simple program to read/write a text file

jcl/TXTWKR01.jcl

@@ -0,0 +1,7 @@
+//TXTWKR01   JOB (GCC),'File I/O Example',
+//             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
+//

src/TXTWKR01.c

@@ -0,0 +1,40 @@
+#include <stdio.h>
+
+#define BUFFER_SIZE 256
+
+/* Simple program to read and append to a single PDS member via DD name. */
+/* Author: Greg Gauthier */
+/* Date: 2026-02-08 */
+/* NOTE: No single-line comments due to MVS SYSIN/JCL interpretation. */
+
+int main()
+{
+    FILE *fp;
+    char buffer[BUFFER_SIZE];
+
+    /* 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+");
+    if (fp == NULL)
+    {
+        perror("fopen failed");
+        return 1;
+    }
+
+    /* Read and print existing lines (to SYSOUT). */
+    printf("Existing content:\n");
+    while (fgets(buffer, BUFFER_SIZE, fp) != NULL)
+    {
+        printf("%s", buffer);
+    }
+
+    /* After reading to EOF, the file position is at the end, so we can append. */
+    fprintf(fp, "Appended from C program!\n");
+
+    /* Close the file. */
+    fclose(fp);
+
+    printf("Append complete.\n");
+
+    return 0;
+}