slice: extract a part from a file

1 files changed, 74 insertions, 0 deletions

diff --git a/slice.c b/slice.c
new file mode 100644
index 0000000..ec4abd2
--- /dev/null
+++ b/slice.c
@@ -0,0 +1,74 @@
+/*
+ * Extract a part from a file.
+ *
+ * Author: Peter Wu <lekensteyn@gmail.com>
+ * Date: 2012-11-01
+ */
+
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+int main(int argc, char **argv) {
+	if (argc < 4) {
+		fprintf(stderr, "Usage: %s file offset last-byte|+length\n", *argv);
+		return 1;
+	}
+	char *filename = argv[1];
+	unsigned long seekpos = strtoul(argv[2], NULL, 0);
+	unsigned long bytes_end = strtoul(argv[3], NULL, 0);
+
+	if (*argv[3] == '+') {
+		if (bytes_end + seekpos < bytes_end) {
+			fprintf(stderr, "Integer overflow on the last byte\n");
+			return 1;
+		}
+		bytes_end += seekpos;
+	}
+
+	if (bytes_end <= seekpos) {
+		fprintf(stderr, "The last byte must be greater than the first byte.\n");
+		return 1;
+	}
+
+	FILE *fp;
+	if (filename[0] == '-' && filename[1] == 0)
+		fp = stdin;
+	else
+		fp = fopen(filename, "r");
+	if (!fp) {
+		perror(filename);
+		return 1;
+	}
+
+	unsigned long read_bytes = 0;
+	if (!fseek(fp, seekpos, SEEK_SET))
+		read_bytes += seekpos;
+
+	do {
+		char buf[8 * 1024];
+		unsigned int read_count = sizeof(buf);
+		unsigned long skip_bytes = 0;
+		if (bytes_end - read_bytes < read_count)
+			read_count = bytes_end - read_bytes;
+
+		if (read_bytes < seekpos)
+			skip_bytes = seekpos - read_bytes;
+
+		unsigned long n = fread(buf, 1, read_count, fp);
+		if (!n)
+			break; /* EOF? */
+		if (n > skip_bytes)
+			write(STDOUT_FILENO, buf + skip_bytes, n);
+		read_bytes += n;
+	} while (read_bytes < bytes_end);
+
+	if (read_bytes != bytes_end)
+		fprintf(stderr, "Output is truncated, missing %lu bytes.\n",
+				bytes_end - read_bytes);
+
+	fclose(fp);
+
+	return 0;
+}