Manipulating structs with a void function in C - c

Question

so I've been set a task of creating a faux string struct and implementing all the usual string functions on my faux string struct. I'm stuck on the tests of my strcat implementation called append, with the first test failing (segfault) being the 5th line. My function for creating new structs should be OK because it passed all the tests, but I've included it just incase.
I've already been able to successfully implement length, get, set and copy functions for my faux string structs.
The struct:
struct text {
int capacity;
char *content;
};
typedef struct text text;
My function for creating new structs:
text *newText(char *s) {
printf("new Text from %s\n", s);
int sizeNeeded = (strlen(s)+1);
int sizeGot = 24;
while (sizeNeeded > sizeGot) {
sizeGot = sizeGot * 2;
}
text *out = malloc(sizeGot);
char *c = malloc(sizeGot);
strcpy(c, s);
out->content = c;
out->capacity = (sizeGot);
printf("the capacity is %d\n", sizeGot);
return out;
free(c);
}
My append function:
void append(text *t1, text *t2) {
printf("t1 content is %s, t2 content is %d\n", t1->content, *t2->content);
int sizeNeeded = (t1->capacity + t2->capacity);
int sizeGot = 24;
while (sizeNeeded > sizeGot) {
sizeGot = sizeGot * 2;
}
char *stringy = calloc(sizeGot, 32);
stringy = strcat(t1->content, t2->content);
free(t1);
t1 = newText(stringy);
}
and finally the tests:
void testAppend() {
text *t = newText("car");
text *t2 = newText("pet");
append(t, t2);
assert(like(t, "carpet"));
assert(t->capacity == 24);
text *t3 = newText("789012345678901234");
append(t, t3);
assert(like(t, "carpet789012345678901234"));
assert(t->capacity == 48);
freeText(t);
freeText(t2);
freeText(t3);
}

Answer 1

You are allocating memory in the wrong way. You could fix this by using a flexible array member like this:
typedef struct {
int capacity;
char content[];
} text;
text *out = malloc(sizeof(text) + sizeof(something));
strcpy(out->content, str);
...
And obviously code such as this is nonsense:
return out;
free(c);
}
Enable compiler warnings and listen to them.

Answer 2

Och, some errors you have:
Inside text_new you allocate memory for text *out using text *out = malloc(sizeGot); when sizeGot = 24 is a constant value. You should allocate sizeof(*out) or sizeof(text) bytes of memory for it.
I don't know what for int sizeGot = 24; while (sizeNeeded > sizeGot) the loop inside text_new and append is for. I guess the intention is to do allocations in power of 24. Also it mostly looks like the same code is in both functions, it does look like code duplication, which is a bad thing.
Inside append You pass a pointer to t1, not a double pointer, so if you modify the t1 pointer itself the modification will not be visible outside of function scope. t1 = newText(stringy); is just pointless and leaks memory. You could void append(text **t1, text *t2) and then *t1 = newText(stringy). But you can use a way better approach using realloc - I would expect append to "append" the string, not to create a new object. So first resize the buffer using realloc then strcat(&t1->content[oldcapacity - 1], string_to_copy_into_t1).
int sizeNeeded = (t1->capacity + t2->capacity); is off. You allocate capacity in power of 24, which does not really interact with string length. You need to have strlen(t1->content) + strlen(t2->content) + 1 bytes for both strings and the null terminator.
Try this:
size_t text_newsize(size_t sizeNeeded)
{
// I think this is just `return 24 << (sizeNeeded / 24);`, but not sure
int sizeGot = 24;
while (sizeNeeded > sizeGot) {
sizeGot *= 2;
}
return sizeGot;
}
text *newText(char *s) {
printf("new Text from %s\n", s);
if (s == NULL) return NULL;
int sizeNeeded = strlen(s) + 1;
int sizeGot = text_newsize(sizeNeeded);
text *out = malloc(sizeof(*out));
if (out == NULL) {
return NULL;
}
out->content = malloc(sizeGot);
if (out->content == NULL) {
free(out);
return NULL;
}
strcpy(out->content, s);
out->capacity = sizeGot;
printf("the capacity is %d\n", sizeGot);
return out;
}
and this:
int append(text *t1, text *t2) {
printf("t1 content is %s, t2 content is %s\n", t1->content, t2->content);
int sizeNeeded = strlen(t1->content) + strlen(t2->content) + 1;
if (t1->capacity < sizeNeeded) {
// this could a text_resize(text*, size_t) function
int sizeGot = text_newsize(sizeNeeded);
void *tmp = realloc(t1->content, sizeGot);
if (tmp == NULL) return -ENOMEM;
t1->content = tmp;
t1->capacity = sizeGot;
}
strcat(t1->content, t2->content);
return 0;
}
Some remarks:
Try to handle errors in your library. If you have a function like void append(text *t1, text *t2) let it be int append(text *t1, text *t2) and return 0 on success and negative number on *alloc errors.
Store the size of everything using size_t type. It's defined in stddef.h and should be used to represent a size of an object. strlen returns size_t and sizeof also returns size_t.
I like to put everything inside a single "namespace", I do that by prepending the functions with a string like text_.
I got some free time and decided to implement your library. Below is the code with a simple text object storing strings, I use 24 magic number as allocation chunk size.
// text.h file
#ifndef TEXT_H_
#define TEXT_H_
#include <stddef.h>
#include <stdbool.h>
struct text;
typedef struct text text;
text *text_new(const char content[]);
void text_free(text *t);
int text_resize(text *t, size_t newsize);
int text_append(text *to, const text *from);
int text_append_mem(text *to, const void *from, size_t from_len);
const char *text_get(const text *t);
int text_append_str(text *to, const char *from);
char *text_get_nonconst(text *t);
size_t text_getCapacity(const text *t);
bool text_equal(const text *t1, const text *t2);
#endif // TEXT_H_
// text.c file
//#include "text.h"
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <assert.h>
struct text {
size_t capacity;
char *content;
};
text *text_new(const char content[])
{
text * const t = malloc(sizeof(*t));
if (t == NULL) goto MALLOC_ERR;
const struct text zero = {
.capacity = 0,
.content = NULL,
};
*t = zero;
if (content != NULL) {
const int ret = text_append_str(t, content);
if (ret) {
goto TEXT_APPEND_ERR;
}
}
return t;
TEXT_APPEND_ERR:
free(t);
MALLOC_ERR:
return NULL;
}
void text_free(text *t)
{
assert(t != NULL);
free(t->content);
free(t);
}
int text_resize(text *t, size_t newcapacity)
{
// printf("%s %d -> %d\n", __func__, t->capacity, newcapacity);
// we resize in chunks
const size_t chunksize = 24;
// clap the capacity into multiple of 24
newcapacity = (newcapacity + chunksize - 1) / chunksize * chunksize;
void * const tmp = realloc(t->content, newcapacity);
if (tmp == NULL) return -ENOMEM;
t->content = tmp;
t->capacity = newcapacity;
return 0;
}
int text_append_mem(text *to, const void *from, size_t from_len)
{
if (to == NULL || from == NULL) return -EINVAL;
if (from_len == 0) return 0;
const size_t oldcapacity = to->capacity == 0 ? 0 : strlen(to->content);
const size_t newcapacity = oldcapacity + from_len + 1;
int ret = text_resize(to, newcapacity);
if (ret) return ret;
memcpy(&to->content[newcapacity - from_len - 1], from, from_len);
to->content[newcapacity - 1] = '\0';
return 0;
}
int text_append_str(text *to, const char *from)
{
if (to == NULL || from == NULL) return -EINVAL;
return text_append_mem(to, from, strlen(from));
}
int text_append(text *to, const text *from)
{
if (to == NULL || from == NULL) return -EINVAL;
if (text_getCapacity(from) == 0) return 0;
return text_append_str(to, text_get(from));
}
const char *text_get(const text *t)
{
return t->content;
}
const size_t text_strlen(const text *t)
{
return t->capacity == 0 ? 0 : strlen(t->content);
}
size_t text_getCapacity(const text *t)
{
return t->capacity;
}
bool text_equal_str(const text *t, const char *str)
{
assert(t != NULL);
if (str == NULL && t->capacity == 0) return true;
const size_t strlength = strlen(str);
const size_t t_strlen = text_strlen(t);
if (t_strlen != strlength) return false;
if (memcmp(text_get(t), str, strlength) != 0) return false;
return true;
}
// main.c file
#include <stdio.h>
int text_testAppend(void) {
text *t = text_new("car");
if (t == NULL) return -1;
text *t2 = text_new("pet");
if (t2 == NULL) return -1;
if (text_append(t, t2)) return -1;
assert(text_equal_str(t, "carpet"));
assert(text_getCapacity(t) == 24);
text *t3 = text_new("789012345678901234");
if (t3 == NULL) return -1;
if (text_append(t, t3)) return -1;
assert(text_equal_str(t, "carpet789012345678901234"));
assert(text_getCapacity(t) == 48);
text_free(t);
text_free(t2);
text_free(t3);
return 0;
}
int main()
{
text *t1 = text_new("abc");
text_append_str(t1, "def");
printf("%s\n", text_get(t1));
text_free(t1);
printf("text_testAppend = %d\n", text_testAppend());
return 0;
}