Overview of the ISO C Standard Library

The C Standard Library (libc) provides standardized utility functions, macro definitions, and data types built into every compliant ISO C environment.

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1. <stdio.h> — Input & Output Functions

Header for console streams, file operations, and formatted input/output:

• int printf(const char *format, ...): Print formatted text to standard output.
• int scanf(const char *format, ...): Read formatted input from standard input.
• FILE *fopen(const char *path, const char *mode): Open a file stream.
• int fclose(FILE *stream): Close an open file stream.
• char *fgets(char *s, int size, FILE *stream): Read a line safely from a stream.

c (ISO Standard)
Run in Compiler.cCopy
#include <stdio.h>

int main() {
    printf("Standard output formatted string: %d\n", 42);
    return 0;
}

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2. <stdlib.h> — Dynamic Memory & Utilities

Header for heap memory management, conversions, and process lifecycle:

• void *malloc(size_t size): Allocate uninitialized heap memory.
• void *calloc(size_t nmemb, size_t size): Allocate zero-initialized memory.
• void free(void *ptr): Release dynamically allocated memory back to system.
• int atoi(const char *nptr): Convert string to integer.
• void exit(int status): Terminate calling process immediately.

c (ISO Standard)
Run in Compiler.cCopy
#include <stdio.h>
#include <stdlib.h>

int main() {
    int *buffer = (int *)malloc(2 * sizeof(int));
    if (buffer != NULL) {
        buffer[0] = 100;
        buffer[1] = 200;
        printf("Heap value #1: %d, #2: %d\n", buffer[0], buffer[1]);
        free(buffer);
    }
    return 0;
}

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3. <string.h> — String & Memory Manipulation

• size_t strlen(const char *s): Returns length of string excluding null terminator.
• char *strcpy(char *dest, const char *src): Copy string into destination buffer.
• int strcmp(const char *s1, const char *s2): Lexicographical string comparison.
• void *memset(void *s, int c, size_t n): Fill memory block with a constant byte.

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4. <math.h> — Common Mathematical Routines

• double sqrt(double x): Calculate square root.
• double pow(double base, double exp): Calculate power exponentiation.
• double fabs(double x): Absolute value of a floating-point number.

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Related Guides & References

• Look up formatted placeholder tokens in C Format Specifiers Reference.
• Review reserved language tokens in C Reserved Keywords Reference.
• Test standard library functions interactively in the Online C Compiler.