Char vector by reference

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I’m trying to pass a array of char for a Procedure that will be resized (with malloc()), write your content and return to main().

I know that every vector is a pointer and is already passed by reference, but something in my software is not letting this happen, when back to function main() it is with the initial values, follows example code:

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

void proc(char msg[])
{
    unsigned int i;

    msg = malloc(sizeof(char) * 10);

    msg[0] = 'a';
    msg[1] = 'b';
    msg[2] = 'c';
    msg[3] = 'd';
    msg[4] = 'e';
    msg[5] = 'f';
    msg[6] = 'g';
    msg[7] = 'h';
    msg[8] = 'i';
    msg[9] = 'j';

    for (i = 0; i<10; i++) {
        printf("%c\n", msg[i]);
        msg[i] = 'x';
    }

    printf("\n");
}

int main ()
{
    char msg[] = "12345";
    unsigned int i;

    printf("Before proc\n");

    for (i = 0; i<5; i++) {
        printf("%c\n", msg[i]);
    }

    printf("\nin proc ======\n");

    proc( msg );

    printf("After proc\n");

    for (i = 0; i<10; i++) {
        printf("%c\n", msg[i]);
    }

    printf("FIM");
    return 0;
}

The output is as follows:

bash-4.2$ ./a.out
Before proc
1
2
3
4
5

in proc ======
a
b
c
d
e
f
g
h
i
j

After proc
1
2
3
4
5





FIM

Where am I going wrong?

c array pointer arduino char

  • What’s the point? The function is pretty weird, does things kind of meaningless.

  • I did just one example to try to isolate the problem I am having when reading a serial port, what I want is: 1. pass a char array to a Procedure 2. resize that array in the Procedure 3. write values in that array 4. return the main with the dimentional array and with the new values

  • That is, when returning to the main the vector has to be all filled with 'x' and have size of 10 bytes.

  • You start working with a string, then shaken into this, which is right, is a string or not?

  • The correct thing is to work all the time with a string, I may be wrong about this, but it has to be dynamic.

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1 answer

1

There are several errors and complications. I kept the writing to see it working, but obviously this shouldn’t exist there.

If you’re going to work with string do it all the time like this. You need to have space for the terminator.

If you are going to change the size then you need to allocate the array dynamically with malloc().

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

void proc(char msg[]) {
    msg = realloc(msg, 11);
    strcpy(msg, "xxxxxxxxxx");
    printf("\nin proc ======\n%s\n", msg);
}

int main() {
    char *msg = malloc(6);
    strcpy(msg, "12345");
    printf("Before proc\n%s", msg);
    proc(msg);
    printf("After proc\n%s\nFIM", msg);
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

I didn’t release the memory with free() because in theory would use elsewhere.

I didn’t check if the allocation worked, which makes the code less robust. The normal is to do this with malloc() and realloc().

But in certain contexts none of this is the right and accurate solution quite different from this, especially in embedded environments.

  • Thank you, there is some example or reference of how to do this in embedded systems?

  • Depends on what you need to do.

  • I want to handle what comes through the serial but I don’t know the size of what comes in the serial, so I’m making dynamic array.

  • You need to create a buffer, but it seems to me that it is something advanced for your knowledge and depends on various things to decide how to do.

  • If I get a treixo of code I’ll study, but for now thank.

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