working on ELF identification

src/relf/relf.c

@@ -1 +1,61 @@
 #include "relf/relf.h"
+#include "blackjack/debug.h"
+#include <elf.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <string.h>
+
+relf_value_t relf_open(relf_t* relf, const char* path)
+{
+    // reset struct
+    memset(relf, '\0', sizeof(relf_t));
+
+    // try stat file
+    struct stat st = {0};
+    if (stat(path, &stat))
+        return RELF_ERROR(RELF_FAILED_OPEN);
+    
+    // open file and read ELF header
+    relf->fd = open(path, O_RDONLY);
+    if (relf->fd < 0)
+        return RELF_ERROR(RELF_FAILED_OPEN);
+    
+    union {
+        Elf64_Ehdr hdr64;
+        Elf32_Ehdr hdr32;
+    } e;
+
+    // read biggest value by default
+    if (read(relf->fd, &e.hdr64, sizeof(e.hdr64)) < sizeof(e.hdr64))
+    {
+        close(relf->fd);
+        return RELF_ERROR(RELF_FAILED_OPEN);
+    }
+
+    // check magic and decide ELF type
+    // we operate here ELF64 variant since it same as in ELF32
+    if (!memcmp(e.hdr64.e_ident, ELFMAG, sizeof(ELFMAG)))
+    {
+        // not an ELF file at all
+        close(relf->fd);
+        return RELF_ERROR(RELF_NOT_AN_ELF);
+    }
+
+    // 32 bit or 64 bit
+    switch (e.hdr64.e_ident[EI_CLASS])
+    {
+        case ELFCLASS32: relf->type = RELF_32BIT; break;
+        case ELFCLASS64: relf->type = RELF_64BIT; break;
+        default:
+            close(relf->fd);
+            return RELF_ERROR(RELF_UNSUPPORTED);
+    }
+
+    if (e.hdr64.e_ident[EI_DATA] != ELFDATA2LSB)
+    {
+        // not little endian, we can't work with that
+        close(relf->fd);
+        return RELF_ERROR(RELF_UNSUPPORTED);
+    }
+}

src/relf/relf.h

@@ -1,4 +1,46 @@
 #ifndef __RELF_H
 #define __RELF_H
 
+#include <stdint.h>
+
+// composite error type
+typedef enum {
+    RELF_MMAP_FAILED    = -4,   // file memory mapping failed
+    RELF_UNSUPPORTED    = -3,   // big endian or not x86/x86-64 architecture
+    RELF_NOT_AN_ELF     = -2,   // wrong magic
+    RELF_FAILED_OPEN    = -1,   // failed to stat or open file
+    RELF_OK             = 0,
+} relf_error_t;
+
+typedef union {
+    relf_error_t error;
+
+    uintptr_t value;
+} relf_value_t;
+
+// supply relf_value_t type here
+#define RELF_IS_ERROR(v)    (v.error < 0)
+#define RELF_ERROR(e)       ((relf_value_t) {.error = e})
+
+typedef enum {
+    RELF_64BIT,
+    RELF_32BIT
+} relf_type_t;
+
+// relf instance
+typedef struct {
+    int fd;
+    void* image;
+
+    // is it 64 or 32 bit mode
+    relf_type_t type;
+} relf_t;
+
+// opens ELF file, checks ELF magic and maps it into memory
+// may load additional info like string table
+relf_value_t relf_open(relf_t* relf, const char* path);
+
+// closes mapping and file, frees any allocated memory in relf instance
+void relf_close(relf_t* relf);
+
 #endif