| Function | Use |
| abs | Returns the absolute value of an integer |
| pow | Calculates a value raised to a power |
| sqrt | Finds square root |
| floor | Finds largest integer less than or equal to argument |
| ceil | Finds smallest integer greater than or equal to argument |
| round | Rounds floating value to nearest integer value |
| exp | Find exponential value |
| atoi | Converts string to int |
| exit | Ends a program normally |
| abort | Stops a program abnormally |
| assert | Checks if a given expression is true |
| isalpha | Checks if a character is alphabetic or not |
| isdigit | Checks if a character is digit or not |
| islower | Checks if a character is lowercase or not |
| isupper | Checks if a character is uppercase or not |
| tolower | Converts an uppercase character to lowercase |
| toupper | Converts a lowercase character to uppercase |
| strcat | Concatenates one string to another |
| strcmp | Compare one string to another |
| strcmpi | Compares two strings without regard to case |
| strcpy | Copies one string to another |
| strdup | Duplicates a string |
| strchar | Finds first occurance of a given characternin a string |
| strlen | Finds length of a string |
| strlwr | Converts a string to lowercase |
| strupr | Converts a string to uppercase |
| strrev | Reverse a string |
| close | Closes a file |
| fclose | Closes a file |
| feof | Detects end-of-file |
| getc | Reads a character from a file |
| fgetc | Reads a character from a file |
| getchar | Reads a character from keyboard (macro version) |
| fgetchar | Reads a character from keyboard (function version) |
| gets | Reads a line from keyboard |
| fgets | Reads a string from a file |
| getch | Reads a character from the keyboard |
| getche | Reads a character from keyboard and echoes it |
| open | Opens a file |
| fopen | Opens a file |
| printf | Writes formatted data to screen |
| fprintf | Writes formatted data to a file |
| sprintf | Writes formatted output to a string |
| putc | Writes a character to a file (macro version) |
| fputc | Writes a character to a file |
| putchar | Writes a character to screen (macro version) |
| fputchar | Writes a character to screen (function version) |
| puts | Writes a line to file |
| fputs | Writes a string to file |
| putch | Writes a character to the screen |
| scanf | Reads formatted data from keyboard |
| fscanf | Reads formatted data from a file |
| sscanf | Reads formatted input from a string |
| fseek | Repositions file pointer to given location |
| lseek | Repositions file pointer to a given location |
| tell | Gets current file pointer position |
| ftell | Gets current file pointer position |
| kbhit | Checks for a keystroke at the keyboard |
| read | Reads data from a file |
| fread | Reads data from the file and stores it in a buffer. |
| write | Writes data to a file |
| fwrite | Writes data to a file |
| rewind | Repositions file pointer to beginning of a file |
| calloc | Allocates a block of memory |
| malloc | Allocates a block of memory |
| realloc | Reallocates a block of memory |
| free | Frees a block allocated with malloc |