Check out example codes for "c value set to zero __memmove_avx_unaligned_erms". It will help you in understanding the concepts better.
Code Example 1
//The issue is that you probably have an uninitialized variable.
//Instead of:
char *fnam;
int nn = sprintf(fnam,"%s/%s",cwd,filnam);
//Try:
char fnam[500];
int nn = sprintf(fnam,"%s/%s",cwd,filnam);
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