modify one line content in a file

#include

void modifyFile();

void modifyFile()
{
    FILE *fp = NULL;
    char key[19];
    char curkey[16];
    fpos_t pos;
    long offset = 0;
    fp = fopen("a.txt", "r+");
    if (fp == NULL)
    {
        printf("fp is NULL\n");
        return;
    }
    while ( fgets(key, 20, fp) != NULL )
    {
        printf("key is %s\n", key);
        if (strlen(key) != 0)
        {
            if (key[strlen(key)-1] == '\n')
                key[strlen(key)-1] = '\0';
            strncpy(curkey, key, 16);
            curkey[16] = 0;
            printf("curkey is %s\n", curkey);
            fgetpos(fp, &pos);
            offset += strlen(key);
            printf("fp position is %ld\n", pos);
            if (strncmp(curkey, "2000000155FF1235", 16)==0)
            {
                fseek(fp, offset, SEEK_CUR);
                fprintf(fp, "%s=%d\n", "2000000155FF1237", 4);
                break;
            }
        }
    }
    fclose(fp);
}

void main()
{
    FILE *fp = NULL;
    fp = fopen("a.txt", "w");
    fprintf(fp, "%s=%d\n", "2000000155FF1234", 1);
    fprintf(fp, "%s=%d\n", "2000000155FF1235", 2);
    fprintf(fp, "%s=%d\n", "2000000155FF1236", 3);
    fclose(fp);
    modifyFile();
}