add mmaptest.c

Author: roman10

mmaptest.c

@@ -0,0 +1,52 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <errno.h>
+
+int main(int argc, char *argv[])
+{
+    int fd, offset;
+    char *data;
+    struct stat sbuf;
+	FILE *f;
+
+    if (argc != 2) {
+        fprintf(stderr, "usage: mmapdemo offset\n");
+        exit(1);
+    }
+
+	f = fopen("./h1_1280_720_5m.mp4_mbstpos_gop1.txt", "w");
+	fprintf(f, "sssssssssssssssssssssssssss");
+	fflush(f);
+	//this line will cause no such device error
+	printf("fd: %d\n", fd);
+	if (fd = open("./h1_1280_720_5m.mp4_mbstpos_gop1.txt", O_RDONLY) == -1) {			//line caused bug
+    //if ((fd = open("./h1_1280_720_5m.mp4_mbstpos_gop1.txt", O_RDONLY)) == -1) {			//correct source code
+        perror("open");
+        exit(1);
+    }
+	printf("fd: %d\n", fd);
+    if (stat("./h1_1280_720_5m.mp4_mbstpos_gop1.txt", &sbuf) == -1) {
+        perror("stat");
+        exit(1);
+    }
+	printf("file size: %ld", sbuf.st_size);
+    offset = atoi(argv[1]);
+    if (offset < 0 || offset > sbuf.st_size-1) {
+        fprintf(stderr, "mmapdemo: offset must be in the range 0-%ld\n", sbuf.st_size-1);
+        exit(1);
+    }
+    
+    data = mmap(0, sbuf.st_size, PROT_READ, MAP_SHARED, fd, 0)	;
+    if (data == (-1)) {
+        perror("mmap");
+        exit(1);
+    }
+
+    printf("byte at offset %d is '%c'\n", offset, data[offset]);
+
+    return 0;
+}