These functions are the most commonly used useful Linux functions

Chapter 1 Primary I/O Functions

• close function: close the opened file
• creat function: create a file
• dup function: Duplicate file descriptors
• dup2 function: copy the file descriptor to the specified location
• fcntl function: change the state of the file
• fsync function: write buffer data back to disk file
• Lseek function: move the read and write position of the file
• open function: open a file read function: read the data of the file
• sync function: write all open files back to disk
• write function: output content to a file

Chapter 2 File Management Operation Functions

• access function: determine whether the process has permission to access the file
• alphasort function: sort directory structure alphabetically
• chdir function: change the current working directory
• chmod function: change file permissions
• chown function: change the owner of a file
• chroot function: change the root directory of the process
• closedir function: close the directory file
• fchdir function: use the file descriptor to change the current working directory
• fchmod function: change the permissions of an open file
• fstat function: get the status of an open file
• truncate function: change the size of an open file
• ftw function: traverse a directory tree
• get_current_dir_name function: get the current working directory
• getcwd function: get the current working directory of the process
• lchown function: change the owner of a symbolic link file
• link function: create a hard link
• lstat function: get the file status of the symbolic link
• nftw function: traverse a directory tree
• opendir function: open directory file
• readdir function: read the contents of a directory file
• readlink function: read the contents of the symbolic link file itself
• realpath function: convert a relative directory path to an absolute path
• remove function: delete a file or directory
• rename function: change the name or location of a file
• rewinddir function: rewind directory file
• seekdir function: file positioning function
• stat function: get file status
• symlink function: create a symbolic link
• telldir function: get the reading position of the directory file
• truncate function: change file size
• umask: Set the permission mask to use when creating files
• unlink function: delete a hard link to a file

Chapter 3 Standard I/O Functions

• clearerr function: clear the error flag
• fclose function: close the stream file descriptor
• fdopen function: convert file descriptor to file pointer
• feof function: end-of-file judgment function
• fflush function: flush the flow file read and write cache
• fgetc function: read a character from a stream file
• fgetpos function: returns the current read and write position of the stream file
• fgets function: read a line of data from a stream file
• fileno function: get the file descriptor of the stream file
• fopen function: open file
• fputc function: write characters to stream file
• fputs function: write string to stream file
• fread function: read data from a stream file
• freopen function: open a file
• fseek function: move the read and write position of the stream file
• fsetpos function: set the current read and write position of the stream file
• ftell function: get the read position of the stream file
• fwrite function: write data to the stream file
• getc function: read a character from a file
• getchar function: read a character from standard input
• gets function: read a line of data from standard input
• mktemp function: generate the filename of the temporary file
• putc function: write characters to file
• putchar function: output characters to standard output
• puts function: print a string to standard output
• rewind function: move the read and write position of the stream file to the beginning of the file
• setbuf function: set the file buffer
• setbuffer function: set the file buffer
• setlinebuf function: set the line buffer mode
• setvbuf function: set custom file buffer
• tmpfile function: create a temporary binary file
• ungetc function: write characters back to file

Chapter 4 Formatting Input and Output Functions

• fprintf function: formatted output to file
• fscanf function: format to read data from a file
• printf function: formatted output
• scanf function: formatted input
• snprintf function: format character input
• sprintf function: formatted string printing
• sscanf function: format string value
• vfprintf function: variadic output function
• vfscanf function: formatted input
• vprintf function: formatted output function
• vscanf function: formatted input function
• vsnprintf function: format string copy
• vsprintf function: format string copy
• vsscanf function: format character input

Chapter 5 Process Control

• abort function: process abnormal termination function
• atexit function: process termination handler
• execl function: execute file function
• execle function: the function that executes the file
• execlp function: find files from the PATH environment variable and execute
• execv function: execute the file
• execve function: execute the file
• execvp function: execute the file
• exit function: function to end the process running
• _exit function: function to end the process running
• fork function: create a new process
• getgid function: get the actual process group ID
• getegid function: get the effective process group ID
• getpid function: get the effective group ID
• getppid function: get the process ID of the parent process
• getpriority function: get the process execution priority
• longjmp function: jump to the stack environment stored by the original setjmp
• on_exit function: Set the function to be called before the normal end of the program
• setjmp function: store the current stack environment
• setgid function: set the process group ID of the process
• setuid function: set the process group ID
• setpriority function: set the program process priority
• siglongjmp function: jump to the stack environment stored in the original sigset jmp
• sigsetjmp function: store the current stack environment and masked signal set
• system function: execute shell commands
• vfork function: create a new process
• wait function: wait for the child process to interrupt or end
• waitpid function: wait for the child process to interrupt or end
• wait3 function: wait for the process to end and output child process statistics

Chapter 6 Interprocess Communication Functions

• ftok function: get the IPC key value related to the project
• msgctl function: message queue operation function
• msgget function: create or open a message queue
• msgrcv function: read message queue
• msgsnd function: write a message to the message queue
• semctl function: semaphore operation function
• semget function: create or open a semaphore
• semop function: release or acquire semaphore
• shmat function: import shared memory
• shmctl function: shared memory operation function
• shmdt function: detach shared memory from the program
• shmget function: create shared memory
• mkfifo function: create fifo pipeline function
• pclose function: close the pipe file
• pipe function: create an anonymous pipe
• popen function: create a pipeline file

Chapter 7 Signal Functions

• alarm function: kernel timer function
• kill function: signal sending function
• pause function: the process pauses the execution function
• raise function: send a signal function to the process itself
• sigaction function: advanced signal processing method setting function
• sigaddset function: add signal to signal set function
• sigdelset function: delete the corresponding signal function in the signal set
• sigemptyset function: clear signal set function
• sigfillset function: fill signal set function
• sigismember function: test whether a signal is in the signal set
• signal function: signal processing method setting function
• sigpause function: Pause the process until the signal arrives
• sigpending function: query pending signals
• sigprocmask function: signal masking function
• sigsuspend function: suspend until the signal arrives
• sleep function: let the process sleep for a period of time

Chapter 8 User, Group Functions, and Environment Variable Functions

• clearenv function: delete the value of all environment variables
• cuserid function: get username function
• endgrent function: close the system group file function
• endpwent function: close the system password function
• endutent function: close the utmp file
• fgetgrent function: read group data from file
• fgetpwent function: read password format data from file
• getegid function: get the effective gid group identifier
• getenv function: get the value of the environment variable
• geteuid function: get the effective uid user identifier
• getgid function: get the group identifier
• getgrent function: read group information from the file /etc/group
• getgrgid function: find group information from the file /etc/group
• getgrnam function: find group information from the file /etc/group
• getgroups function: returns the list of groups to which the current user belongs
• getlogin function: Get the account name of the current user
• getpw function: get the user information of the specified uid
• getpwent function: read the data in the /etc/passwd file
• getpwnam function: get user information by user name
• getpwuid function: get user information by user ID
• getuid function: Get the user ID to which the current process belongs
• getutent function: get login information
• getutid function: Get the login information of the specified user ID
• getutline function: find the specified login information
• initgroups function: initialize group information
• logwtmp function: add login record information
• putenv function: change the value of an environment variable
• pututline function: add login information
• setegid function: set the effective group ID
• setenv function: change the value of an environment variable
• seteuid function: set the effective user ID
• setfsgid function: set the group ID of the file system of the process
• setfsuid function: set the user ID of the file system of the process
• setgid function: set the group ID of the process
• setgrent function: move the read and write position of the file etc/group to the starting position
• setgroups function: set the group identifier of the current process
• setpwent function: move the read and write position of the file etc/passwd to the starting position
• setregid function: set the real and effective group ID of the current process
• setreuid function: set the real and effective user ID of the current process
• setuid function: set the user ID of the process
• setutent function: move the read and write position of the file utmp to the starting position
• unsetenv function: clear the value of the environment variable
• updwtmp function: write login data records to utmp file
• utmpname function: set the absolute position of the file utmp

Chapter 9 Error Handling and Memory Allocation Functions

• brk function: change the size of the data segment
• calloc function: memory allocation function
• ferror function: file error judgment
• free function: release dynamically allocated memory function
• getpagesize function: get the operating system memory page size function
• malloc function: dynamic memory allocation function
• mmap function: file mapping function
• munmap function: release the mapped memory address
• perror function: error cause output function
• realloc function: memory interception function
• sbrk function: increase data space
• strerror function: error code query

Chapter 10 Logging, Dynamic, and Regular Expression Functions

• closelog function: close the information record
• dlclose function: close the dynamic library file
• dlerror function: dynamic function error handling
• dlopen function: open dynamic library file
• dlsym function: find functions in dynamic library files
• openlog function: open log information
• regcomp function: compile regular expressions
• regerror function: compile error judgment function
• regexec function: perform regular matching
• regfree function: release regular pattern string resources
• syslog function: output logging information

Chapter 11 Time Functions

• asctime function: string time manipulation function
• clock function: Calculate the running time function
• ctime function: string time function
• difftime function: Calculate the time difference function
• ftime function: get system time
• gettimeofday function: get the current time of the system
• gmtime function: calendar time format function
• localtime function: get the current system time
• mktime function: system time conversion function
• settimeofday function: system time setting function
• strftime function: time formatted output function
• time function: get the current time of the system
• tzset function: set the system time zone variable tzname

Chapter 12 Socket Related Functions

• accept function: processing socket request function
• bind function: bind the socket descriptor to a socket
• connect function: connect to the remote host
• endprotoent function: the read function that ends the network protocol
• endservent function: close the file /etc/services
• gethostbyaddr function: get host information by IP
• gethostbyname function: get host information by host name
• getprotobyname function: get protocol data by protocol name
• getprotobynumber function: get protocol data by protocol number
• getprotoent function: read the network protocol data in the file /etc/rotoent
• getservbyname function: get network service protocol data
• getservbyport function: get network service protocol data
• getservent function: read host network service data
• getsockopt function: get the status of the specified socket descriptor
• herror function: error cause output function
• hstrerror function: error code query
• htonl function: address byte order conversion
• htons function: address byte order conversion
• inet_addr function: network address type conversion
• inet_aton function: network address type conversion
• inet_ntoa function: network address type conversion
• listen function: network socket listening
• ntohl function: address byte order conversion
• ntohs function: address byte order conversion
• recv function: receive message
• recvfrom function: receive message
• recvmsg function: read data from multiple buffers
• send function: send data to the remote host
• sendto function: send data to the remote host
• setprotoent function: open the network protocol file
• setservent function: open the network service file
• setsockopt function: set the state of the socket descriptor
• shutdown function: end socket socket
• socket function: create a network socket

Chapter 13 Character Test Functions

• isalnum function: determine whether the character is an English letter or a number
• isalpha function: determine whether the character is an English letter
• isascii function: determine whether a character is an ASCII character
• isblank function: determine whether the character is a blank character
• iscntrl function: determine whether the character is an ASCII control character
• isdigit function: determine whether the character is an Arabic numeral
• isgraph function: determine whether a character is a printable character
• islower function: determine whether the character is a lowercase English letter
• isprint function: determine whether a character is a printable character
• isspace function: determine whether the character is a space character
• ispunct function: determine whether a character is a punctuation mark
• isupper function: determine whether the character is an uppercase English letter
• isxdigit function: determine whether a character is a hexadecimal number

Chapter 14 Data Conversion Functions

• atof function: convert string to floating point number
• atoi function: convert string to integer
• atol function: convert string to long integer
• ecvt function: convert floating point number to string
• fcvt function: convert floating point number to string
• gcvt function: convert float to string
• strtod function: convert string to float
• strtol function: convert string to long integer
• strtoul function: convert string to unsigned long integer
• toascii function: convert integers to legal ASCII characters
• tolower function: convert uppercase letters to lowercase letters
• toupper function: convert lowercase letters to uppercase letters

Chapter 15 String Handling Functions

• bcmp function: compare the contents of two blocks of memory
• bcopy function: copy the contents of memory
• bzero function: clear the contents of a block of memory to zero
• ffs function: find the first true bit in an integer
• index function: find the first occurrence of a specified character in a string
• memccpy function: copy the contents of memory
• memchr function: find a specified character in a specified range of memory
• memcmp function: compare the contents stored in memory
• memcpy function: copy the contents of a block of memory to another block
• memfrob function: recode a memory area
• memmove function: copy memory contents
• memset function: fill a value into a memory area
• rindex function: find the last occurrence of a specified character in a string
• strcasecmp function: compare strings ignoring case
• strcat function: concatenate one string to the tail of another string
• strchr function: find the specified character in a string
• strcmp function: compare two strings
• strcoll’s function: compare strings according to the current environment information
• strcpy function: copy the contents of one string into another string
• strcspn function: Calculate the length of a substring in a string consisting of characters other than the specified charset
• strdup function: copy string content
• strfry function: Randomly reorganize a string
• strlen function: returns the length of the string
• strncasecmp function: compare two strings ignoring case
• strncat function: concatenate the first n characters of one string to the end of another string
• strncmp function: compare two strings
• strncpy function: copy string
• strpbrk function: find the first occurrence of a specified character in a string
• strrchr function: finds the last occurrence of a specified character in a string
• strspn function: Calculate the length of a substring composed of characters of the specified character set in a string

Chapter 16 Mathematical Computation Functions

• abs function: Calculate the absolute value of an integer
• acos function: Calculate the value of the arc cosine
• asin function: Calculate the value of the arc sine
• atan function: Calculate the value of the arctangent
• atan2 function: Calculate the value of the arctangent
• ceil function: computes the smallest integer greater than or equal to a given argument
• cos function: Calculate the value of the cosine
• cosh function: Calculate the cosine of a hyperbola
• div function: Calculate the quotient and remainder after dividing two integers
• exp function: Calculate the exponent of a floating point number
• fabs function: Calculate the absolute value of a floating point number
• frexp function: Compute the mantissa and exponent of a floating-point number
• hypot function: Calculate the length of the hypotenuse of a right triangle
• labs function: Calculate the absolute value of a long integer
• ldexp function: Calculate the product of a floating point number and 2 to the exp power
• ldiv function: Calculate the quotient and remainder after dividing two long integers
• log function: calculates the logarithmic value to the base e
• log10 function: Calculates the logarithmic value to base 10
• modf function: decompose a floating point number into fractional and integer parts
• pow function: calculate the power value
• sin function: take the value of the sine
• sinh function: Calculate the sine of a hyperbola
• sqrt function: Calculate the square root of a floating point number
• tan function: Calculate the tangent of an angle
• tanh function: Calculate the value of the tangent function of a hyperbola

Chapter 17 Random Numbers and Data Encryption Functions

• crypt function: use the DES algorithm to encode a string
• drand48 function: generate a positive random floating point number
• erand48 function: generate a positive random floating point number
• initstate: Create a random number state array
• jrand48 function: generate a long integer random number
• lcong48 function: set the random number seed for 48-bit operations
• lrand48 function: generate a positive long integer random number
• mrand48 function: generate a long integer random number
• nrand48 function: generate a positive long integer random number
• rand function: generate pseudo-random numbers
• random function: generate random numbers
• seed48 function: set the random number seed for 48-bit operations
• setstate function: create a random number state array
• srand function: set the random number seed
• srand48 function: set the random number seed for 48-bit operations
• srandom function: set the random number seed
• getpass function: get the user input password string

Chapter 18 Data Structure Functions

• bsearch function: binary search data in sorted array
• hcreate function: create a hash table
• hdestroy function: delete hash table
• hsearch function: find key data in hash table
• lfind function: linear search
• lsearch function: linear search
• qsort function: Sort an array using quicksort
• tdelete function: delete data from a binary tree
• tfind function: search binary tree
• tsearch function: search a binary tree
• twalk function: traverse a binary tree

Chapter 19 Other Functions

• getopt function: analyze command parameters
• isatty function: determine whether the file descriptor is a terminal
• pselect function: multiplexing I/O
• select function: multiple I/O transfer
• ttyname function: returns the name of the terminal