fix file access issue

jcl/TXTWKR01.jcl

@@ -1,8 +1,7 @@
 //TXTWKR01   JOB (GCC),'File I/O Example',
-//             NOTIFY=@05054,CLASS=A,MSGCLASS=H,
+//             NOTIFY=@05054,CLASS=A,MSGCLASS=A,
 //             MSGLEVEL=(1,1),REGION=4M,TIME=1440
 //* Compile and Go Step
 //STEP1 EXEC GCCCG,INFILE='@05054.SRCLIB.C(TXTWKR01)'
-//GO.TESTDD DD DSN=@05054.DATA(TESTMEM),DISP=OLD
-//
-
+//GO.TESTDD DD DSN=@05054.DATA(TESTMEM),DISP=SHR
+//

src/TXTWKR01.c

@@ -14,10 +14,11 @@ int main()
 
     /* Open the file in update mode (read and write). */
     /* The filename is "dd:DDNAME" where DDNAME is defined in JCL. */
-    fp = fopen("dd:TESTDD", "r+");
+    fp = fopen("dd:TESTDD", "a+");
     if (fp == NULL)
     {
-        perror("fopen failed. Insure the member exists.");
+        perror("fopen failed.");
+        printf("errno = %d\n", errno);
         return 1;
     }