Essential C Functions

These are the big ones, the ones you just gotta know. Anybody writing any amount of C code would do well to be familiar with these guys. Having these in the front of your brain can often save you a lot of time.

This is not a list of every function, but rather, those that you and I are most likely to use on a daily basis.

Each function has a short description after it. Click the function name for a writeup with more detailed information.

• abs() Returns absolute value of an int.
• atoi() Converts ASCII string to an int.
• calloc() Allocates contiguous storage in heap.
• exit() Terminates program.
• exp() Returns value of e^x.
• fclose() Closes file.
• fgetc() Reads a single character (from file).
• fgets() Reads a string (from file).
• fopen() Opens a file
• fprintf() Writes formatted output
• fputc() Writes a character (to file).
• fputs() Writes a string (to file).
• free() Frees storage.
• fscanf() Reads formatted input.
• getchar() Reads a character (from file).
• isalnum() Alphanumeric?
• isalpha() Alphabetic character?
• isdigit() Decimal digit?
• islower() Lowercase character?
• isupper() Uppercase character?
• log() log_ex.
• log10() log₁₀x.
• malloc() Allocates storage in heap.
• pow() x^y.
• printf() Write formatted output.
• putchar() Write a character.
• puts() Write a string.
• rand() Generate random integer.
• scanf() Read formatted input.
• sqrt() Square root.
• srand() Seeds random number generator.
• strncat() Concatenate strings.
• strncmp() Compare strings.
• strncpy() Copy string.
• strstr() Find substring.
• system() Execute a command (USEFUL!)
• tolower() Convert from uppercase to lowercase.
• toupper() Convert from lowercase to uppercase.

If you think I've made poor decisions in my function choices, /msg me and let me know.

Huge thanks to wharfinger for his E2 List Formatter