Programming for All: C Edition for Python Programmers

Chapter 5: C Libraries

A C library is a collection of precompiled functions and definitions that you can use to add functionality to your programs. To program efficiently in C, you need to know how to use these libraries and how to create your own.

5.1 Standard library functions

The C Standard Library has functions for input/output, string manipulation, mathematical computation, and more. Some of the most commonly used ones follow.

5.1.1 String handling

The functions for handling strings are in the <string.h> library.

Common functions:

• strlen: Returns the length of a string.
• strcpy: Copies one string to another.
• strcat: Concatenates two strings.
• strcmp: Compares two strings.

Example:

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

int main() {
    char str1[20] = "Hello";
    char str2[20] = "World";
    char str3[20];

    // Copy str1 to str3
    strcpy(str3, str1);
    printf("strcpy: %s\n", str3);

    // Concatenate str1 and str2
    strcat(str1, str2);
    printf("strcat: %s\n", str1);

    // Length of str1
    printf("strlen: %zu\n", strlen(str1));

    // Compare str1 and str2
    int cmp = strcmp(str1, str2);
    printf("strcmp: %d\n", cmp);

    return 0;
}

5.1.2 Mathematical functions

Mathematical functions are available in the <math.h> library.

Common functions:

• sqrt: Computes the square root.
• pow: Raises a number to a power.
• sin, cos, tan: Trigonometric functions.
• fabs: Computes the absolute value of a floating-point number.

Example:

#include <stdio.h>
#include <math.h>

#define PI 3.14159265358979323846

int main() {
    double num = 16.0;

    printf("Square root of %.2f: %.2f\n", num, sqrt(num));
    printf("2 raised to the power 3: %.2f\n", pow(2, 3));
    printf("Sine of 45 degrees: %.2f\n", sin(45 * PI / 180)); // Convert degrees to radians

    return 0;
}

Standard C has no constant for π. Many systems define M_PI in <math.h>, but that comes from POSIX, not the C standard, so the example defines its own PI. On Linux you also have to link the math library by adding -lm at the end of the command, as in gcc math_example.c -o math_example -lm. On macOS the math functions are part of the standard C library, so -lm is optional there.

5.1.3 Time and date functions

For time and date functions, use the <time.h> library.

Common functions:

• time: Returns the current time.
• ctime: Converts time to a string.
• difftime: Computes the difference between two times.
• strftime: Formats date and time.

Example:

#include <stdio.h>
#include <time.h>

int main() {
    time_t current_time;
    time(&current_time);

    printf("Current time: %s", ctime(&current_time));

    return 0;
}

5.2 Writing and using header files

A header file declares functions and variables so that several source files can share them. Header files usually have a .h extension.

To create a header file:

1. Create a file named myheader.h.
2. Declare functions and variables in it.

myheader.h:

// Function prototype
void greet();

To use the header file, include it in your source file with #include.

main.c:

#include <stdio.h>
#include "myheader.h"

// Function definition
void greet() {
    printf("Hello from the header file!\n");
}

int main() {
    greet();
    return 0;
}

Explanation:

• #include "myheader.h" includes the header file.
• The greet function is declared in the header file and defined in the source file.

5.3 Linkers and makefiles

Linkers and makefiles are essential for managing larger C projects. The linker combines several object files into a single executable, and a makefile automates the build process.

5.3.1 Using the linker

When you compile several source files, the linker can combine them into a single executable.

Example:

1. Create two source files, file1.c and file2.c.

file1.c:

#include <stdio.h>

void function1() {
    printf("This is function1 from file1.\n");
}

file2.c:

void function1();

int main() {
    function1();
    return 0;
}

2. Compile and link the files:

gcc -c file1.c
gcc -c file2.c
gcc file1.o file2.o -o myprogram
./myprogram

Explanation:

• gcc -c file1.c compiles file1.c into an object file file1.o.
• gcc -c file2.c compiles file2.c into an object file file2.o.
• gcc file1.o file2.o -o myprogram links the object files into an executable myprogram.

5.3.2 Writing a makefile

A makefile automates the build by spelling out how to compile and link the program.

Example Makefile:

# Makefile for building myprogram

# Compiler
CC = gcc

# Compiler flags
CFLAGS = -Wall

# Target executable
TARGET = myprogram

# Source files
SRCS = file1.c file2.c

# Object files
OBJS = $(SRCS:.c=.o)

# Build the target
$(TARGET): $(OBJS)
	$(CC) $(CFLAGS) -o $(TARGET) $(OBJS)

# Compile source files into object files
%.o: %.c
	$(CC) $(CFLAGS) -c $< -o $@

# Clean up object files and executable
clean:
	rm -f $(OBJS) $(TARGET)

Each command line under a rule (the recipe) must start with a tab character. Make rejects lines indented with spaces, with an error such as missing separator.

Explanation:

• CC = gcc sets the compiler to gcc.
• CFLAGS = -Wall turns on most of the common compiler warnings. Despite the name it doesn’t enable every warning; -Wextra adds more.
• TARGET = myprogram sets the name of the executable.
• SRCS = file1.c file2.c lists the source files.
• OBJS = $(SRCS:.c=.o) converts source file names to object file names.
• $(TARGET): $(OBJS) defines the target and its dependencies.
• $(CC) $(CFLAGS) -o $(TARGET) $(OBJS) links the object files to create the executable.
• %.o: %.c compiles each source file into an object file.
• clean: removes the object files and executable.

Building with Make:

make
./myprogram
make clean

Explanation:

• make compiles and links the program.
• ./myprogram runs the executable.
• make clean removes the object files and executable.