < argc) seems to not have any way to finish since you never increase commandAmount. This can cause undefined behavior and with such, your program may crash or do just about anything. I suggest making a for-loop to fix that.

Example:

#include <stdio.h>

void printString(int commandAmount, char* argv[]) {
    printf("the word is %s,", argv[commandAmount]);
}

int main(int argc, char* argv[]) {
    for(int commandAmount = 1; commandAmount < argc;   commandAmount) {
        printString(commandAmount, argv);
    }
}

or in the way you structured it:

#include <stdio.h>

void printString(int commandAmount, char* argv[]); // corrected

int main(int argc, char* argv[]) {
    int commandAmount = 1;
    while (commandAmount < argc) {
        printString(commandAmount, argv);
          commandAmount;                           // needed
    }
}

void printString(int commandAmount, char* argv[]) {
    printf("the word is %s,", argv[commandAmount]);
}

CodePudding user response：

For starters this function declaration

void printString();

does not provide a function prototype. So the compiler determines the type of the parameters of the function from the function call

printString(commandAmount, &argv);

However the expression &argv used in this call

printString(commandAmount, &argv);

has the type char *** due to the declaration of the identifier argv

int main (int argc, char* argv[]) {
                    ^^^^^^^^^^^^

But the corresponding parameter in the definition of the function printString has the type char ** due to adjusting by the compiler parameters having array types to pointers to array element type.

That is this function declaration

void printString(int commandAmount, char* argv[]) {

is adjusted by the compiler to

void printString(int commandAmount, char** argv) {
                                    ^^^^^^^^^^^

Thus there are incompatible types of the argument expression and of the parameter. As a result this call

printf("the word is %s," , argv[commandAmount]);

invokes undefined behavior.

Moreover this loop in main

    int commandAmount = 1;
    while (commandAmount < argc) {
    printString(commandAmount, &argv);
    }

in general is an infinite loop because the variable commandAmount is not changed within the loop.

Firstly you should provide the function prototype before main to make your program more safer

void printString(int commandAmount, char** argv);

and call the function like

printString(commandAmount, argv);
                           ^^^^

Of course you need also to change the loop in main.

Pay attention to that as the value of the parameter commandAmount is not outputted within the function then in fact it is redundant. You could pass to the function the pointer to the string itself. For example

#include <stdio.h>

void printString( const char *s );

int main( int argc, char* argv[] ) 
{
    for ( int commandAmount = 1; commandAmount < argc; commandAmount   ) 
    {
        printString( argv[commandAmount] );
    }
    putchar( '\n' );

    return 0;
}

void printString( const char *s ) 
{
    printf( "the word is %s, " , s );
}