I'm trying to build a 2 dimensional array by using str.cpy, but the program fails. The code receives 4 arrays add copies their content to their matching arrays.
int InsertStudent(char *firstName, char* lastName, char* dynCourses, char *dynGrades,
char firstNames[50][20],
char familyNames[50][20], char courses[50][5][20],
char grades[50][5])
{
int set,
int cset = 0;
for (set = 0; set <= (50); set++)
{
if (firstNames[set][cset] == '\0')
{
strcpy(firstNames[set][cset], firstName);
strcpy(familyNames[set], lastName);
for (cset = 0; cset <= 5; cset++)
{
strcpy(courses[set], dynCourses);
strcpy(grades[set], dynGrades);
}
}
}
return 0;
}
Well clearly the error is using strcpy wrongly.
The correct way would be
strcpy(firstNames[set], firstName);
Also in the loop it should be
for (cset = 0; cset < MAX_COURSES; cset++)
{
strcpy(courses[cset], dynCourses);
strcpy(grades[cset], dynGrades);
}
Note that the idiomatic C loop is for (int i = 0; i < MAX; i++), using < and not <=.
The signature of the strcpy function is
char *strcpy(char * restrict s1, const char * restrict s2);
Earlier you passed in place of s1 a char instead of char*. You must have got some warning (if enabled). If not then turn all compiler flags -Wall -Werror.
if (firstNames[set][cset] == '\0')
But if you are initially checking an uninitilized value with \0. This will higly unlikely will turn out to be false. There is no gurantee that the char array which doesn't contain strings will be initialized with 0 automatically. So make sure you have initialized the char arrays in the callee function like this
char arr[20][50]={0};
The loop is from 0 to MAX_STUDENTS. You are invoking an undefined behavior looping on array index out of bound if MAX_STUDENTS is greater than or equal to 50. Same goes for MAX_COURSES. More clearly the looping would be for (set = 0; set < (50); set++).
Again there will be a lot better way to deal with it if you put the initialization and copy part seperate. Otherwise this will be uneasy to maintain.
Seeing your use courses it is obvious that you want to declare it like this
char courses[5][20];
same goes for the grades array. Here you were trying to copy a string into into a 2d array. Compiler would complain about type incompatibility.
Also in the function you didn't return anything meaningful. The correct way would be to return the index value on which new name or information is added.
Here you are copying content of dynGrades and dynCourses to the array. So they will all contain the same values. Is this what you want? Because then what's the use of keeping 5 seperate char arrays - one could serve the purpose pretty well.
Related
I'm working on a minisql code in C and i having some issues to allocate array of strings. I made a function called "alocaString" to do this (bc i'm using that a lot), but i don't think is working.
When the code reaches the line "strncpy(lista[qtnPalavras], splitStr, 100);" in the function "listaPalavras" (that have the purpose of split a string in different types of characters) a file named "strcpy-avx2.S" is created, one of the arguments of that function (**lista) is allocated with "alocaString" so i think the problem is in that function.
I already try to use valgrind and shows "is used uninitialized in this function [-Werror=uninitialized]" to all arrays of strings that i tried to use on that function, but i'm initializing them inside of the function
int alocaString (char **string, int tamanho, int posicoes){
string = malloc (posicoes * sizeof(char*));
for (int i = 0; i < posicoes; i++){
string [i] = malloc (tamanho * sizeof(char));
if (string[i] == NULL){return 0;}
}
return **string;
}
void desalocaString (char **string, int posicoes){
for (int i = 0; i < (posicoes); i++){
free (string[i]);
}
free (string);
}
int listaPalavras(char *entrada, char **lista, char *separador){ // lista as palavras
char *splitStr;
int qtnPalavras = 0;
splitStr = strtok(entrada, separador);
while (splitStr != NULL){
strncpy(lista[qtnPalavras], splitStr, 100);
qtnPalavras++;
splitStr = strtok(NULL, separador);
}
return qtnPalavras;
}
I assume that you are using these functions like this:
alocaString(lista, tamanho, posicoes);
listaPalavras(some_string, lista, some_delimiters);
desalocaString(arr);
Even without looking at the code, it seems logically wrong to allocate an array of strings first and then populate it if you do not already know how many strings it will need to fit. If you happen to allocate an array of n strings, but your listaPalavras() functions splits the provided string into n+1 or more substrings, you're going to overflow your previously allocated array. Nonetheless, this can be done taking the appropriate precautions, like carrying around sizes and checking them to avoid overflow.
The only sane way to achieve what you want is therefore to either (A) count the number of delimiters in the string first to know in advantage how many pointers you will need or (B) dynamically allocate the needed amount in listaPalavras() while splitting. You seem to be going with something similar to option A, but your code is flawed.
The desalocaString() is the only function that seems correct.
A correct implementation of alocaString() would return the allocated array (or NULL in case of failure), but you are returning **string which is just the first character of the first string. Needless to say, this does not make much sense. You don't need to take a char ** parameter, just the sizes. Secondly, in case of failure of any of the calls to malloc() you should free the previously allocated ones before returning NULL.
char **alocaString (unsigned tamanho, unsigned posicoes) {
char **lista = malloc(posicoes * sizeof(char*));
if (lista == NULL)
return NULL;
for (unsigned i = 0; i < posicoes; i++) {
lista[i] = malloc(tamanho * sizeof(char));
if (lista[i] == NULL) {
for (unsigned j = 0; j < i; j++)
free(lista[j]);
free(lista);
return NULL;
}
}
return lista;
}
As per listaPalavras(), which has the job of splitting the given string into other strings and copying them into the previously allocated array, to avoid overflowing the given array of strings you will need to also provide its length as well as the length of the previously allocated strings as argument (let's call them posicoes and tamanho like for the above function). Moreover, strncpy() will not add a NUL-terminator (\0) to the destination string if it is not found in the source string within the first n characters (n being the third argument), so you will need to add it yourself to make sure your strings are correctly terminated.
unsigned listaPalavras(const char *entrada, char *separador, char **lista, unsigned posicoes, unsigned tamanho) {
char *splitStr;
unsigned qtnPalavras = 0;
splitStr = strtok(entrada, separador);
while (qtnPalavras < posicoes && splitStr != NULL){
strncpy(lista[qtnPalavras], splitStr, tamanho);
lista[qtnPalavras][tamanho - 1] = '\0';
qtnPalavras++;
splitStr = strtok(NULL, separador);
}
return qtnPalavras;
}
Finally the code of the caller should look something like this:
char **lista;
unsigned tamanho = 100;
unsigned posicoes = 10;
unsigned palavras;
lista = alocaString(tamanho, posicoes);
if (lista == NULL) {
// handle the error somehow
}
palavras = listaPalavras(YOUR_STRING, YOUR_DELIMITERS, lista, posicoes, tamanho);
desalocaString(lista);
This should work fine, however you are limited by the fact that:
You cannot know beforehand the number of substrings that strtok() will find.
You cannot know beforehand the length of any of those substrings.
Therefore, allocating the needed lista dynamically inside listaPalavras() would make more sense.
Finally, as a side note, the names of your functions are misleading: if you need to allocate an array of strings, you might want to choose a better name than alocaString() which seems to imply that you are allocating a single string. Maybe alocaLista() and dealocaLista() would be better choices.
I have one char strings pointers array, lets call it "str_array".
I also have 3 strings:
1."Hello"
2."World"
3."Today"
(Every string ends with \0)
And I have this function, that receives our str_array, the size of it, and another pointers arr that is not releveant for my question.
The problem that I encounter, is that function "scanString" receives NULL, or garbage values, instead of the strings inside str_array.
unsigned int RemoveFromStrArray(char*** str_array, unsigned int str_array_size, char** ptr_to_chars_array)
{
int k = 0;
while (k != str_array_size)
{
// This is the part where im trying to scan str_array strings.
scanString(*str_array[k]);
str_array++;
k++;
}
}
int scanString(char* string)
{
int c = 0;
int counter = 0;
while (string[c] != '\0')
{
if (string[c] == 1)
{
moveOneBack(string, c);
c--;
counter++;
}
c++;
}
return c;
}
I've been trying multiple alternative ways to scan str_array string arrays.
But all of my times I just had to deal with garbage values or NULL strings.
How do I reach str_array strings, that would be passed by reference to scanString?
Picture of what I'm talking about:
BIG THANKS IN ADVANCE!
unsigned int RemoveFromStrArray(char*** str_array
You've got at least one too many indirections there. From your drawing, str_array refers to an array, so it's effectively a pointer, and the values in the array are pointers to char. You don't seem to be changing the array itself, e.g. you're not making str_array refer to some other array of strings, so there's no need to pass the address of the array. So char ** is closer to the type you want.
str_array++;
I'm not sure why you're incrementing str_array and using k as an index into the array. Although it's safe to modify parameters locally like that, it's nice to avoid it so that you can refer to them while debugging. Consider this:
unsigned int RemoveFromStrArray(char** str_array,
unsigned int str_array_size)
{
int k = 0;
while (k != str_array_size)
{
char *string = str_array[k];
scanString(string);
k++;
}
}
That is, the values in str_array are of type char *, so str_array[k] has that type. Copying it into a temporary variable string makes it a little easier to see what's going on. (I removed the last parameter, ptr_to_chars_array, to keep things simple and because it's not used. You'll add it back if you have plans for it, I'm sure.)
A for loop would be a little more compact but otherwise equivalent:
unsigned int RemoveFromStrArray(char** str_array,
unsigned int str_array_size)
{
for (int k = 0; k < str_array_size; k++)
{
char *string = str_array[k];
scanString(string);
}
}
So I have this code:
char inte[10];
while(j<noinput) {
fscanf(circuit,"%s",inte);
vararray[count]=inte;
count++;
j++;
}
However when I print the contents of the array like this:
for (h=0;h<noinput+2;h++){
printf("%dth variable: %s\n",h,vararray[h]);
}
The elements past the first two (which are reserved for special elements) are all equal to the LAST string that I had taken in from fscanf earlier. I have no idea how one of the strings from fscanf could be equal to multiple slots in the array when I am only setting
vararray[count]=inte;
Shouldn't this mean that each element of the array will be different since I am incrementing count every time? I am so confused. I also tried doing:
fscanf(circuit,"%s",vararray[count]);
But this also did not work and gave me null elements for certain indexes.
you are doing something too wrong. By "vararray[count]=inte;" you are doing pointer assignment so all of your vararray is getting filled by same string. I am guessing you are new to C so I will answer due to that. Correct way would look something like below
Fixed size solution:
char vararray[ROWCOUNT][BUFFERSIZE];
for(count=0; j<noinput; ++count, ++j) {
fscanf(circuit,"%s",(char*)vararray[count]);
}
With dynamic memory management
char * vararray[ROWCOUNT];
for(count=0; j<noinput; ++count, ++j) {
vararray[count] = (char*)malloc(BUFSIZE);
fscanf(circuit,"%s", vararray[count]);
}
I want to warn you in the way of becoming an expert on C nowadays is somewhat madness , i mean unless you have another choice. Examples below I put and the thing you wrote are completely unsafe and unsecure...
You're not copying the string. Here's what's happening:
char *vararray[462]; // declare an array of string pointers
char inte[10]; // declare a char array (to function as a string)
for (int i = 0; i < 462; i += 1) {
// do something
vararray[i] = inte;
}
This is causing all of the items of vararray to point to the memory also referred to as inte... but you're overwriting that each time! Instead, do something like this:
#include <string.h> // write me at the top, outside main()!
char vararray[462][10]; // declare an array of strings (max length 9)
char inte[10]; // declare a char array (to function as a string)
for (int i = 0; i < 462; i += 1) {
fscanf(circuit,"%10s",inte); // use buffer size to make xscanf safer
strncpy(vararray[i], inte, 9); // copy inte, making sure not to buffer overflow!
vararray[i][9] = '\0'; // if inte is too big, a null byte won't be added to the end of the string; make sure that it's there
}
This copies the string! Your problem should go away when you do this.
I'm working on the exercises in K&R's book, and I've run into a weird bug while trying to extend 04-06 to allow for variables with string names. Truthfully, I've actually managed to fix the bug (pretty simple - explained below), but I'd like to know why the error was occuring in the first place.
For those unfamiliar with the problem, you're basically asked to create a command-line calculator (using Polish notation) that can store and recall variables with character names.
Here's the relevant code where the issue occurs:
#define MAXOPLEN 1000
int varCount = 1;
char **keys;
char **values;
// changing the declaration to:
// char strOps[][STROPCOUNT] = { ... };
// fixed the issue
char *strOps[STROPCOUNT] = { "dupe", "swap", "del", "print",
"clr", "sin", "cos", "tan",
"exp", "pow", "ln", "log",
"mem", "re"};
main() {
keys = malloc(varCount * sizeof(char[MAXOPLEN]));
keys[0] = "ans";
values = malloc(varCount * sizeof(char[MAXOPLEN]));
values[0] = "0.0";
... // Other stuff related to the program
}
// flag is unrelated to the problem I'm asking about. It just checks to see
// if the variable name used to store value n is 'ans', which is where
// the last returned value is stored automatically
void memorize(char s[], double n, bool flag) {
... // small conditional block for flag
for (i = 0; i < varCount; i++) {
if (equals(keys[i], s)) {
found = True;
// Next line is where the program actually breaks
snprintf(values[i], MAXOPLEN, "%f", n);
break;
}
}
if (!found) {
i = varCount;
varCount++;
keys = realloc(keys, varCount * sizeof(char*));
keys[i] = malloc(sizeof(char[MAXOPLEN]));
keys[i] = s;
values = realloc(values, varCount * sizeof(char*));
values[i] = malloc(sizeof(char[MAXOPLEN]));
snprintf(values[i], MAXOPLEN, "%f", n);
}
}
After compiling and running, the first time you enter in an equation to calculate, everything seems to run smoothly. However, while debugging, I found out that the first three char* in strOps were oddly made to point to different addresses. When trying to save the return value of the equation to "ans", it enters the for-loop in memorize() that tries to see if string s had been used as a key name already. It correctly finds keys[0] to point to a string matching s's value ("ans"), then attempts to convert double n to a string and save it in values[0].
While inside the snprintf() function, the first three char* in strOps are made to point elsewhere inside this method in corecrt_stdio_config.h:
_Check_return_ _Ret_notnull_
__declspec(noinline) __inline unsigned __int64* __CRTDECL __local_stdio_printf_options(void)
{
// Error occurs after this next line:
static unsigned __int64 _OptionsStorage;
return &_OptionsStorage;
}
As commented in the code above, making strOps a 2D array of characters (rather than an array of char pointers) fixed the issue. This makes sense because arrays of characters can't have the values of individual characters changed, but what I don't understand is why the that method in corecrt_stdio_config.h was changing the values of those three pointers in the first place.
Thanks!
Your initializations are incorrect and are causing the change:
keys[0] = "ans";
values[0] = "0.0";
Both "ans" and "0.0" are string literals and cannot be used to initialize the arrays, you need to use strcpy after you allocate.
strcpy (keys, "ans");
strcpy (values, "0.0");
Your other option is to assign one character at a time:
size_t i;
char *p = "ans";
for (i = 0; i < strlen (p); i++)
keys[i] = p[i]; /* copy to keys */
p[i] = 0; /* nul-terminate */
note: these are examples of your errors, you do the same thing throughout your code.
gcc 4.4.4 c89
warning assignment makes integer from pointer without a cast
**devices = device_buff;
warning: value computed is not used
*devices++;
I get the above warnings with the code below. What I am trying to do is get an input from the user. And assign that char array to an array of pointers. So my array of pointers will contain all the devices entered. However, I am getting a UB on this line:
**devices = device_buff;
Many thanks for any advice,
static void device_input()
{
#define DEVICE_SIZE 80
char device_buff[DEVICE_SIZE] = {0};
char **devices = NULL;
size_t i = 0;
for(i = 0; i < 3; i++) {
printf("Enter device name: ");
fgets(device_buff, (size_t)DEVICE_SIZE, stdin);
**devices = device_buff;
*devices++;
}
/* NULL terminate last element */
*devices = NULL;
printf("Display devices\n");
while(*devices != NULL) {
printf("Device [ %s ]\n", *devices++);
}
}
**devices is a char, device_buff is an array of char. The two types are incompatible.
Even if you fix the compiler errors (as described by others), what you are trying to do won't work. You are calling fgets() on the same device_array each time, so each time it's called, it will overwrite what was stored there previously.
Possible solutions include using multiple character arrays (e.g. char device_buff[3][DEVICE_SIZE]) or one long array, and advancing a pointer each time you call fgets().
You are dereferencing a null pointer. Nothing good can come out of that
char** devices = NULL;
initializes the pointer to NULL. It's never set to anything else and then dereferenced (twice).
Pointers are considered hard and it's rather impossible to use them, if one doesn't understand exactly, what he/she is doing. I think there are two options in your scenario. You can store the names in one char array, one adjacent to another and keep an array of pointers pointing to the beginnings of those names or you can use an array of char arrays (two dimensional array) to store the names "separately" each one in another array. I think the second way is much simpler and you should start from getting it working.
You can define the array like this
#define NUM_OF_NAMES 3
char devices[NUM_OF_NAMES][DEVICE_SIZE] = {0};
now devices[0], devices[1] and devices[2] are all char arrays of type char[DEVICE_SIZE].You can use each of them, like the buffer previously.
You must use dynamic or predefined allocation for your Buffer-ARRAY.
The Endmarker in the example is an empty String not a NULL-Pointer.
#define DEVICE_SIZE 80
typedef char DBuff[DEVICE_SIZE];
static void device_input()
{
#define MAXB 3
DBuff device_buff[MAXB+1];
DBuff *devices=device_buff;
size_t i = 0;
for(i = 0; i < MAXB; i++,devices++) {
printf("Enter device name: ");
fgets(*devices, (size_t)DEVICE_SIZE, stdin);
}
**devices=0;
devices=device_buff;
printf("Display devices\n");
while( **devices ) {
printf("Device [ %s ]\n", *devices++);
}
}