1GETAUXVAL(3) Linux Programmer's Manual GETAUXVAL(3)
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6 getauxval - retrieve a value from the auxiliary vector
7
9 #include <sys/auxv.h>
10 unsigned long getauxval(unsigned long type);
12
14 The getauxval() function retrieves values from the auxiliary vector, a
15 mechanism that the kernel's ELF binary loader uses to pass certain
16 information to user space when a program is executed.
17 Each entry in the auxiliary vector consists of a pair of values: a type
19 that identifies what this entry represents, and a value for that type.
20 Given the argument type, getauxval() returns the corresponding value.
21 The value returned for each type is given in the following list. Not
23 all type values are present on all architectures.
24 AT_BASE
26 The base address of the program interpreter (usually, the
27 dynamic linker).
28 AT_BASE_PLATFORM
30 A string identifying the real platform; may differ from AT_PLAT‐
31 FORM (PowerPC only).
32 AT_CLKTCK
34 The frequency with which times(2) counts. This value can also
35 be obtained via sysconf(_SC_CLK_TCK).
36 AT_DCACHEBSIZE
38 The data cache block size.
39 AT_EGID
41 The effective group ID of the thread.
42 AT_ENTRY
44 The entry address of the executable.
45 AT_EUID
47 The effective user ID of the thread.
48 AT_EXECFD
50 File descriptor of program.
51 AT_EXECFN
53 Pathname used to execute program.
54 AT_FLAGS
56 Flags (unused).
57 AT_FPUCW
59 Used FPU control word (SuperH architecture only). This gives
60 some information about the FPU initialization performed by the
61 kernel.
62 AT_GID The real group ID of the thread.
64 AT_HWCAP
66 An architecture and ABI dependent bit-mask whose settings indi‐
67 cate detailed processor capabilities. The contents of the bit
68 mask are hardware dependent (for example, see the kernel source
69 file arch/x86/include/asm/cpufeature.h for details relating to
70 the Intel x86 architecture; the value returned is the first
71 32-bit word of the array described there). A human-readable
72 version of the same information is available via /proc/cpuinfo.
73 AT_HWCAP2 (since glibc 2.18)
75 Further machine-dependent hints about processor capabilities.
76 AT_ICACHEBSIZE
78 The instruction cache block size.
79 AT_PAGESZ
81 The system page size (the same value returned by
82 sysconf(_SC_PAGESIZE)).
83 AT_PHDR
85 The address of the program headers of the executable.
86 AT_PHENT
88 The size of program header entry.
89 AT_PHNUM
91 The number of program headers.
92 AT_PLATFORM
94 A pointer to a string that identifies the hardware platform that
95 the program is running on. The dynamic linker uses this in the
96 interpretation of rpath values.
97 AT_RANDOM
99 The address of sixteen bytes containing a random value.
100 AT_SECURE
102 Has a nonzero value if this executable should be treated
103 securely. Most commonly, a nonzero value indicates that the
104 process is executing a set-user-ID or set-group-ID binary (so
105 that its real and effective UIDs or GIDs differ from one
106 another), or that it gained capabilities by executing a binary
107 file that has capabilities (see capabilities(7)). Alterna‐
108 tively, a nonzero value may be triggered by a Linux Security
109 Module. When this value is nonzero, the dynamic linker disables
110 the use of certain environment variables (see ld-linux.so(8))
111 and glibc changes other aspects of its behavior. (See also
112 secure_getenv(3).)
113 AT_SYSINFO
115 The entry point to the system call function in the vDSO. Not
116 present/needed on all architectures (e.g., absent on x86-64).
117 AT_SYSINFO_EHDR
119 The address of a page containing the virtual Dynamic Shared
120 Object (vDSO) that the kernel creates in order to provide fast
121 implementations of certain system calls.
122 AT_UCACHEBSIZE
124 The unified cache block size.
125 AT_UID The real user ID of the thread.
127
129 On success, getauxval() returns the value corresponding to type. If
130 type is not found, 0 is returned.
131
133 ENOENT (since glibc 2.19)
134 No entry corresponding to type could be found in the auxiliary
135 vector.
136
138 The getauxval() function was added to glibc in version 2.16.
139
141 For an explanation of the terms used in this section, see
142 attributes(7).
143 ┌────────────┬───────────────┬─────────┐
145 │Interface │ Attribute │ Value │
146 ├────────────┼───────────────┼─────────┤
147 │getauxval() │ Thread safety │ MT-Safe │
148 └────────────┴───────────────┴─────────┘
150 This function is a nonstandard glibc extension.
151
153 The primary consumer of the information in the auxiliary vector is the
154 dynamic linker ld-linux.so(8). The auxiliary vector is a convenient
155 and efficient shortcut that allows the kernel to communicate a certain
156 set of standard information that the dynamic linker usually or always
157 needs. In some cases, the same information could be obtained by system
158 calls, but using the auxiliary vector is cheaper.
159 The auxiliary vector resides just above the argument list and environ‐
161 ment in the process address space. The auxiliary vector supplied to a
162 program can be viewed by setting the LD_SHOW_AUXV environment variable
163 when running a program:
164 $ LD_SHOW_AUXV=1 sleep 1
166 The auxiliary vector of any process can (subject to file permissions)
168 be obtained via /proc/[pid]/auxv; see proc(5) for more information.
169
171 Before the addition of the ENOENT error in glibc 2.19, there was no way
172 to unambiguously distinguish the case where type could not be found
173 from the case where the value corresponding to type was zero.
174
176 secure_getenv(3), vdso(7), ld-linux.so(8)
177
179 This page is part of release 4.16 of the Linux man-pages project. A
180 description of the project, information about reporting bugs, and the
181 latest version of this page, can be found at
182 https://www.kernel.org/doc/man-pages/.
183GNU 2017-09-15 GETAUXVAL(3)