What is the difference between malloc and calloc?

malloc(size) allocates uninitialized memory of the given byte count. calloc(n, size) allocates an array of n elements, zero-initializes the whole block, and checks for multiplication overflow internally. Use calloc when you need zeroed memory or are allocating an array.

Why is gets dangerous and what replaces it?

gets reads a line with no bound on length, so any input longer than your buffer overflows it — a classic security hole. It was removed in C11. Use fgets, which takes a maximum size, or getline on POSIX systems.

What does memcpy require that memmove does not?

memcpy gives undefined behaviour if the source and destination regions overlap. memmove handles overlap correctly by copying as if through a temporary buffer. Use memmove whenever the regions might overlap, even though memcpy is slightly faster.

How do I check if a stdio call failed?

Most stdio functions signal errors through their return value: fopen returns NULL, fread returns a short count, and many return EOF. After a stream error use ferror to test the stream and feof to distinguish end-of-file from a real error.

Are these functions portable across platforms?

The C99 and C11 standard functions are portable to any conforming compiler. A few entries noted as POSIX, such as strdup before C23 and getline, are guaranteed only on POSIX systems like Linux and macOS, not on plain standard C.

C Standard Library Reference

The C standard library is split across a handful of headers, each grouping related functions for I/O, memory, strings, math, and time. Calling the wrong one — or ignoring a return value — is a frequent source of bugs. This tool is a searchable reference covering each function’s prototype, purpose, and the pitfalls that bite C programmers.

How it works

Every function belongs to a header you must #include. The reference is grouped by header:

stdio.h — printf, scanf, fopen, fgets, fread, fwrite, fprintf, snprintf, fclose.
stdlib.h — malloc, calloc, realloc, free, atoi, strtol, qsort, bsearch, rand, exit.
string.h — strlen, strcpy, strncpy, strcmp, strcat, memcpy, memmove, memset, strstr.
ctype.h — isdigit, isalpha, isspace, toupper, tolower.
math.h — sqrt, pow, floor, ceil, fabs, fmod.
time.h — time, clock, strftime, localtime.

Worked example

#include <stdio.h>
#include <string.h>

char buf[16];
fgets(buf, sizeof buf, stdin);   // bounded read — safe
buf[strcspn(buf, "\n")] = '\0';  // strip trailing newline

char dst[8];
snprintf(dst, sizeof dst, "%s", buf); // bounded copy, always NUL-terminated

fgets and snprintf take an explicit size, which is why they are preferred over gets and strcpy.

Notes

Return values carry the error signal: fopen returns NULL, allocation functions return NULL, and many stdio calls return EOF. Always check.
strncpy does not guarantee a terminating '\0' if the source is exactly as long as the buffer — terminate manually.
Prefer strtol/strtod over atoi/atof: they report conversion errors, while the ato* family silently returns 0 on bad input.
Functions tagged POSIX (strdup pre-C23, getline) are not part of plain standard C.

C Standard Library Reference

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How it works

Worked example

Notes