C

Linking

Linking a library (.a or .so file)

To link a lib file of name libexample.a you include the flag -lexample After the name of the output file is specified.

The -L. flag specifies to search for libraries in the current directory.

You also must include the header file in your c file which contains the function prototypes e.g.

#include "example.h"

Makefile

Compile commands in makefile are usually structured as following

${CC} ${CFLAGS} inputfile1.c inputfile2.c -o outputfile.o -L. -lmylib

Setting PHONY targets

Phony targets unlike normal targets aren’t named after a output file and are just utility commands.

 
.PHONY: clean
 
clean:
    #clean command here

Automatic Variables

• $@ Gets name of rule target
• $< Gets first name of first prerequisit
• $^ Gets all prerequesits
• $? Gets all prerequesits newer than the target

Wildcards

% is similar to * in bash. For example %.c means any file ending with .c and f% means any file starting with f

Example Makefile

# Configuration
CC=clang
CFLAGS=-Wall
LDFLAGS=
EXE=example
OBJ=example.o circles.o output.o
 
 
# Default target: link the object code into the executable
${EXE}: ${OBJ}
   ${CC} ${LDFLAGS} -o $@ $^
 
 
# Compile object code from source code
%.o: %.c
   ${CC} ${CFLAGS} -c $<
 
 
# Remove build products
.PHONY: clean
clean:
   rm *.o ${EXE}
 

GDB

Adding debug to a object file during compilation

Add the -g flag to your clang command e.g.

clang -g program.c -o 

GDB Commands

Invoking GDB

gdb --tui executable

Running the program

• Use run use the run or r command

Progressing the Program

• To run until breakpoint/ watch point use continue or c
• To run one line use step or s (note: step allows for entering into functions)
• To run one line without entering functions use next or n

Stopping a Program

• To end a program’s execution use the kill or k command

Breakpoints

There are different contexts to use the break command:

• break line_number breaks at specified line number
• break function_name breaks when specified function is called
• break filename:function_name breaks when function from a specific file is called
• info breakpoints gets info on all breakpoints (including IDs)
• delete deletes all breakpoints
• delete ID deletes breakpoint with that ID

Watchpoints

• watch VARIABLE make program pause every time VARIABLE is updated
• watch CONDITION pauses the program when a condition is true e.g. watch i>12

Examining Functions

• print VARIABLE shows the value of the variable
• print &VARIABLE prints the address of a variable
• print *VARIABLE prints the value at the address of a variable
• print VARIABLE[n] prints the nth element of an array
• ptype VARIABLE prints the datatype of a variable

Strings

string.h

Useful String Functions

• strcmp(string1, string2) - 0 if identical
• strlen(string1)
• strcpy(destination, source)
• strcat(string1, string2)

Format String Specifiers

• %d or %i signed integer
• %u unsigned integer
• %f float
• %lf double
• %c character
• %s string
• %hd signed short int
• %hu unsigned short int
• %hhu short short unsigned int

Commmand Line args in C

int main(int argc, char* argv) {
    printf("Hello %s", argv[1]);
}

File operations C

stdio.h

Initalising the file pointer

FILE *fp = fopen("filename.txt", "w")

File Open Modes

• r for read mode
• w for write mode
• a for append mode
• b after any of the above to do this in binary mode
• r+, w+ for read and write mode
• a+ for both reading and appending

Reading from a file

fscanf(fp, "formatted string %d %d", &var1, &var2);

Writing to a file

fprintf

fprintf(fp, "formatted string %d %d", &var1, &var2);

fprintf prints a formated string to a file

fwrite

fwrite(&var, sizeof(var), 1, fp);

Writes only a variable to a file.

An array can be written to a file using

int array[100];
int n = sizeof(array)/sizeof(int); //calculates the length of arr
fwrite(&arr, sizeof(int), n, fp);

Reading user input

scanf("%d", &var1);