Dynamic reallocation an array of structures with malloc and realloc

Question

I am really having a difficult time trying to create an array of structures using malloc and realloc. I have posted mostly the entire code base, or at least the relevant information pertaining to the question below.

struct _section {
    char *sectName;
    int start_addr;
    int end_addr;
    char *bytes;
};

struct _section *get_exe_sections(struct _section *exe_sect, Elf *elf, GElf_Ehdr *ehdr, GElf_Shdr *shdr, Elf_Data *data) { 
    exe_sect->sectName = elf_strptr(elf, ehdr->e_shstrndx, shdr->sh_name);
    exe_sect->start_addr = shdr->sh_addr;
    exe_sect->end_addr = shdr->sh_addr + shdr->sh_size;
    exe_sect->bytes = (unsigned char *)data->d_buf;

    return exe_sect;  
}

int main(int argc, char *argv[]) {
    Elf *elf;
    int fd;

    //process input file
    int sections_count = count_sections(elf);
    GElf_Ehdr ehdr_mem;
    GElf_Ehdr *ehdr = gelf_getehdr(elf, &ehdr_mem);

    struct _section *exe_sect = (struct _section *)malloc(sizeof(struct _section));
    for(int cnt = 0; cnt < sections_count; cnt++) {
        Elf_Scn *scn = elf_getscn(elf, (size_t)cnt);
        GElf_Shdr shdr_mem;
        GElf_Shdr *shdr = gelf_getshdr(scn, &shdr_mem);
        Elf_Data *data = elf_getdata(scn, NULL);

        if(ehdr == NULL || shdr == NULL)
            exit(1);

        if(strcmp(header_name(SECT_TYPE, GELF_ST_TYPE(shdr->sh_type)), "PROGBITS") == 0) {
            if(strcmp(flag_name(SECT_FLAGS, shdr->sh_flags), "ALLOC & EXECUTE") == 0 || \
                strcmp(flag_name(SECT_FLAGS, shdr->sh_flags), "EXECUTE") == 0) {

                exe_sect = get_exe_sections(exe_sect, elf, ehdr, shdr, data);
                struct _section *nxt_sect = (struct _section *)realloc(exe_sect, 2*sizeof(*exe_sect));
                if(nxt_sect != NULL)
                    exe_sect = nxt_sect;
            }
        }
    }
    return 0;
}

What I am having trouble with is creating an array of structures dynamically and using malloc and realloc to resize the structure to fit more data into it. If I were to place a handful of print statements are the bottom of main the output will give me the last data that was input to the structure. How would I go about accessing individual entries that were made during each call to get_exe_section ? From prior posts and other resources I thought this would work, but I cannot create an array in this manner. Any form of help would be great. Thanks.

Answer 1

You could add another element in your struct that points to the next section. This way you could create a linked list

struct _section {
    char *sectName;
    int start_addr;
    int end_addr;
    char *bytes;
    struct _section *next; // pointer to next section
};

You could use another Struct to point to the head of your list.

and then instead of using realloc . you could just do

exe_sect->next = (struct _section *)malloc(sizeof(struct _section));

Here is how I would change the main function :

int main(int argc, char *argv[]) {
    Elf *elf;
    int fd;

    //process input file
    int sections_count = count_sections(elf);
    GElf_Ehdr ehdr_mem;
    GElf_Ehdr *ehdr = gelf_getehdr(elf, &ehdr_mem);

    struct _section *exe_sect = (struct _section *)malloc(sizeof(struct _section));

    for(int cnt = 0; cnt < sections_count; cnt++) {
        Elf_Scn *scn = elf_getscn(elf, (size_t)cnt);
        GElf_Shdr shdr_mem;
        GElf_Shdr *shdr = gelf_getshdr(scn, &shdr_mem);
        Elf_Data *data = elf_getdata(scn, NULL);

        if(ehdr == NULL || shdr == NULL)
            exit(1);

        if(strcmp(header_name(SECT_TYPE, GELF_ST_TYPE(shdr->sh_type)), "PROGBITS") == 0) {
            if(strcmp(flag_name(SECT_FLAGS, shdr->sh_flags), "ALLOC & EXECUTE") == 0 || \
                strcmp(flag_name(SECT_FLAGS, shdr->sh_flags), "EXECUTE") == 0) {

                exe_sect = get_exe_sections(exe_sect, elf, ehdr, shdr, data);
                exe_sect->next = (struct _section *)malloc(sizeof(struct _section));

                if(exe_sect->next != NULL)
                    exe_sect = exe_sect->next;
            }
        }
    }
    return 0;

PS : To access all the individual entries, add another struct that consists of a pointer to the head of the list, and a variable that keeps count.