Segmentation error handling strcmp

I have the following code:

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

#define RCVBUFSIZE 1024

void insertarOperacion(char *op){
        char operacion[RCVBUFSIZE], *retrString = "RETR "; //<
        bool operacionError = false;

        printf("Para solicitar un fichero escriba get <nombreFechero>, para salir escriba quit\n");
        do{
            if(operacionError)
                printf("Opcion invalida\n");
            printf("Operacion: ");
            fgets(operacion, sizeof(operacion), stdin);
            operacion[strlen(operacion) - 1] = '\0';
            operacionError = true; printf("000000000\n");
        } while( ((strcmp(operacion, "quit") == 0) + (strncmp(operacion, "get", 3) == 0)) != 1);
        printf("111111111\n");
        if (strcmp(operacion, "quit") == 0){
            strcpy(operacion, "QUIT\r\n");
            operacion[strlen(operacion) - 1] = '\0';
            strcpy(op, operacion);
        }
        else if(strncmp(operacion, "get",3) == 0){printf("DALEEEKLAJDSFK");
            strcat(retrString, operacion + 4);printf("33333333333\n");
            strcat(retrString, "\r\n"); //>
            retrString[strlen(retrString) - 1] = '\0';
            printf("333333333334\n");
            strcpy(op, retrString);
            printf("333333333335\n");
        } printf("PORFI?");
    //  send(clientSock, retrString, RCVBUFSIZE, 0);
}
int main(){

    //int i = 10, j=20;
    char jiji[RCVBUFSIZE] = "lkajsdfkjdsf";
    insertarOperacion(jiji);
    printf("%s\n\n", jiji);
    insertarOperacion(jiji);
    printf("%s", jiji);



return 0;
}

The problem is that the "quit" section works perfectly. The "get" section does not work. If I input for example: "get rekt.txt". I got the following output:

000000000

111111111

Segmentation fault (core dumped)

This:

char *retrString = "RETR ";

is a string literal, which means that the memory this pointer points to is read-only, thus it can not be modified/written.

Your "get" section violates exactly the above, when it tries to modify that string literal:

strcat(retrString, operacion + 4);
strcat(retrString, "\r\n"); //>
retrString[strlen(retrString) - 1] = '\0';

This results to a segmentation fault.

In order to get around this issue, just declare the char pointer, then allocate space for it, populate it and then modify it at will.

Example:

char* retrString = NULL;
retString = malloc(sizeof("RETR ") * sizeof(char) + 1); // +1 for the null terminator
strcpy(retrString, "RETR ");
// do your get thing then
strcat(retrString, operacion + 4);
strcat(retrString, "\r\n"); //>
retrString[strlen(retrString) - 1] = '\0';