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Diffstat (limited to 'md/notes')
| -rw-r--r-- | md/notes/undefined_c/titles.md | 126 |
1 files changed, 121 insertions, 5 deletions
diff --git a/md/notes/undefined_c/titles.md b/md/notes/undefined_c/titles.md index a09f0a7..014035b 100644 --- a/md/notes/undefined_c/titles.md +++ b/md/notes/undefined_c/titles.md @@ -393,20 +393,136 @@ Run pointer in while loop incrementing pointer. It will stop only when segfaults Dont initialize pointer and it will have random value. + + +### Allocate memory + +From programs perspective memory allocation is adding address range to executable that can be addressed. + +malloc should be accompanied with free statement, otherwise it will have memory leaks. + +```c +#include <stdio.h> +#include <stdlib.h> +#include <string.h> + +int main() { + char *c = malloc(16); + memset(c,0,16); + int *arr = malloc(16*sizeof(int)); + memset(arr,0,16*sizeof(int)); + free(c); + free(arr); +} +``` + +### Signed/Unsigned + +Signed and unsigned variables differ just in one bit interpretation. But they have different behavior on minimal and maximal values. + + +```c +#include <stdio.h> +#include <limits.h> +int main() +{ + int i=INT_MAX; + unsigned int u=UINT_MAX; + + printf("i=%d\n",i); + printf("u=%u\n",u); + + i++; + u++; + printf("i=%d\n",i); + printf("u=%u\n",u); + i=0; + u=0; + i--; + u--; + printf("i=%d\n",i); + printf("u=%u\n",u); + +} +``` + ### Endianess + + +```c +#include <stdlib.h> +#include <stdio.h> +#include <fcntl.h> +#include <unistd.h> + +int main() { + int arr[4] = {0x00112233,0x44556677,0x8899AABB, 0xCCDDEEFF}; + printf("%08x\n",arr[0]); + printf("%08x\n",arr[1]); + printf("%08x\n",arr[2]); + printf("%08x\n",arr[3]); + + FILE *f = fopen("int.hex","w+"); + fprintf(f,"%08x",arr[0]); + fprintf(f,"%08x",arr[1]); + fprintf(f,"%08x",arr[2]); + fprintf(f,"%08x",arr[3]); + fclose(f); + + int fd=open("int.bin",O_CREAT|O_RDWR,S_IWUSR|S_IRUSR|S_IRGRP|S_IRWXO); + write(fd,arr,sizeof(arr)); + close(fd); + + int i; + fd = open("int.bin2",O_CREAT|O_RDWR,S_IWUSR|S_IRUSR|S_IRGRP|S_IRWXO); + for (i=0;i<4;i++) { + uint32_t val = (arr[i]>>16) &0x0000ffff; + val += (arr[i]<<16)&0xffff0000; + write(fd,&val,sizeof(uint32_t)); + } + close(fd); +} +``` + +While saving formated values to file you will get what you expect +``` +$ cat int.hex +00112233445566778899aabbccddeeff +``` + +Saving just memory dump of all values, will give you different result +``` +$ hexdump int.bin +0000000 2233 0011 6677 4455 aabb 8899 eeff ccdd +0000010 +``` + +Need to swap 16bit pairs to look same as value memory dump +``` +$ hexdump int.bin2 +0000000 0011 2233 4455 6677 8899 aabb ccdd eeff +0000010 +``` + +### Compiler flags + +Compiler have whole list of command line arguments that you can enable for different purposes, lets look into some of them +https://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html + + + + ### Static binary ### Dynamic binary -### Styles -### Compiler flags -### Allocate memory ### stdin,stdout,stderr -### Signed/Unsigned +### Styles + ## Basic usage -### File manipulation +### File manipulation with libc ### File manipulation with syscalls ## Base usage |