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.

What is the C Standard Library Reference?

Searchable C standard library reference organized by header — stdio.h, stdlib.h, string.h, math.h, ctype.h, time.h — with each function's prototype, purpose, and a usage note covering common pitfalls. It runs free in your browser on Gera Tools, with nothing uploaded.

C Standard Library Reference

Name: C Standard Library Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Common pitfalls by header

stdio.h pitfalls

scanf with %s has no length limit — it overflows exactly like gets. Use %15s (or whatever your buffer size minus one is) to bound the read. printf with a user-supplied format string is a format-string vulnerability; always use printf("%s", str) not printf(str).

fread and fwrite return the number of items transferred, not bytes. Checking ferror after a short count distinguishes a real error from end-of-file.

stdlib.h pitfalls

malloc returns NULL on allocation failure; dereferencing without checking is undefined behaviour that often manifests as a crash on a memory-scarce system, not during development. Always:

int *p = malloc(n * sizeof *p);
if (!p) { /* handle */ }

qsort comparator must return negative, zero, or positive — not just true/false. Returning a - b for integers silently overflows for large values; use (a > b) - (a < b) instead.

string.h pitfalls

strcpy and strcat are unsafe because they have no buffer-size parameter. Replace them with snprintf for concatenation or strlcpy/strlcat on platforms that provide them.

memcpy with overlapping regions is undefined. Use memmove when the regions might overlap.

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.