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The second problem is that you hide the pointer in the typedef. You should not hide pointer, this made the code hard to read. And the user could think that this is not a pointer and try to write StringBuilder *foo;. A answer about this is available on stackoverflow.

A detail about your call of malloc(), you could write struct StringBuilder *ret = malloc(sizeof *ret);, this is more readable and you can read this answer about it. You don't need to use sizeof char because by definition the is equal to 1.

The second problem is that you hide the pointer in the typedef. You should not hide pointer, this made the code hard to read. And the user could think that this is not a pointer and try to write StringBuilder *foo;. A answer about this is available on stackoverflow.

A detail about your call of malloc(), you could write struct StringBuilder *ret = malloc(sizeof *ret);, this is more readable and you can read this answer about it. You don't need to use sizeof char because by definition the is equal to 1.

Next I will talk about your function sb_new(), first I think you should not allocate the structure StringBuilder, this made a lot of overhead. You should separate your function into two sb_new() and sb_init(), one if the user want allocate the structure and one his the user take care of it.

Plus, you use calloc(), this add a lot of useless overhead. You should use malloc(), if you user want to affect the string by zero you should implement an other function for it, like sb_unused_zero().

And, if the user want a zero size string you send NULL, I don't see the point. Maybe you should this value for tell to the function to use a default value like 42.

Next I will talk about your function sb_new(), first I think you should not allocate the structure StringBuilder, this made a lot of overhead. You should separate your function into two sb_new() and sb_init(), one if the user want allocate the structure and one if the user take care of it.

Plus, you use calloc(), this add a lot of useless overhead. You should use malloc(). If the user want to affect the string by zero you should implement an other function for it, like sb_unused_zero().

And, if the user want a zero size string you send NULL, I don't see the point. Maybe you should use this value for tell to the function to use a default value like 42.

A detail about your call of malloc(), you could write struct StringBuilder *ret = malloc(*ret);, this is more readable and you can read this answer about it. You don't need to use sizeof char because by definition the is equal to 1.

struct StringBuilder *sb_new(size_t init_cap) {
 struct StringBuilder *sb = malloc(*sb);
 if (!sb) return NULL;
 if (!sb_init(sb, init_cap)) {
 free(sb);
 return NULL;
 }
 return sb;
}
bool sb_init(struct StringBuilder *sb, size_t init_cap) {
 init_cap = init_cap == 0 ? 42 : init_cap;
 sb->mem = malloc(init_cap);
 if (!sb->mem) return false;
 sb->count = 0;
 sb->cap = init_cap;
 return true;
}
void sb_unused(struct StringBuilder *sb, char c) {
 memset(sb->mem + to->count, c, sb->cap - sb->count);
}

A detail about your call of malloc(), you could write struct StringBuilder *ret = malloc(sizeof *ret);, this is more readable and you can read this answer about it. You don't need to use sizeof char because by definition the is equal to 1.

struct StringBuilder *sb_new(size_t init_cap) {
 struct StringBuilder *sb = malloc(sizeof *sb);
 if (!sb) return NULL;
 if (!sb_init(sb, init_cap)) {
 free(sb);
 return NULL;
 }
 return sb;
}
bool sb_init(struct StringBuilder *sb, size_t init_cap) {
 init_cap = init_cap == 0 ? 42 : init_cap;
 sb->mem = malloc(init_cap);
 if (!sb->mem) return false;
 sb->count = 0;
 sb->cap = init_cap;
 return true;
}
void sb_unused(struct StringBuilder *sb, char c) {
 memset(sb->mem + to->count, c, sb->cap - sb->count);
}