Functions Reference: Standard Library

Concept

C's standard library is split into headers. Include the right one before using a function: <stdio.h> for input/output, <string.h> for strings, <stdlib.h> for memory and conversions, <math.h> for maths, <ctype.h> for characters.

Many string functions don't check buffer sizes — prefer the n-versions (strncpy, snprintf) and always leave room for '\0'.

When using <math.h> with gcc, link the maths library with -lm.

Cheat sheet

Code (C17)

#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
    char line[] = "asha,92,hyderabad";
    char *tok = strtok(line, ",");
    while (tok) {
        printf("[%s] ", tok);
        tok = strtok(NULL, ",");
    }

    char name[] = "ravi";
    name[0] = (char)toupper(name[0]);
    printf("\n%s has %zu letters\n", name, strlen(name));

    int marks = atoi("88");
    printf("sqrt=%.2f pow=%.0f\n", sqrt(marks), pow(2, 5));

    int a[] = {0};
    memset(a, 0, sizeof a);
    return 0;
}
/* gcc demo.c -o demo -lm */

Remember

• strcmp returns 0 when strings are equal.
• strtok changes the original string.
• Link <math.h> functions with -lm.

Interview questions