What are built-in functions in c programs?

Answer 1

There are no built-in functions in C as such. What we call built-in functions are actually part of the C standard function library, which is just a function library like any other, but one that ships with all implementations of C.

The functions we specifically regard as being built-in are those functions that do not require us to include any specific library headers. These functions are imported by default, hence they are all considered built-in. They are as follows:

The string management functions (strcpy, strncpy, strcmp, strncmp, strlen, strcat, strncat, strchr, strrchr, strstr and strtok), memory management functions (malloc, calloc, realloc and free), buffer manipulation functions (memcpy, memcmp, memchr, memset and memmove), character functions (isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, tolower and toupper) and error handling functions (perror, strerror).