implement relf file check function to get ELF type

src/relf/relf.c

@@ -10,6 +10,39 @@
 #include <stdlib.h>
 #include <errno.h>
 
+int relf_file_check(const char* path)
+{
+    uint8_t e_ident[EI_NIDENT] = {0};
+
+    int fd = open(path, O_RDONLY);
+    if (fd < 0) return -1;
+
+    if (read(fd, e_ident, EI_NIDENT) < EI_NIDENT)
+    {
+        close(fd);
+        return -1;
+    }
+
+    if (!memcmp(e_ident, ELFMAG, sizeof(ELFMAG)))
+    {
+        close(fd);
+        return -1;
+    }
+
+    // now we need to read ELF type
+    // e_type comes right after e_ident in ELF header
+    uint16_t e_type;
+    if (read(fd, &e_type, sizeof(e_type)) < sizeof(e_type))
+    {
+        close(fd);
+        return -1;
+    }
+
+    close(fd);
+    // return ELF type
+    return e_type;
+}
+
 // returns 1 if size not suitable for 32 bit host
 static int check_32bit_limit(off_t st_size)
 {

src/relf/relf.h

@@ -4,12 +4,11 @@
 #include <stdint.h>
 #include <stddef.h>
 
-// composite error type
 typedef enum {
     RELF_MMAP_FAILED    = -5,   // file memory mapping failed
     RELF_UNSUPPORTED    = -4,   // big endian or not x86/x86-64 architecture
-    RELF_NOT_AN_ELF     = -3,   // wrong magic
-    RELF_TOO_BIG        = -2,   // file is over size_t limit
+    RELF_TOO_BIG        = -3,   // file is over size_t limit
+    RELF_NOT_AN_ELF     = -2,   // wrong magic
     RELF_FAILED_OPEN    = -1,   // failed to stat or open file
     RELF_OK             = 0,
 } relf_error_t;
@@ -93,6 +92,13 @@ typedef struct {
     unsigned symbol_num;
 } relf_t;
 
+// tries to open ELF file, checks its magic and if everything ok
+// returns ELF type. In case of error, return -1
+// if "e_ident" is non-null, e_ident of ELF would be saved here,
+// in that case "e_ident" must provide at least EI_NIDENT bytes size place,
+// if "fd" is non-null, file won't be closed and file descriptor would be placed here
+int relf_file_check(const char* path);
+
 // opens ELF file, checks ELF magic and maps it into memory
 // may load additional info like string table
 relf_error_t relf_open(relf_t* relf, const char* path);

src/relf_test/relf_test.c

@@ -4,6 +4,8 @@
 
 int main(int argc, char** argv)
 {
+    printf("relf_file_check %u\n", relf_file_check(argv[1]));
+
     relf_t dummy;
     relf_open(&dummy, argv[1]);