Important C standard libraries

`<string.h>`

The <string.h> library provides functions for manipulating C strings (null-terminated character arrays) and for general memory block operations.

String manipulation functions

These functions operate on null-terminated strings.

size_t strlen(const char *str);
- Calculates the length of the string str (excluding the null terminator).
  caution
  Undefined behavior if str is not a pointer to a null-terminated array.
int strcmp(const char *lhs, const char *rhs);
- Compares two strings lhs and rhs lexicographically.
- Returns:
  - 0 if strings are equal.
  - a negative value if lhs comes before rhs.
  - a positive value if lhs comes after rhs.
- Comparison is based on ASCII values.
  caution
  Undefined behavior if lhs or rhs are not pointers to null-terminated arrays.
int strncmp(const char *lhs, const char *rhs, size_t count);
- Compares up to count characters of strings lhs and rhs.
- Return value is similar to strcmp.
  caution
  Undefined behavior if access goes beyond the end of either array if count is larger than string lengths.
char *strchr(const char *str, int ch);
- Searches for the first occurrence of the character ch (cast to unsigned char) in the string str.
- Returns a pointer to the first occurrence, or NULL if not found.
- The null terminator can also be searched.
char *strrchr(const char *str, int ch);
- Searches for the last occurrence of the character ch in str.
- Returns a pointer to the last occurrence, or NULL if not found.
char *strstr(const char* str, const char* substr);
- Searches for the first occurrence of the substring substr within the string str.
- Returns a pointer to the beginning of the first occurrence in str, or NULL if substr is not found.
- If substr is an empty string, returns str.
  caution
  Undefined behavior if str or substr are not null-terminated.
char *strcpy(char *dest, const char *src);
- Copies the string src (including the null terminator) to dest.
- Returns dest.
  danger
  Undefined behavior if dest is not large enough. Undefined behavior if strings overlap in memory. Undefined behavior if src is not null-terminated.
char *strcat(char *dest, const char *src);
- Appends (concatenates) the string src to the end of dest. The first character of src overwrites the null terminator of dest.
- Returns dest.
  danger
  Undefined behavior if dest is not large enough to hold its original content, src, and the new null terminator. Undefined behavior if strings overlap. Undefined behavior if dest or src are not null-terminated.
char *strncpy(char *dest, const char *src, size_t count);
- Copies up to count characters from src to dest.
  danger
  If src has fewer than count characters, dest is padded with null characters up to count characters. If src has count or more characters, dest might not be null-terminated if no null terminator is found within the first count characters of src. Always performs count writes.
- Returns dest.
char *strncat(char *dest, const char *src, size_t count);
- Appends up to count characters from src to dest, plus a null terminator.
- dest must be large enough for strlen(dest) + count + 1 characters.
- Always null-terminates the resulting string.
- Returns dest.

Memory manipulation functions

These functions operate on generic blocks of memory (arrays of char or void*) and do not assume null termination. They require the size of the memory block to be specified.

void *memset(void *dest, int ch, size_t count);
- Fills the first count bytes of the memory area pointed to by dest with the constant byte ch (cast to unsigned char).
- Returns dest.
  caution
  Undefined behavior if dest is NULL or if count exceeds the allocated size of dest.
void *memchr(const void* ptr, int ch, size_t count);
- Searches for the first occurrence of ch (cast to unsigned char) within the first count bytes of the memory area pointed to by ptr.
- Returns a pointer to the matching byte or NULL if not found.
  caution
  Undefined behavior if ptr is NULL or if count exceeds accessible memory.
int memcmp(const void* lhs, const void* rhs, size_t count);
- Compares the first count bytes of memory areas lhs and rhs.
- Return value is similar to strcmp (0 for equal, negative if lhs < rhs, positive if lhs > rhs).
  caution
  Undefined behavior if access goes beyond allocated memory for lhs or rhs.
void *memcpy(void *dest, const void *src, size_t count);
- Copies count bytes from memory area src to memory area dest.
- Returns dest.
  danger
  Undefined behavior if memory areas src and dest overlap. Use memmove for overlapping regions.
  caution
  Undefined behavior if dest or src are NULL or if count leads to access beyond allocated memory.
void *memmove(void *dest, const void *src, size_t count);
- Copies count bytes from src to dest. Handles overlapping memory regions correctly (as if using a temporary buffer).
- Returns dest.
  caution
  Undefined behavior if dest or src are NULL or if count leads to access beyond allocated memory.

`<assert.h>`

The <assert.h> library provides a single macro, assert, used for debugging.

void assert(scalar expression);
- If expression evaluates to false (0), assert prints an error message to stderr (including the expression, source file name, and line number) and then calls abort() to terminate the program.
- If expression is true (non-zero), assert does nothing.
- NDEBUG macro: if the macro NDEBUG is defined at the point <assert.h> is included, the assert macro is disabled and expands to ((void)0), meaning it has no effect. NDEBUG is typically defined for release builds to remove assertion checks.

// Example:
#include <assert.h>
// #define NDEBUG // Uncomment to disable asserts

int main(void) {
    int x = -5;
    // assert(x > 0); // This would trigger an assertion failure if not NDEBUG
    return 0;
}

`<float.h>` and `<limits.h>`

These libraries define macros that represent various limits and characteristics of fundamental data types. They are crucial for writing portable code.

`<float.h>`

Defines macros for floating-point types (float, double, long double). Examples:

FLT_MIN, FLT_MAX: minimum and maximum representable finite float values.
DBL_MIN, DBL_MAX: for double.
LDBL_MIN, LDBL_MAX: for long double.
FLT_EPSILON: smallest x such that 1.0 + x != 1.0.
FLT_DIG, DBL_DIG, LDBL_DIG: number of decimal digits of precision.
FLT_MANT_DIG: number of base-FLT_RADIX digits in the mantissa.
FLT_MIN_EXP, FLT_MAX_EXP: minimum and maximum integer exponents.

Using these macros ensures that the code uses correct values for the specific architecture and compiler.

`<limits.h>`

Defines macros for integer types. Examples:

CHAR_BIT: number of bits in a char.
SCHAR_MIN, SCHAR_MAX: min/max for signed char.
UCHAR_MAX: max for unsigned char.
SHRT_MIN, SHRT_MAX: min/max for short int.
USHRT_MAX: max for unsigned short int.
INT_MIN, INT_MAX: min/max for int.
UINT_MAX: max for unsigned int.
LONG_MIN, LONG_MAX: min/max for long int.
ULONG_MAX: max for unsigned long int.
LLONG_MIN, LLONG_MAX: min/max for long long int (C99 and later).
ULLONG_MAX: max for unsigned long long int (C99 and later).

`<time.h>`

The <time.h> library provides functions for working with dates and times.

Time representation

time_t: an arithmetic type capable of representing time. Often a long long representing seconds since the Epoch (January 1, 1970, 00:00:00 UTC).

struct tm: a structure holding a calendar date and time broken down into components:

struct tm {
    int tm_sec;   // seconds after the minute - [0, 60] (60 for leap second)
    int tm_min;   // minutes after the hour - [0, 59]
    int tm_hour;  // hours since midnight - [0, 23]
    int tm_mday;  // day of the month - [1, 31]
    int tm_mon;   // months since January - [0, 11]
    int tm_year;  // years since 1900
    int tm_wday;  // days since Sunday - [0, 6]
    int tm_yday;  // days since January 1 - [0, 365]
    int tm_isdst; // Daylight Saving Time flag (>0 if DST is in effect, 0 if not, <0 if information is not available)
};

Core functions

time_t time(time_t *timer);
- Returns the current calendar time as a time_t value.
- If timer is not NULL, the return value is also stored in the object pointed to by timer.
- Common usage: time_t now = time(NULL);

Conversions: `time_t` to `struct tm`

These functions convert a time_t value into a struct tm. The _r versions are thread-safe (reentrant) and part of POSIX, standardized in C23.

struct tm *gmtime_r(const time_t *timer, struct tm *result);
- Converts time_t to struct tm expressed in Coordinated Universal Time (UTC).
- Stores the result in the user-provided result buffer and returns it. Returns NULL on error.
struct tm *localtime_r(const time_t *timer, struct tm *result);
- Converts time_t to struct tm expressed in local time (considering timezone and DST).
- Stores the result in result and returns it. Returns NULL on error.
- (Note: older, non-thread-safe versions gmtime() and localtime() exist but return pointers to static internal buffers, making them problematic in multithreaded code or with multiple calls.)
- Microsoft provides gmtime_s and localtime_s which have a different signature but similar purpose.

Conversions: `struct tm` to string

errno_t asctime_s(char *buf, rsize_t bufsz, const struct tm *timeptr); (C11 Annex K - bounds-checking interfaces)
- Converts struct tm to a string of the form "Www Mmm dd hh:mm:ss yyyy\n".
- buf is the user-provided buffer, bufsz is its size.
- Returns zero on success.
- (The older asctime() is not bounds-checked and returns a pointer to a static buffer.)
size_t strftime(char *s, size_t maxsize, const char *format, const struct tm *timeptr);
- Formats the struct tm timeptr into the string s according to the format string.
- maxsize is the maximum number of characters to write to s (including null terminator).
- format contains conversion specifiers (e.g., %Y for year, %m for month, %d for day, %H for hour, %M for minute, %S for second).
- Example format: "%d/%m/%Y %H:%M:%S" might produce "15/11/2023 08:46:20".
- Returns the number of characters written (excluding null terminator) on success, or 0 if maxsize was too small.

High-precision time (C11)

struct timespec: represents time with nanosecond precision.

struct timespec {
    time_t tv_sec;  // Seconds
    long   tv_nsec; // Nanoseconds [0, 999999999]
};

int timespec_get(struct timespec *ts, int base);
- Gets the current calendar time with nanosecond resolution.
- ts points to the struct timespec to be filled.
- base must be TIME_UTC. Other values are for future extensions or implementation-defined bases.
- Returns base on success, 0 on failure.

// Example:
#include <time.h>
#include <stdio.h> // For printf

// For gmtime_r/localtime_r on systems that might not have C23 versions directly
// (e.g., using Microsoft's _s versions as a fallback)
#if defined(_MSC_VER)
struct tm* my_gmtime_r(const time_t* timer, struct tm* buf) {
    return gmtime_s(buf, timer) == 0 ? buf : NULL;
}
struct tm* my_localtime_r(const time_t* timer, struct tm* buf) {
    return localtime_s(buf, timer) == 0 ? buf : NULL;
}
#else
#define my_gmtime_r gmtime_r
#define my_localtime_r localtime_r
#endif

int main(void) {
    time_t now_t = time(NULL);
    struct tm now_tm_utc, now_tm_local;
    char time_str[100];

    if (my_gmtime_r(&now_t, &now_tm_utc)) {
        strftime(time_str, sizeof(time_str), "%Y-%m-%d %H:%M:%S UTC", &now_tm_utc);
        printf("UTC Time: %s\n", time_str);
    }

    if (my_localtime_r(&now_t, &now_tm_local)) {
        strftime(time_str, sizeof(time_str), "%Y-%m-%d %H:%M:%S Local", &now_tm_local);
        printf("Local Time: %s\n", time_str);
    }

    struct timespec ts;
    if (timespec_get(&ts, TIME_UTC)) {
        printf("High-res seconds: %ld, nanoseconds: %ld\n", (long)ts.tv_sec, ts.tv_nsec);
    }
    return 0;
}

`<stdlib.h>` (additional features)

Beyond memory allocation (malloc, free, etc.), <stdlib.h> offers various utility functions.

Pseudo-random number generation

int rand(void);
- Returns a pseudo-random integer in the range [0, RAND_MAX]. RAND_MAX is a macro defined in <stdlib.h> (guaranteed to be at least 32767).
void srand(unsigned int seed);
- Initializes (seeds) the pseudo-random number generator used by rand().
- Using the same seed will produce the same sequence of random numbers.
- To get different sequences on different runs, seed with a value that changes, e.g., current time:
```
#include <stdlib.h>
#include <time.h> // For time()
// ...
srand((unsigned int)time(NULL)); // Seed once at program start
int random_num = rand();
```

Program utilities

void exit(int status);
- Terminates the calling process immediately. Any open streams are flushed, temporary files are removed, and functions registered with atexit() are called.
- status is an integer value returned to the parent process or operating system.
EXIT_SUCCESS: macro expanding to an integer value (typically 0) indicating successful termination.

EXIT_FAILURE: macro expanding to an integer value (typically 1 or non-zero) indicating unsuccessful termination.

// Example:
if (/* error condition */) {
    fprintf(stderr, "An error occurred.\n");
    exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);

Integer arithmetic

int abs(int n);
long labs(long n);
long long llabs(long long n); (C99)
- Return the absolute value of n.
div_t div(int numer, int denom);
- Computes numer / denom and numer % denom simultaneously.
- Returns a div_t struct:
```
typedef struct {
    int quot; // quotient
    int rem;  // remainder
} div_t;
```
- Similar functions ldiv_t ldiv(long numer, long denom); and lldiv_t lldiv(long long numer, long long denom); (C99) exist.

String to number conversion

These functions convert strings to numerical types. They are more robust than older functions like atoi(), atof().

Floating-point conversion:
- double strtod(const char *restrict nptr, char **restrict endptr);
- float strtof(const char *restrict nptr, char **restrict endptr); (C99)
- long double strtold(const char *restrict nptr, char **restrict endptr); (C99)
  - nptr: the string to convert.
  - endptr: if not NULL, it's set to point to the character in nptr after the last character used in the conversion. This is useful for error checking or parsing multiple numbers from a string.
  - Skips leading whitespace.
  - Returns the converted value. If no conversion could be performed, 0 is returned. If the value is out of range, HUGE_VAL, HUGE_VALF, or HUGE_VALL (possibly with a sign) is returned, and errno is set to ERANGE.
Integer conversion:
- long int strtol(const char *restrict nptr, char **restrict endptr, int base);
- long long int strtoll(const char *restrict nptr, char **restrict endptr, int base); (C99)
- unsigned long int strtoul(const char *restrict nptr, char **restrict endptr, int base);
- unsigned long long int strtoull(const char *restrict nptr, char **restrict endptr, int base); (C99)
  - nptr, endptr: same as for floating-point versions.
  - base: the numerical base (2-36). If base is 0, the base is auto-detected from the string prefix: 0 for octal, 0x or 0X for hexadecimal, otherwise decimal.
  - Returns the converted value. If no conversion, 0 is returned. If out of range, LONG_MIN/MAX, LLONG_MIN/MAX, ULONG_MAX, ULLONG_MAX is returned, and errno is set to ERANGE.

// Example strtol:
#include <stdlib.h>
#include <stdio.h>
#include <errno.h> // For errno

int main(void) {
    const char *str = "  12345 and some text";
    char *endptr;
    long val;

    errno = 0; // Clear errno before call

    // Convert the string starting at pointer p to a long integer, 
    // interpreting the number in base 10 (decimal).
    // endptr will point to the first character after the number in the string.
    val = strtol(str, &endptr, 10);
    // val == 12345
    // endptr points to " and some text"

    if (endptr == str) {
        printf("No digits were found.\n");
    } else if (*endptr != '\0') {
        printf("Converted value: %ld\n", val);
        printf("Remaining string: \"%s\"\n", endptr);
    } else {
        printf("Converted value: %ld (entire string consumed)\n", val);
    }

    if (errno == ERANGE) {
        printf("Value out of range.\n");
    }
    return 0;
}

Converted value: 12345
Remaining string: " and some text"

`<math.h>`

The <math.h> library provides common mathematical functions. Most functions operate on double arguments and return double. Versions for float (suffixed with f) and long double (suffixed with l) also exist (e.g., sinf(), sinl()).

Trigonometric functions

(Angles are in radians)

double sin(double x);
double cos(double x);
double tan(double x);
double asin(double x); (arc sine, result in [-PI/2, PI/2])
double acos(double x); (arc cosine, result in [0, PI])
double atan(double x); (arc tangent, result in [-PI/2, PI/2])
double atan2(double y, double x); (arc tangent of y/x, using signs to determine quadrant, result in [-PI, PI])

Exponential and logarithmic functions

double exp(double x); (computes e^x)
double log(double x); (natural logarithm, ln(x))
double log10(double x); (base-10 logarithm)
double log2(double x); (base-2 logarithm, C99)
double exp2(double x); (computes 2^x, C99)

Power and root functions

double pow(double base, double exponent); (computes base^exponent)
double sqrt(double x); (square root, x >= 0)
double cbrt(double x); (cube root, C99)
double hypot(double x, double y); (computes sqrt(x² + y²), C99)

Rounding and remainder functions

double ceil(double x); (smallest integer not less than x)
double floor(double x); (largest integer not greater than x)
double trunc(double x); (rounds x towards zero, C99)
double round(double x); (rounds to nearest integer, halfway cases away from zero, C99)
long int lround(double x); (rounds to nearest long int, C99)
long long int llround(double x); (rounds to nearest long long int, C99)
double fmod(double x, double y); (floating-point remainder of x/y. Result has same sign as x.)

Absolute value

double fabs(double x); (absolute value for double)
float fabsf(float x);
long double fabsl(long double x);

NaN and infinity (C99)

Macros:

NAN: represents a "Not-a-Number" quiet NaN value.
INFINITY: represents a positive infinity value. Functions to check:
int isinf(x): returns non-zero if x is positive or negative infinity.
int isnan(x): returns non-zero if x is NaN.
int isfinite(x): returns non-zero if x is a normal, subnormal, or zero value (not NaN or infinity).
int isnormal(x): returns non-zero if x is a normal number (not zero, subnormal, NaN, or infinity).

`<errno.h>`

The <errno.h> library provides a way for C library functions to report error conditions.

errno: a macro that expands to a modifiable lvalue of type int. It's a global variable (or a macro expanding to a function call that returns a pointer to it, especially in multithreaded environments) that is set by system calls and some library functions when an error occurs.
info
errno is only set on error. It is not cleared on success. Therefore, you must set errno = 0; before calling a function that might set it, and then check its value after the call.
Error code macros: <errno.h> defines symbolic names for common error codes. Examples:
- EDOM: domain error (e.g., sqrt(-1)). Used by <math.h>.
- ERANGE: range error (e.g., result of a function is too large or too small to be represented). Used by <math.h> and string conversion functions in <stdlib.h>.
- EILSEQ: illegal byte sequence (relevant for multibyte character conversions).
char *strerror(int errnum); (defined in <string.h>)
- Takes an error number (errnum, typically the value of errno) and returns a pointer to a human-readable string describing that error.
- The string returned should not be modified by the program.

// Example using errno with sqrt:
#include <math.h>
#include <errno.h>
#include <stdio.h>
#include <string.h> // For strerror

int main(void) {
    double result;

    errno = 0; // Clear errno before the call
    result = sqrt(-1.0);

    if (errno != 0) {
        printf("Error calculating sqrt(-1.0): %s (errno code: %d)\n", strerror(errno), errno);
        if (errno == EDOM) {
            printf("This was a domain error.\n");
        }
    } else {
        printf("sqrt(-1.0) = %f\n", result); // Will likely print nan
    }

    errno = 0;
    result = exp(1000.0); // Might cause a range error
    if (errno == ERANGE) {
         printf("Error calculating exp(1000.0): %s (errno code: %d)\n", strerror(errno), errno);
         printf("Result: %f\n", result); // Will likely print inf
    }

    return 0;
}

<string.h>​

String manipulation functions​

Memory manipulation functions​

<assert.h>​

<float.h> and <limits.h>​

<float.h>​

<limits.h>​

<time.h>​

Time representation​

Core functions​

Conversions: time_t to struct tm​

Conversions: struct tm to string​

High-precision time (C11)​

<stdlib.h> (additional features)​

Pseudo-random number generation​

Program utilities​

Integer arithmetic​

String to number conversion​

<math.h>​

Trigonometric functions​

Exponential and logarithmic functions​

Power and root functions​

Rounding and remainder functions​

Absolute value​

NaN and infinity (C99)​

<errno.h>​