Manual

The C functions, system calls and commands the articles cite.

C library

atoi(3)Converts the start of a string to an int. atoi reports no errors: “abc” and “0” both give 0, and an overflow is undefined behavior.
calloc(3)Allocates an array of nmemb elements of size bytes, sets every byte to zero and checks that nmemb * size does not overflow. Returns NULL on failure.
execvp(3)Replaces the program in the current process with a command looked up in PATH, its arguments passed in an array terminated by NULL.
fflush(3)Forces an output stream to write the bytes it is holding in memory. Needed before a possible crash or a fork, with no guaranteed effect on stdin.
fgets(3)Reads a line into an array without going past its size, keeps the \n and always terminates the string. Returns NULL at end of file or on error.
free(3)Returns a block from malloc, calloc or realloc to the heap. free(NULL) is safe; freeing twice, or a pointer that was never allocated, is undefined behavior.
malloc(3)Reserves a block of memory on the heap and returns its address, or NULL if the allocation fails. The contents are uninitialized; the block is yours until free.
memcpy(3)Copies n bytes from one memory area to another and returns the destination. The two areas must not overlap, or the result is undefined.
memmove(3)Copies n bytes from one area to another and gets the right result even when the two overlap. Returns the destination, like memcpy.
memset(3)Fills the first n bytes of a memory area with one byte value and returns the address of the area. Handy for setting a buffer to zero.
printf(3)Writes formatted text to standard output. The formats to know (%d, %ld, %zu, %p, %s), what printf returns and the -Wformat warnings.
putchar(3)Writes one character to standard output, converted to unsigned char. Returns that character or EOF, and goes through the stdout buffer like printf.
puts(3)Writes a string to standard output, then a newline. No format is interpreted, which makes it simpler than printf for printing fixed text.
realloc(3)Resizes a heap block and keeps its contents up to the smaller of the two sizes. On failure it returns NULL and leaves the old block allocated.
scanf(3)Reads standard input according to a format and stores the values at the addresses you pass. Returns the number of values read, to check on every call.
strcmp(3)Compares two strings byte by byte and returns a negative, zero or positive int. Why == does not compare strings, and why only the sign counts.
strcpy(3)Copies a string, null byte included, into a buffer you provide. strcpy ignores the size of the destination: guaranteeing it is your job.
strdup(3)Allocates a copy of a string with malloc and returns its address, or NULL. Part of standard C since C23; the copy must be released with free.
strlen(3)Returns the number of bytes in a C string, not counting the final null byte. How it differs from sizeof, bytes versus characters, the size_t trap.
strncpy(3)Copies at most n bytes and pads with null bytes. If the source is too long, the destination is left unterminated: strncpy is not a safe strcpy.
strtol(3)Converts the start of a string to a long in the base you choose, tells you where parsing stopped through endptr and reports overflow with errno.

System calls

Shell commands

man(1)Shows the manual page of a command or a C function. Sections 1, 2 and 3, man 3 printf, man -k to search, and how to read a page.

Development tools

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