Refactored file reading into finction.

src/common.c

@@ -0,0 +1,53 @@
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include "common.h"
+
+char * read_first_line(const char * filename) {
+    int fd;
+    char *endptr;
+    char *cipherstring;
+
+    // open file
+    fd = open(filename, O_RDONLY, (mode_t)0600);
+    if (fd == -1) {
+        perror("Error opening file");
+        exit(EXIT_FAILURE);
+    }
+    // get length
+    struct stat fileInfo = {0};
+    if (fstat(fd, &fileInfo) == -1) {
+        perror("Error getting the file size");
+        exit(EXIT_FAILURE);
+    }
+    if (fileInfo.st_size == 0) {
+        fprintf(stderr, "Error: File is empty, nothing to do\n");
+        exit(EXIT_FAILURE);
+    }
+    // mmap file
+    char *map = mmap(0, fileInfo.st_size, PROT_READ, MAP_SHARED, fd, 0);
+    if (map == MAP_FAILED)
+    {
+        close(fd);
+        perror("Error mmapping the file");
+        exit(EXIT_FAILURE);
+    }
+    // find first line
+    endptr = strchrnul(map, 0x0a);
+
+    size_t cipherstring_len = endptr - map;
+    cipherstring = malloc(cipherstring_len+1);
+    strncpy(cipherstring, map, cipherstring_len);
+
+    // munmap and close file
+    if (munmap(map, fileInfo.st_size) == -1)
+    {
+        close(fd);
+        perror("Error un-mmapping the file");
+        exit(EXIT_FAILURE);
+    }
+    close(fd);
+
+    return cipherstring;
+}