isequal: Add options for better handling

--binary: handled binary files
--empty:  check if file is empty

--skipleft=<n>: Skip <n> lines at the start of the left (first) file
--skipright=<n>: Skip <n> lines at the start of the right (second) file

Note that --binary, --empty and one or both of --skipXXX are mutual
exclusive; that is, you cannot specify more than one of them at the same
time, or the behaviour will be unpredictable.

test/isequal.c

@@ -3,6 +3,97 @@
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
+
+static int binary = 0;
+static int empty = 0;
+static int skiplines_left = 0;
+static int skiplines_right = 0;
+static char * filename_left = 0;
+static char * filename_right = 0;
+
+int handleargparameter(int offset, char * parameter)
+{
+    long number = -1;
+    char * endptr = NULL;
+
+    if (parameter[offset++] != '=') {
+        return -1;
+    }
+
+    number = strtol(parameter + offset, &endptr, 10);
+
+    if (endptr == NULL || *endptr != 0) {
+        return -1;
+
+    }
+    return number;
+}
+
+int handleparameter(int argc, char *argv[])
+{
+    static const char opt_binary[]    = "--binary";
+    static const char opt_empty[]     = "--empty";
+    static const char opt_skipleft[]  = "--skipleft";
+    static const char opt_skipright[] = "--skipright";
+
+    static const char len_skipleft    = sizeof opt_skipleft - 1;
+    static const char len_skipright   = sizeof opt_skipright - 1;
+
+    int argindex = 1;
+
+    if (argc < 2) {
+        return -1;
+    }
+
+    while (argindex < argc && argv[argindex] && argv[argindex][0] == '-') {
+        if (strcmp(argv[argindex], opt_binary) == 0) {
+            if (empty || skiplines_left || skiplines_right) {
+                fprintf(stderr, "--empty cannot go with other options.\n");
+                exit(1);
+            }
+            binary = 1;
+        }
+        else if (strcmp(argv[argindex], opt_empty) == 0) {
+            if (binary || skiplines_left || skiplines_right) {
+                fprintf(stderr, "--binary cannot go with other options.\n");
+                exit(1);
+            }
+            empty = 1;
+        }
+        else if (strncmp(argv[argindex], opt_skipleft, len_skipleft) == 0) {
+            if (binary || empty) {
+                fprintf(stderr, "%s cannot go with other options.\n", opt_skipleft);
+                exit(1);
+            }
+            skiplines_left = handleargparameter(len_skipleft, argv[argindex]);
+            if (skiplines_left < 0) {
+                fprintf(stderr, "%s: you must specify the number of lines\n", opt_skipleft);
+            }
+        }
+        else if (strncmp(argv[argindex], opt_skipright, len_skipright) == 0) {
+            if (binary || empty) {
+                fprintf(stderr, "%s cannot go with other options.\n", opt_skipright);
+                exit(1);
+            }
+            skiplines_right = handleargparameter(len_skipright, argv[argindex]);
+            if (skiplines_right < 0) {
+                fprintf(stderr, "%s: you must specify the number of lines\n", opt_skipright);
+            }
+        }
+        ++argindex;
+    }
+
+    if (argc + empty - argindex != 2) {
+        fprintf(stderr, "filenames are missing!\n");
+        return -1;
+    }
+
+    filename_left  = argv[argindex++];
+    filename_right = argv[argindex++];
+
+    return 0;
+}
 
 /* get the next character from FILE and convert commonly used line-endings all
    into the same value (0x0a, as used on *nix systems)
@@ -17,7 +108,7 @@
 int getnext(FILE *f)
 {
     int c = fgetc(f);
-    if (c == 0x0d) {
+    if (!binary && c == 0x0d) {
         if (!feof(f)) {
             int n = fgetc(f);
             if (n != 0x0a) {
@@ -30,25 +121,65 @@ int getnext(FILE *f)
     return c;
 }
 
+void skiplines(FILE *f, int skipcount)
+{
+   int c;
+
+   while (skipcount > 0) {
+       c = getnext(f);
+       if (feof(f)) {
+           return;
+       }
+
+       if (c == 0x0a) {
+           --skipcount;
+       }
+   }
+}
+
 int main(int argc, char *argv[])
 {
     FILE *f1, *f2;
-    if (argc < 3) {
+
+    if (handleparameter(argc, argv) < 0) {
         return EXIT_FAILURE;
     }
-    f1 = fopen(argv[1], "rb");
-    f2 = fopen(argv[2], "rb");
-    if ((f1 == NULL) || (f2 == NULL)) {
+
+    f1 = fopen(filename_left, "rb");
+    if (f1 == NULL) {
         return EXIT_FAILURE;
     }
-    for(;;) {
-        if (feof(f1) && feof(f2)) {
-            return EXIT_SUCCESS;
-        } else if (feof(f1) || feof(f2)) {
+
+    if (empty) {
+        fseek(f1, 0, SEEK_END);
+        if (ftell(f1) != 0) {
             return EXIT_FAILURE;
         }
-        if (getnext(f1) != getnext(f2)) {
+        else {
+            return EXIT_SUCCESS;
+        }
+    }
+    else {
+        if (skiplines_left) {
+            skiplines(f1, skiplines_left);
+        }
+
+        f2 = fopen(filename_right, "rb");
+        if (f2 == NULL) {
             return EXIT_FAILURE;
         }
+        if (skiplines_right) {
+            skiplines(f2, skiplines_right);
+        }
+        for(;;) {
+            if (feof(f1) && feof(f2)) {
+                return EXIT_SUCCESS;
+            } else if (feof(f1) || feof(f2)) {
+                return EXIT_FAILURE;
+            }
+            if (getnext(f1) != getnext(f2)) {
+                return EXIT_FAILURE;
+            }
+        }
     }
 }