#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", "a+");
    if (fp == NULL)
    {
        perror("fopen failed.");
        printf("errno = %d\n", errno);
        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;
}